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Thermal Desktop Advanced Modeling Techniques

1. mmemrnee 2 6 24 2 Example Autoclaven sisemaa lats aial a tee 2 11 2 4 3 Example Milled Flow Channels imieeemmeevneenneennneenneeneee 2 16 3 Importing CAD Geometry scicescsiscccccssscsacsscesesaccncanscastivesscsctessssdcestestecee 3 1 31 SEP AP203 o dant edad Cea a 3 1 32 A ANSA k ma kaa else eae eae deans Ries 3 1 33 ACIS maksa Aen eae da KAKS SS a ew es Re esd Os 3 2 NO 3 2 4 Working with CAD Geometry eeeeveoooesosoooesooooooenaooenavooo esse nee 4 1 4 1 Simplifying CAD Geometry sii daa 4 1 AEL AUOCAD A kama 4 1 412 SN O 4 3 413 A ae onan T an aa ce ath 4 4 4 2 Snapping TD Objects to CAD Geometry coooocooccoccccooncconoconanonnnconccnonocnnn cono nconncnns 4 6 Finite Element Models inicia 5 1 5 1 Importing FE Model as Nodes and ElementS ooooocnnccnnnccnncccooncconoconcconncconocnnos 5 1 5 2 Importing FE Model as Graphics id 5 2 53 Mapping Results SAA AA 5 2 Meshing A A A 6 1 6 1 Final Preparation for MeshiN8 ooconccnincononoconononnnconncconocnnnconnoconocnnn cono ncon coco nacnnss 6 2 02 DM a a o les 6 3 6 2 1 Generating a Preview Mesh Controller o ooonoccninnnnncicicncnoncnnncconccnnns 6 5 6 2 2 Controlling Mesh Resolution indi 6 6 6 2 3 Slivers Cracks and other Problems in Parts 6 11 6 2 4 Generating the FEM Mesh ooioocononononconnninnnonnnconninnn cono ncno non cacon nono non 6 12 0 2 57 Viewing the Mesh a dd tr Aeon 6 14 6 2 6 Editing the Mesh using the Mes
2. Importing of IGES information is significantly different than all the other Import and Export options The IGES translator is written by Autodesk and is available through Au toCAD Mechanical or Autodesk Mechanical Desktop bundled with Autodesk Inventor Suite Users who purchase CAD functionality along with Thermal Desktop have access to the IGES translator The IGES translator is accessed typing the command IGESIN in the command line IGES import functionality only imports geometrical information such as lines points arcs circles etc This information may then be used to construct Thermal Desktop type surfaces and elements using these entities and the key points on these entities IGES import is a way to get CAD data from another CAD system such as PRO ENGI NEER I deas CATIA UNIGRAPHICS and others The user must EXPORT an IGES file from those CAD systems The export command on most CAD systems will contain options Importing CAD Geometry 3 1 for the user Generally it is best to try several different options to see what works best for the system being used once it is imported into AutoCAD Some of the options may produce so much information that the system may not be able to handle it in terms of memory and graphics limitations When exporting from I deas some users have determined that it is best to set the Surface Types to Export option to none This option will export just the wireframe geometry which can easily be used as refe
3. 0 125 units a 2D approximation of the plate is questionable Also it is convenient to use a geometrically faithful realistic representation of the flow channels as long as the mesh resolution is not too high such that FloCAD can calculate the flow areas wetted perimeter etc Therefore this part will be modeled as a FEM solid Both AutoCAD and Inventor will be used to create the plate in order to highlight the differences in approach between the two tools The drawings and part files are available for 15 inspection so only a brief description is provided here 15 These demonstrations are certainly not intended to teach efficient or even proper usage of either AutoCAD or Inventor Like the intended reader the authors are thermal fluid engineers and not CAD experts 2 16 Creating CAD Geometry Figure 2 11 Coolant Channels Milled into a Cold Plate 2 4 3 1 Creating the Cold Plate in AutoCAD The first step is to create a series of construction points placed at critical locations through the drawing at opposite corners of the plate at key locations where the centerlines of the flow channels will pass including tangents of any arcs at the centers of holes or where the Creating CAD Geometry 2 17 race track shaped manifold reliefs will be located Approximately 20 such points are re quired These points will make construction of objects circles polylines etc easy and accurate by acting as snap points A solid r
4. 6 2 8 Copying the Mesh A mesh can be copied by selecting the Mesh Controller alone using either Ctrl C Ctrl V or the AutoCAD Copy command A copy of the part preview and mesh will be placed wherever the user selects New layers as described in Section 6 2 5 2 will also be created 6 2 9 Attaching the Mesh to an Articulator Meshes generated using TDMesh can be attached to articulators assemblies or trackers The Mesh Controller is the only mesher entity part preview or mesh that must be attached to an articulator for the mesh to move with the articulator Attaching only the nodes and elements or only the part will not cause the mesh to move with the articulator A best practice however would be to attach both the Mesh Controller and the part to the articulator This ensures that the mesh remains aligned appropriately with the articulator If only the Mesh Controller were attached to the articulator and the articulator repositioned then regenerating the mesh from the preview would cause the mesh to realign with the part even though the mesh would still be attached to the articulator 6 16 Meshing 6 3 TDMesh Extrude For meshing arbitrary solid CAD parts TDMesh described in Section 6 2 is applicable Arbitrary CAD surfaces can also be meshed and if they are sufficiently thin they can be modeled as planar elements whose thickness and perhaps insulation properties can be assigned for thermal response even though the thickness and
5. 0 CSR TECHNOLOGIES Thermal Desktop Advanced Modeling Techniques User s Guide Version 5 2 July 2008 This manual as well as the software described in it is furnished under license and may be used or copied only in accordance with the terms of such license The content of this manual is furnished for informational use only is subject to change without notice and should not be construed as a commitment by Cullimore amp Ring Technologies Cullimore amp Ring Technologies assumes no responsibility or liability for any errors or inaccuracies that may appear in this book Prepared distributed and supported by Cullimore and Ring Technologies Inc 9 Red Fox Lane Littleton Colorado 80127 303 971 0292 www crtech com Authors Douglas P Bell Timothy D Panczak Brent A Cullimore Table of Contents Table of Contents ARA AAA 1 v L MECA CO a 1 1 tel IPPO os ms ado t laam imik ni 1 1 172 Terminology aii haa ekaa 1 1 E A A A ta dima cial la E 1 1 LA E dde de e Saar 1 2 1 5 UC ia 1 3 2 Creating CAD GEOMELF Vi sccsacssssacssseauendsctscsdendscdanessdonssachuecsusnaaevsvosseues 2 1 21 lt AMI ADD articles 2 1 2AL Capabilities ula aaa aaa aatal 2 1 2 1 2 TEAME RESOUTESS aaa E OS 2 4 22 Autodesk Vii 2 4 DIA AAD ADIN A aiad aaa aa 2 4 2 22 A tia ae a ala mulli laama eo 2 5 23 Summary of AutoCAD vs Inventor iia 2 5 A alata alal 2 6 2 4 1 Example Electronic Equipment Enclosure
6. 1 1 Advanced Modeling Process Map 1 5 Guidance The key to using the advanced modeling techniques described in this manual and ob taining a usable thermal model is planning As with any analytical task the analyst must understand the goal of the analysis understand the necessary detail of the model plan the appropriate path through the process and have a preliminary simple analysis completed Introduction 1 3 The quickest way to have troubles with a model is to over build from the start Including unnecessary detail i e small features which lead to too many nodes can slow or stall the solution obscure important results or make troubleshooting impossible Note C amp R Technologies has created TDMesh as a middle ground between building models with Thermal Desktop surfaces and solids and meshing in a dedicated mesher This tool can be very useful but if abused will create models that are difficult to maintain 1 4 Introduction 2 Creating CAD Geometry This section provides an overview of how to create analytic geometry in AutoCAD and Autodesk Inventor Analytic geometry means thermally appropriate surfaces and solids perhaps suitable for subsequent meshing Section 6 Ifa thermal analyst has been supplied a CAD drawing or structural model refer instead to Section 3 on how to import such data and Section 4 on how to prepare it for thermal modeling tasks This section presumes that no such CAD drawing is available an
7. 6 16 TD Mesh Extruder Parameters New elements will be extruded in a direction normal to the surface elements Layers refers to the number of solid sections The total number of copies of the base surface mesh including the original base mesh will be one plus the number of solid layers requested These two values can be redefined at any time but doing so will force a regeneration of the preview mesh and this action will in turn cause the deletion of any FEM meshes that have been created based on those previews the same effects that are caused by changing mesh resolution within the base part This form is similar that of the full 3D mesher in terms of mesh resolution adjustments but it differs in that the user can selectively omit the preview mesh for any or all of the three surfaces base end side associated with the extrusion This means the mesh extruder can create open shells e g sides only and other objects that cannot be created using the full TDMesh For extrusion and revolving operations an option exists to give each copy of the base mesh layer the same node IDs For an extrusion this would make a three dimensional thick finite element mesh but the resulting network of nodes and conductors would be two di mensional in SINDA FLUINT This may be a convenient way of modeling thick parts that perhaps must attach to other parts on both sides but it is desired to keep only a two dimen sional resolution Likewis
8. FEM Mesh Release from Controller Does not exist Done Help Figure 6 15 TD Mesh Extruder and Revolver Form Selecting Generate Preview invokes the preview parameter options shown in Figure 6 16 This form is similar that of the full 3D mesher in terms of mesh resolution adjustments but it also contains fields for defining the distance to extrude in model units can be positive or negative and the number of layers to employ The number of layers can be specified to be broken down in equal size layers or a parametric list of boundaries may be input The parametric distance is 0 at the base and 1 at the end of the extrusion Intermediate positions are specified as number between 0 and 1 Since the first and last boundaries will always exist they are not input Only the intermediate layer boundary locations need to be supplied 6 18 Meshing Mesh Extrusion Parameters A xl Base Surface Element Size Fraction of Max Dimension 0 1 Absolute Size 0 6124 Max Turning Angle 45 r Extrusion Subdivision Extrusion Distance 3 5 m Equal Size Layers 5 C Parametric Layer List Base end and side sur faces can be generated or omitted separately r Meshed Faces Interior IV Generate Surface Mesh Preview at Base IV Generate Surface Mesh Preview at End of Extrusion IV Generate Surface Mesh Preview on Sides of Extrusion Generate Solid Mesh Preview Cancel Help Figure
9. Figure 6 2 Mesh Generation Options for a Surface Part Meshing 6 5 When Generate Preview is invoked for a solid part the following form Figure 6 3 appears Again mesh resolution controls are described in Section 6 2 2 The two options that become active at the bottom of the form when the Mesh Controller is applied to a solid will now be described Mesh Generation Parameters Element Size Preview Mesh 9 Fraction of Max Dimension jor Resolution Control O Absolute Size 39 370078740157 in Max Turning Angle 45 Generate surface mesh Generate Surface Mesh Preview Generate Solid Mesh Preview Generate solid mesh Figure 6 3 Mesh Generation Options for a Solid Part If the user requires a thermal model of the surface to which radiation convection etc can be applied check the Generate Surface Mesh Preview button If the thermal re sponse within the solid is required also check Generate Solid Mesh Preview Having both options checked is equivalent to surface coating a solid FE mesh It is possible to neglect either the solid preview or the surface preview which means that one of the corresponding FEM meshes will not be generated in later steps only an uncoated solid or only a hollow surface model is needed For example the surface preview mesh can be skipped if no surface area effects such as radiation convection or contact will be needed which would be unusual More commonly the solid geometric mesh
10. Instead simple 2D Thermal Desktop surfaces will suffice as long as the nodes along the edges can be mathematically welded made coincident then merged There is little benefit therefore in starting with an Inventor part drawing an AutoCAD surface based model is best In fact the sides and top of the box can be represented using Thermal Desktop FDM rectangles with edge nodes for ease of merging shared nodes with adjacent rectangles The top and sides of the box are therefore constructed using FDM rectangles The bolt flange itself could also be constructed with such FDM methods However such a choice would require setting the resolutions i e node locations with extreme care such that adjacent nodes are coincident for both the side walls and the corners 7 When the material is continuous merging shared nodes is preferred Using a high value of contact conductance is a poor alternative it leads to both inaccuracies and numerical inefficiencies 2 8 Creating CAD Geometry YS KAS ETS ae A lt SIS Figure 2 5 Meshing a Thin Solid top versus Meshing a Surface bottom Therefore TDMesh will be used instead to create a FEM model of the flange A outer rectangular region is subtracted from an inner rectangular region in AutoCAD then meshed As experimentation will reveal the mesher creates an even spacing of nodes on the inner periphery of sucharegion even if it s not square and even if the mes
11. for completeness Creating CAD Geometry 2 13 2 4 2 2 AutoCAD and TDMesh Revolve Model of AutoClave Axisymmetric For a full 3D model the O ring channel should be removed and the bolt holes suppressed But for an axisymmetric model based on the cross section the full Inventor file is acceptable since all that are needed are snap points To create this model use Inventor Link or import an SAT file created by Inventor and snap a polyline representing the cross section in the XY plane as shown at the left of Figure 2 8 a simplified Inventor SAT file has been used for visual clarity The region formed by this closed polyline can then be meshed in 2D at the right of Figure 2 8 noting a TDmesh is shown rather than the actual TDMesh Revolve object again for visual clarity This mesh is then revolved about the Y axis where a construction line has been placed using TDmesh Revolve Section 6 4 resulting in the solid surface model shown in Figure 2 9 A sufficient number of layers 36 as shown has been chosen to make KI l AK A EN E Figure 2 8 Polyline Cross Section Snapped on Simplified Inventor Model Meshed the resulting volumes and surface areas more accurate Since the layers all contain the same nodes which is what makes the model axisymmetric the cost of this high resolution is minimal 2 4 2 3 Thermal Modeling of the Autoclave How should the copper cladding be handled It is difficult to assign different m
12. insulation are not depicted geometrically For objects that are thick enough or thermally resistant enough such that through thick ness temperature gradients are important or when a definite geometric depiction of their overall thickness is required the TD Mesh Extrude option may be applicable Many types of solids are most efficiently formed by extruding an arbitrary CAD surface through a length with the mesh generated on that original surface also extruded at the same time The resulting mesh is no longer a tri tet mesh Figure 6 14 shows a pentagon the red base part that has been meshed in the original plane then extruded into three solid layers The same triangular surface mesh exists at all four planes between these three layers but at the outsides the surface elements are quadrilaterals and pentahedral elements extruded triangles are used to represent the 3D elements Figure 6 14 Example Extrusion Pentagon Extruded into 3 Layers In addition to using higher level finite elements an extruded mesh has several important advantages over a 3D tetrahedral mesh that would result if the full TDMesh had been applied to the final extruded solid All of these advantages relate to the fact that the key surfaces in the resulting mesh can be generated and addressed separately which greatly facilitates application of boundary conditions that vary according to the face This capability is con trasted with that of the full TD
13. mesh preview appear in Figure 6 2 Since the part has no curved edges the Max Turning Angle is irrelevant for now it will be described later Therefore the resolution of the resulting mesh is controlled by one of two options shown at the top of that form as selected by the radio buttons e The fraction of the maximum dimension of the part e The absolute size of the largest element length along any edge Note that the default is 1 meter 39 37 inches This is usually not a good starting value and should be modified appropriately by the user Normally the Fraction of Max Dimension option should be used as it automatically adjusts the size of the element to the size of the part The alternative the Absolute Size option is intended to help make meshes that directly correspond to other meshed parts such that subsequent merging operations are facilitated because nodes will be coincident For example the Absolute Size option could be used to ensure that edges of a box whose faces are meshed as separate parts can be merged since the nodes in each face will then be coincident along the common edges Note that changing the resolution of either part s mesh will then force the user to repeat the merge operation Meshing 6 7 The default value for the relative mesh fraction is 0 1 meaning that 10 of the largest dimension of the object will dictate the element size For the part shown above Figure 6 4 the default settings result i
14. of the O ring channel and the outside edge of the autoclave The sketch is finished and the Hole command is used consuming Sketch2 into Holel Inventor automatically uses the center point of the previous sketch The 7ermination is switched to Through All and the diameter is chosen Now that a hole exists the Circular Pattern command is used The feature selected is the newly created hole then the y axis is selected as the axis of revolution and the number of occurrences is selected generating Circular Pattern which references Hole1 The copper cladding is added to the bottom by creating a new sketch Sketch3 on the bottom face of the autoclave The Offset command is used to create a slightly smaller version of the circle that forms the base of the pressure vessel The Extrude command creates the geometry of the copper disk consuming Sketch3 into the feature Extrusion1 12 The O ring and bolt holes are added for the sake of completeness of the resulting artwork and to illustrate their suppression or elimination in later steps These extra steps also demonstrate more Inventor features Of course a model generated by thermal analysts purely for their own use would not include such details 13 The small ledge formed by this step is largely irrelevant to a thermal model and only causes the mesher to use a small element size as needed to resolve it So again this does not represent good modeling practices but rather it is demonstrated
15. to these benefits the analyst also maintains control of the resolution of the thermal model and the types of objects used for the thermal analysis 5 2 Finite Element Models 6 Meshing The Thermal Desktop meshing utility TDMesh is a basic meshing tool developed to bridge the gap between building models using native surfaces and solids and importing meshes from third party applications e g structural analysis software TDMesh can be used for quickly creating simple 2D or 3D finite element meshes using CAD geometry that is either created within AutoCAD or imported into AutoCAD TDMesh is recommended if the part geometry is too complex to be represented by Thermal Desktop finite difference surfaces or solids Native conic surfaces are generally preferred when the geometry matches these surfaces since the surface curvature is mathe matically precise rather than approximated by flat elements Native surfaces also allow for a coarser nodal breakdown while still maintaining geometric fidelity when a coarse break down is acceptable for accuracy However for geometry that does not fit well with the standard set of conic primitives TDMesh provides a convenient and quick method for generating thermal models While TDMesh provides the ability to model geometry that is not easily described by the standard set of conic primitives and finite difference solids overly complex geometry is problematic as well First it might not be successfully mes
16. with CAD Geometry enclose the solid object Examples where this command would be useful would be unnec essary bolt holes fillets or chamfers In some instances an object may not be healable When this happens AutoCAD will not delete the selected face and note that the objects could not be healed Certain faces may need to be deleted in a specific order for healing to take place Figure 4 2 shows an electronics cover with some unnecessary details which are high lighted in red and some potentially unnecessary details highlighted in yellow These are all faces that may be deleted but the chamfer around the edge of the base cannot be deleted in one step since it is not one face Along each straight edge is a face and along each curved edge is a face If any one of these faces is deleted a discontinuity will occur at the next face casuing AutoCAD to fail the healing process The solution to this is to remove the rounded corners of the base followed by the chamfer faces at each corner and then the chamfers along the straight edges can be removed as seen on the left of Figure 4 2 The other highlighted surfaces could be deleted directly Figure 4 2 Simplifying a solid 4 1 2 Autodesk Inventor The modification features described below will be in slightly different locations depend ing on whether Inventor is in Part mode or Assembly mode 4 1 2 1 Select and Modify Subobjects In Inventor a selection tool provides options for which item
17. 5 views 6 15 generating 6 5 mesher 1 2 1 4 2 9 6 1 6 22 W extrude 2 8 4 1 6 2 6 6 6 17 6 19 revolve 2 14 6 2 6 6 6 20 6 22 meshing 6 1 6 22 final preparation for 6 2 model thermal 1 1 1 3 2 6 2 12 5 1 6 3 6 6 wireframe 6 1 AutoCAD 2 1 P part 6 4 6 7 controlling visibility 6 15 problems in 6 11 preview controlling visibility 6 15 generating 6 5 R region AutoCAD 2 2 2 3 6 1 6 3 6 5 6 18 6 20 command 2 2 resolution mesh controlling 6 6 S SAT See ACIS slivers 6 11 subtract 2 2 surface AutoCAD 2 2 2 5 6 1 6 3 Thermal Desktop 4 6 6 1 6 2 T TDMesh 1 2 1 4 2 9 6 1 6 22 extrude 2 8 4 1 6 2 6 6 6 17 6 19 revolve 2 14 6 2 6 6 6 20 6 22 Thermal Desktop commands tdmesh 6 5 thermal model 1 1 1 3 2 6 2 12 5 1 6 3 6 6 U upstream tools 4 4 V visibility controlling for parts meshes and pre
18. Mesh The bottom two thirds of the form depicted in Figure 6 1 control the generation of nodes and elements The middle third described in Section 6 2 4 1 is used to define thermal submodels numbering initial temperatures material properties radiation contact insula tion and other thermal properties The bottom third described in Section 6 2 4 2 invokes the generation of the FEM mesh based on the preview geometric mesh and on the thermal properties 6 2 4 1 Nodes and Properties Thermal properties may be assigned within the Mesh Controller middle third of Figure 6 1 independent of the preview mesh and independent of whether or not the FEM mesh has yet been generated In other words customizations made in these locations are preserved as long as the Mesh Controller itself is not deleted Newly created FEM meshes will use these thermal properties If a FEM mesh exists and the thermal properties are edited the FEM mesh will be updated accordingly Edit Node Properties By default a new thermal submodel named Sid will be created for each part s mesh where id is the TD assigned designation for the Mesh Controller e g S99 S1c45 etc and the nodes will be numbered starting with 1 The user can alter 6 12 Meshing these choices along with the initial temperature of the nodes using the TD FEM Mesh Node Properties form Figure 6 12 These designations will be preserved if either the preview or the FEM mesh is regener
19. Mesh which can only generate and address all the surfaces Meshing 6 17 as a single entity In other words the full TDMesh recognizes only one outer surface and it can make no distinctions based on faces which might not even exist as is true in the case of a sphere The usage of the TD Mesh Extruder is almost identical to that of the full TDMesh explained in Section 6 2 so this section will only highlight important differences To use the mesh extruder type tdmeshextrude in the command prompt then highlight an AutoCAD region or appropriate CAD surface Figure 6 15 shows the resulting form Note that the base end and side surfaces can be edited separately Base refers to the surface formed by the original CAD part surface end refers to the analogous surface formed at the far end of the extrusion opposite the base and perpendicular to the direction of extrusion and side collectively refers to all other surfaces parallel to the direction of extrusion TD FEM Extrude Revolve Controller xj M Mesh Preview Generate Preview Preview Status Set Label Display Preferences Does not exist TD FEM Mesh Properties Edit Nodes Edit Base Surfaces Edit End Surfaces Edit Side Surfaces Edit Solids Base end and side sur faces can be customized separately TD FEM Mesh Management Generate TD FEM Mesh from Preview TD FEM Status Delete TD
20. ack of medical articles for sterilization The wall is fabricated from stainless steel but has been clad at the bottom with a copper disk to help spread the heat from the burner At the top a bolted flange plate contains a slot for an O ring seal Figure 2 7 depicts the geometry o 9 Figure 2 7 Copper clad Stainless Steel Autoclave with Sloped Sides 9 See also the treatment of the copper cladding in the next example Section 2 4 2 In both cases defining a com mon 2D surface between adjacent parts would allows the mesher to assure alignment of nodes for the purposes of assigning contact or for merging coincident nodes 10 Essentially a high pressure and therefore higher temperature commercial pressure cooker that can include a drying cycle Creating CAD Geometry 2 11 A transient thermal model is needed to predict both start up and shut down times in cluding heat losses off the bottom and sides the lid is omitted from this discussion for simplicity In particular various water fill levels will be investigated including investigat ing an over temperature scenario in which the water is depleted accidently as needed to investigate burner control strategies Because of the thick and varying walls and the low conductivity of stainless steel a 3D thermal model or at least 2D axi symmetric is required 2D surfaces will not adequately represent this device The thermally irrelevant bolt holes and O ring slots represent targe
21. acted This complex region can now be meshed in 2D or extruded lofted etc then meshed in 3D Figure 2 1 depicts the region after having been extruded 2 In some cases it would be better to leave them as wireframes convert them into regions then use the TD op tions for extruding or revolving a mesh as explained in Section 6 3 and Section 6 4 See Section 2 4 2 as an example 3 Meshes in this instance refers to the method used by AutoCAD to create the surface Nodes and elements do not exist at this point 2 2 Creating CAD Geometry Figure 2 1 Plate with Hole subtracted circle Extruded Other boolean operations include union encompass all surfaces or volumes of all se lected objects as the new region or solid and intersect consider only the common area or volume as the new region or solid For example to make a race track shape union a rectangular region with two circular regions Figure 2 2 depicts such a race track after having been revolved 20 degrees around an axis parallel to the straight sides Figure 2 2 Rectangle with End Circles Unioned and Revolved 20 Degrees Creating CAD Geometry 2 3 2 1 1 3 Solids Section 2 1 1 1 described how to create a solid by starting with a wireframe Solids can also be created directly using various options under Draw gt Modeling including toroids cones and polysolids a solid version of a polyline Once created by whatever means solids can be unioned inter
22. aster Visibility Figure 6 13 Editing Mesh Display and Controlling Visibility using the Toolbar 6 2 5 2 Accessing and Controlling Visibility of Parts Previews and FEM Meshes If a solid is meshed and both the surfaces and interior portions are used as the basis for generating FEM meshes then at least 4 layers and 3 AutoCAD groups will have been generated automatically For example if such a solid were meshed and the resulting Mesh Controller ID was assigned by Thermal Desktop to be 8BE and if the user had assigned the label name myblock then the following AutoCAD layers and groups would have been created Layers TDFEM_PRT_MYBLOCK 8BE containing the part CAD surface solid TDFEM_MC_MYBLOCK 8BE containing the Mesh Controller TDFEM 2D MYBLOCK 8BE containing the 2D surface elements TDFEM 3D MYBLOCK 8BE containing the 3D solid elements Groups TDFEM OD MYBLOCK 8BE containing the thermal nodes TDFEM 2D MYBLOCK 8BE containing the 2D surface elements TDFEM 3D MYBLOCK 8BE containing the 3D solid elements The part layer is automatically rendered invisible when the Mesh Controller is invoked with the tdMesh command and by having generated a preview Otherwise the visibilities of these layers can be individually controlled using AutoCAD techniques or they can be controlled using the TD toolbar options shown at the right of Figure 6 13 For example a common choice is to turn off visibility of a
23. ated as long as the Mesh Controller is not deleted x Submodel 5173 y Initial temp 29915 K Starting ID 1 I Repeat Base ID s for 2D Symmetr Figure 6 12 FEM Mesh Node Properties Form Edit Surface Properties allows the user to set the properties of all the 2D surface elements that are generated either from a part that is a surface or from the surface mesh generated on the outsides of a solid This is the same form used for editing triangular or quadrilateral elements in Thermal Desktop Optical Properties Analysis Groups Conduc tion Capacitance data including non displayed thickness etc can all be set in this form If the FEM mesh has already been generated then this form edits all of those surfaces If the FEM mesh has not yet been generated or if it has been deleted then this form specifies the properties for any newly created surface elements generated from the preview mesh in the future Edit Solid Properties allows the user to set the properties of all the 3D solid elements that are generated This is the same form used for editing solid elements in Thermal Desktop This form can be used to set the materials material orienters conductor submodels and density conductivity scaling factors If the FEM mesh has already been created this form modifies the parameters of those solid elements If the solid elements have not yet been created or if they have been deleted then this form specifies the properties for any new
24. aterial properties within a single meshed part at least unless an uneven interface element by element can be tolerated On the other hand if two different parts are constructed then they must be thermally mated using large contact conductances which can be inaccurate and computationally inefficient Future versions of Thermal Desktop will offer methods for 2 14 Creating CAD Geometry Figure 2 9 Axi symmetry Model of Autoclave constraining meshes such that merging nodes of adjacent parts is facilitated or such that contact between adjacent nodes is more accurate and efficient e g the bonded fin plate on the north side of the box in Section 2 4 1 Until then an assumption of a thin copper plate will be applied in this case warranted by the lack of gradients in the high conductivity copper The bottom surface elements of the two models will be selected using a careful cursor drag and a thickness corresponding to the copper cladding will be assigned In other words conduction in the 3D elements representing the stainless steel will be augmented by superimposing 2D within surface conduction representing the copper cladding and the extra capacitance of the copper will also be appended to the nodes on the bottom disk Similarly other surfaces must be selected perhaps by placing into AutoCAD groups to impose other boundary conditions such as external natural convection on the sides pool boiling of water in the inside bottom a heat
25. can be skipped if a hollow object is desired keeping in mind that a mathematical not displayed thickness can be assigned to any TD surface When a new Mesh Controller is invoked the controller is placed on a layer named TDFEM MC label id and the underlying part surface or solid is placed ona layer named TDFEM PRT label id with visibility turned off where label is the optional user as signed label Section 6 2 5 1 and id is the TD designated Mesh Controller ID e g 348 or 1A4 Section 6 2 5 2 describes how to use these layers to easily control the visibility of each component of a TD mesh If a Mesh Controller is deleted the part will be restored to its original layer 6 2 2 Controlling Mesh Resolution The methods for controlling the resolution of meshes will be provided by example A 2D planar example will be described in detail This discussion is also applicable for a 2D mesh on an arbitrary curved surface or 3D mesh on an arbitrary solid For the TD Mesh Extrude and Revolve options Section 6 3 and Section 6 4 respectively the mesh resolution is applied first to the base surface object and then extruded or revolved into a solid 6 6 Meshing The example surface to be meshed is displayed in Figure 6 4 Note that this drawing has units of inches with the largest dimension of the part shown being approximately 1 2 inch Figure 6 4 Example Surface to be Meshed The default options for the
26. can be considerable even if some lines are hidden per the options described in Section 6 2 5 1 To avoid this effect in AutoCAD use the Tools gt Options gt Selection option or type options at the command prompt and choose the Se lection tab then turn off Selection Preview at least When no command is active For Mechanical Desktop users use Assist gt Options gt Selection to reach the equivalent form The preview may be displayed as a wireframe outline of just the edges of the mesh a wireframe of the elements with internal faces hidden a wireframe of internal and external elements a solid shaded view of just the external faces or a solid shaded view of both internal and external elements Different display options may be selected for post processing mode and in regular model mode See Section 6 2 5 1 The default mode when model building is to show a wireframe of internal and external elements and the default mode in post processing is to show a solid shaded view of the external faces When in model building mode after the resolution of the mesh has been examined and deemed acceptable the display option may be changed to show a wireframe of just the edges to increase graphics performance The preview will post process data in post processing mode Therefore the nodes and elements that have been created from the preview may be hidden for better graphics perfor mance 6 2 TDMesh TDMesh is used to create triangular finite
27. curve and imprint it on the solid To ensure a node is created at a specific location create a line that ends at the point and imprint that line onto the surface 6 10 Meshing 6 2 3 Slivers Cracks and other Problems in Parts Numerical inaccuracies and truncations in the geometry engines used to build represen tations of complex solid modeling parts are not uncommon Slivers cracks and other defects can cause problems for meshers which cannot assume that such mistakes were accidental Figure 6 10 presents an example of a crack introduced in the previous example In this 1IDA4 Figure 6 10 Irregular Mesh Density Indicates a Possible Problem in the Part case the mesher does not fail per se but rather returns a unusual distribution of element sizes that should draw the user s attention toward a possible problem in the underlying part Meshing 6 11 If the defect is severe enough the mesher can fail mathematically preventing the mesh from being drawn at all Ifit fails it will draw red crosses on potential problem areas Figure 6 11 shows one such failed part with the blue circle has been added to indicate the location CD Figure 6 11 Mesh Failure Points Indicated in the Part blue circle has been added of the real defect which would be visible if sufficiently zoomed The red indicators can be cleared by resetting Thermal Desktop graphics MI 6 2 4 Generating the FEM
28. d that one or more parts must be modeled in 2D and or 3D and that the complexity of these parts exceeds the applicability of the native suite of Thermal Desktop FDM surfaces and solids It is not the purpose of this section to provide detailed instructions on 2D and 3D mod eling capabilities Training and reference resources are available elsewhere some such references are summarized below Rather this chapter seeks to provide initial guidance to the thermal modeler whose CAD experience is limited 2 1 AutoCAD Although both AutoCAD and AutoCAD Mechanical offer nearly the same features some of the 3D modeling options are only available as text commands or in specific toolbars in more recent versions of AutoCAD Mechanical In some cases the primary menu may be different the Draw menu in AutoCAD is Design in AutoCAD Mechanical Therefore this section focuses on AutoCAD To choose AutoCAD use the Start gt All Programs gt Au todesk gt AutoCAD 2008 to choose AutoCAD 2008 the exact names and paths will vary by version year Or go to Tools gt Options and choose lt VANILLA gt in the profiles tab For simplicity both profiles are referred to herein as AutoCAD 2 1 1 Capabilities 2 1 1 1 Wireframes AutoCAD can be used to create various wireframe objects lines polylines arcs rect angles polygons circles etc Most of the wireframe objects are accessible under the Draw menu 1 Autodesk Inventor is available as pa
29. des and conductors This use of the term covers surfaces solids and elements with associated nodes but does not refer to boundary conditions heat loads or conductance defined between objects These latter objects are thoroughly documented in the Thermal Desktop User s Manual 1 3 Scope This document covers e Creation of thermal models based on geometry that has been created using CAD Introduction 1 1 techniques or imported from external sources importing finite element nodes and elements from other sources is noted but coverage is brief due to its simplicity Guidance for creating or importing geometry as a basis of the thermal model e Meshing of CAD geometry using TDMesh This document does not cover e Basic use of Thermal Desktop A thorough understanding of Thermal Desktop and basic modeling techniques as covered in the Thermal Desktop User s Manual is assumed and will not be covered in this document Application of boundary conditions heat loads and thermal connections between model components These topics are covered in the Thermal Desktop User s Manual 1 4 Process The advanced modeling techniques consist of two basic steps defining the geometry and building the thermal model In some situations importing a mesh or a thermal model defines the geometry and creates the thermal model this of course limits the analyst s ability to control the resolution of the model The Advanced Modeling Process Map Fi
30. ditions When the finite element file is imported the mesh appears as a graphical object By editing the graphical object select the mesh and choose Thermal gt Edit the user may choose the display options for the mesh displayer such as wireframe outline wireframe exterior faces etc This graphical object can be used for object snapping for either Thermal Desktop objects or AutoCAD wireframes surfaces or solids to be used for generating geometry that can be meshed using TDMesh If the results of the thermal model will be mapped back to the structural model then the post processing data mapper is recommended as described in the following section 5 3 Mapping Results Using the Post Processing Data Mapper is described in the Thermal Desktop Users Manual but for purposes of Advanced Modeling the data mapper object can be used similar to importing a finite element model as graphics as described in Section 5 2 The benefits of using the data mapper are e a graphical representation of the finite element mesh will be diplayed e the mesh graphics may be relocated translated and or rotated e the thermal model can be verified to represent the same geometry represented by the structural model e the mesh may be reimported e the thermal model results may be mapped back to the structural analysis mesh in its own format accounting for all translations and rotations the mapped results can be verified on the mapper In addition
31. e Cold Plate in Inventor A rectangle is created constrained at one corner to the origin and dimensioned 10 x 20 This forms Sketchl It is extruded 0 5 units which consumes Sketch into the Extrusion feature A new work plane Sketch2 is created on the surface of the plate and two 1 x 1 squares representing part of the manifold reliefs are sketched in opposite corners They are dimen sioned and located relative to the edges of the plate Two circles are added to each end of the squares their centers constrained to the middle of each side of the squares and their diameter constrained to the sides of the squares completing the two race track shapes Lines representing the periphery of the outermost two channels are sketched on Sketch2 being careful to avoid snapping to the manifold race track objects and using parallel per pendicular constraints to position the lines The lines are dimensioned relative to the edges and the distance between them the channel width is set to 0 5 units Extend is used to connect the lines to manifold objects without yet applying fillets nor trimming lines Next four lines of arbitrary length are added to Sketch1 not snapped but parallel to each other in the middle area These will form the channel walls of the middle two passages These lines are then constrained by channel width distance away from previous channels etc Creating CAD Geometry 2 19 Continuing Sketch2 diagonal construction li
32. e basis of that model In fact the AutoCAD model will be based on a slice of the 3D model since it is easier to pick points offan Inventor drawing than it is to create a complex cross section from scratch in AutoCAD 2 4 2 1 Inventor Model of AutoClave Full 3D The completed Inventor model is depicted in Figure 2 7 This section summarizes its construction methods A rudimentary outline of half of the autoclave s cross section is created on Sketch without the O ring channel A Fix constraint is placed to lock the shape to the origin The sketch takes shape as dimensions are added The larger dimensions e g overall radius height are added first then the smaller detail dimensions Once the general cross section is completed the O ring channel is added 12 A U shaped channel is drawn connected to the top edge of the autoclave The channel is dimensioned for size and distance from outer circumference Once the channel is fully constrained the Trim command is used to open the channel The sketch is finished and the Revolve command is used The sketched profile is se lected and the y axis found under Origin in the model window is selected as the axis of revolution The cross section sketch Sketch1 is now consumed by the Revolve command creating the feature Revolution1 A new sketch Sketch2 is created on the top of the part The Point Center Point command is used to create a point on the surface halfway between the outside edge
33. e for revolved parts Using the same node ID s on each layer simulates a two dimensional radially symmetric part Select the Repeat Base ID s for 2D Symmetry option on the Mesh Node Properties form as shown in Figure 6 17 Meshing 6 19 1 s vd Submodel sn y Initial temp kes K Starting ID no IV Repeat Base ID s for 2D Symmetry Cancel Help Figure 6 17 Option for Creating a 2D Symmetric Part When Extruding or Revolving 6 4 TDMesh Revolve For meshing arbitrary solid CAD parts TDMesh described in Section 6 2 is applicable However many types of solids can be formed by revolving an arbitrary CAD surface around an axis with the mesh generated on that original surface also revolved along the arc The resulting mesh is no longer a tri tet mesh Figure 6 18 shows a rectangle with three holes the red base part that has been meshed in the original plane then revolved 90 degrees into six solid layers subtending 15 degrees each The same triangular surface mesh exists at all seven planes between these six solid layers but the sides the surface elements are quadrilaterals and pentahedral elements extruded triangles are used for the solid portions of the model In addition to using higher level finite elements a revolved mesh has several important advantages over a 3D tetrahedral mesh that would result if the full TDMesh had been applied to the final revolved solid All of these advantages relate to the fact that the k
34. element meshes of AutoCAD regions or arbitrary AutoCAD surfaces It may also be applied to arbitrary solids with the option of creating only triangular surface elements or creating only tetrahedral solid elements or both simultaneously which is the default TDMesh can also generate FEM meshes from AutoCAD polygon mesh surfaces Once an region surface or solid the part has been constructed or imported and simplified if necessary TDMesh can be invoked As will be shown the generation of a thermal model from a geometric object is a multi step process Generation of the geometric mesh preview mesh or simply preview from the Meshing 6 3 part with the creation of the corresponding Mesh Controller e Assignment of surface and or solid element thermal material properties and boundary conditions e g insulation radiation e Generation of thermal elements and nodes FEM mesh from the preview Usually these three steps should be performed in the order listed above However a significant feature of the TDMesh design is that any of the three steps can be revisited at any time using the Mesh Controller In fact the underlying part can be moved or modified as well Of course some modifications may require the deletion and regeneration of the FEM mesh TD nodes etc as part of the revision process but the user effort required to reconstruct a new FEM mesh has been minimized Once a preview geometric mesh thermal propert
35. epresenting the outer envelope of the plate 0 5 x 10 x 20 units is created either 16 by using the box option or extruding a rectangular region Next two solid cylinders representing the through holes are created and subtracted from the plate Two are needed since they are consumed in the boolean subtraction opera tion A race track shaped solid is created via a union of a box and two cylinders The thickness is made equal to 0 25 units the depth of the resulting cut This solid is copied and one of each is used to make the manifold relief cuts boolean subtractions near the through holes Four polylines are constructed along the centerlines of each flow channel at the surface of the plate A new UCS is created such that the XY plane is perpendicular to the plate Four rect angular passages 0 5 by 0 25 units are created representing the cross section of the flow channels The sweep operation is then used to generate the solids that will be used to cut the flow channels This is why the four cross section profile rectangles were twice the thickness of the cut required 0 5 by 0 125 units they are swept along the centerline which was at the surface of the plate and so they appear to stand out from the plate 0 125 inside and 0 125 outside Subtraction of these four swept solids from the plate results in the finished plate Figure 2 12 depicts the final object including the mesh preview Construction objects points centerline
36. er reduce model size 2 If the desired object is a complex solid use Inventor especially if the dimensions are subject to change during design iterations Each time dimensions change the Inventor part must be reimported This means that mesh controls and thermal properties and boundary conditions must be reapplied Creating CAD Geometry 2 5 This requirement to repeat set up steps is also present for complex AutoCAD solids so it does not represent a net defect for the Inventor pathway But it can be difficult to change dimensions of a complex e g boolean operated part in AutoCAD often requiring starting over of the part itself much less the thermal attributes This diffi culty is not present in Inventor which accommodates even significant changes to a part with minimal rework 3 If the desired object is a simple solid consider AutoCAD but Inventor may be applica ble as well For simple solids a change to the solid might be made in AutoCAD without losing any associated Mesh Controller data though the user must force a re mesh For example the extrude and rotate operations for the objects in Figure 2 1 and Figure 2 2 respec tively can be adjusted using grip points after the mesh has been made However the size of the hole in Figure 2 1cannot be easily adjusted after the boolean subtract oper ation has been performed whereas the equivalent change is trivial within Inventor Of course if the solid can be described by a T
37. ey surfaces in the resulting mesh can be addressed separately which greatly facilitates application of boundary conditions that vary according to the face This capability is contrasted with that of the full TDMesh which can only generate and address all the surfaces as a single entity making no distinctions based on faces which might not even exist as in the example of a sphere In addition revolving a base mesh can model the same solid geometry with less elements than if it were meshed with tetrahedrons The usage of the TD Mesh Revolver is almost identical to that of the full TDMesh explained in Section 6 2 so this section will only highlight important differences To use the mesh revolver type tdmeshrevolve in the command prompt then select an AutoCAD region or appropriate surface and then select an axis of revolution Figure 6 15 shows the resulting form which is identical to that of the mesh extruder described in Section 6 3 Note that the base end and side surfaces can be edited separately Base refers to the 2 Revolving about an axis that touches one side of the part or that is coincident with it is legal Such a revolution results in some side surface elements that may not be quadrilaterals and in some solid elements that may not be pentahedrons 6 20 Meshing Figure 6 18 Example Revolution 3 holed Rectangle 90 degrees with 6 layers surface formed by the original part end refers to the analogou
38. g the solid This 2D mesh can be assigned a virtual thickness within the TDMesh edit form but ifa geometrically faithful thickness is required perhaps for radiation then the 2D mesh can be extruded Section 6 3 Nonplanar surfaces can also be extruded along local surface normals If the thickness is large enough or if the conductivity low enough several 2D mesh layers can be used If instead the thickness is small enough or conductivity high enough these layers can contain the same node numbers when the TDmesh Extrude option is used resulting in a model which is geometrically faithful but thermally approximate Working with CAD Geometry 4 1 Figure 4 1 Meshing a Thin Solid top versus Meshing a Surface bottom 4 1 1 2 Select and Modify Subobjects By holding the lt CRTL gt key and clicking on edges and faces the edge or face of a solid can be individually selected This same method allows selecting the original objects of a solid created by boolean operations Once selected the grip points ofa subobject will become available and the subobject can be edited e g the radius of a cylinder used to create a hole in a block 4 1 1 3 Delete Face The Modify gt Solid Editing gt Delete Face command allows the user to remove a face from the solid The faces can be selected using the method described in the previous section AutoCAD will attempt to heal the solid fill the gap by extending existing faces to re 4 2 Working
39. geometric mesh or preview and finally the surface and solid FEM elements themselves Section 6 1 contained sugges tions for preparations to avoid excessive time the software spends redrawing meshes and elements This section describes how to modify the visibility and display of the generated objects after they have been created 6 2 5 1 Controlling the Mesh Display Set Label The Mesh Controller label can be optionally defined or redefined using this button on the TD FEM Mesher dialog Figure 6 1 This label will be used as part of the layers and AutoCAD group names used to contain the preview mesh the part and the TD FEM elements Display Preferences Using this button on the TD FEM Mesher dialog the preview mesh display can be customized using the controls shown on the left side of Figure 6 13 These options are especially useful for viewing solid geometric meshes Note that the pre view s visibility can be turned off at the AutoCAD layer level as described in Section 6 2 5 2 6 14 Meshing TDMesh Preview Display Control ORF ORP O023 A AA Part Visibility Edit Mesh Displayer O Wireframe Outline A aa Preview Visibility Wireframe exterior faces hidden internal faces O Wireframe interior and exterior faces Surface Visibility O Shaded exterior faces hidden intemal faces O Shaded interior and exterior faces 3 o Solid Visibility m o Cancel Help az az Ex al M
40. gure 1 1 provides an overview of the paths from geometry to thermal model To use this map start from the block describing where the geometry or mesh has been or will be created and follow the possible paths for trans ferring the data to Thermal Desktop where the thermal model can be built or imported in the case of finite element meshes Defining the geometry is accomplished by creating the geometry in Thermal Desktop Autodesk Inventor or a preferred CAD or CAE tool and importing the geometry if neces sary into Thermal Desktop Depending on the control the user has over the format of ge ometry coming from another CAD tool the geometry file may have to be translated into a format readable by Thermal Desktop Section 2 Section 3 and Section 4 will provide guid ance on defining the geometry The thermal model is then created by either snapping Thermal Desktop objects to the geometry meshing in another CAE tool or using the Thermal Desktop mesher Section 4 Section 5 and Section 6 will provide guidance for these options with Section 6 being the user s guide for using the Thermal Desktop mesher 1 2 Introduction Geometry Geometry in CADICAE tool Autodesk Inventor PKK PPPS Mesh in CAE tool AutoCAD Thermal Desktop M M O TTTTTYTTTYTYITTTTTTTITTITTITYTTITYTTTTTITTITTITTIIITIITYYN o rs Thermal Model Build thermal model Import thermal model into Thermal Desktop Figure
41. h Controller 6 16 61 Moving the Mi al le ar scat lays 6 16 6 2 8 Copying the Mes his ctra lia taastada kavandasid 6 16 6 2 9 Attaching the Mesh to an Articulator tteaes 6 16 63 TDMesh Extr de drr aas summaks eS 6 17 64 TDMeshRevo E taa 6 20 vi vii viii 1 Introduction This document is a user s guide for advanced modeling techniques in Thermal Desktop It has been developed as a road map instead of an instruction manual The exception to this is Section 6 Meshing which contains the instructions for using TDMesh 1 1 Purpose Advanced modeling techniques should be used when e The geometry is complex and cannot be easily modeled with Thermal Desktop surfaces and solids e Simplification of the geometry to be represented by Thermal Desktop surfaces and solids would complicate boundary condition modeling e The geometry has been developed in other CAD tools e The geometry has been developed in other analysis tools Using some of the advanced modeling techniques within this manual such as meshing should be considered a last resort Some reasons are simpler models can be easily checked by hand calculations complex models can be difficult to debug extremely large models like many finite element models can be slow to converge 1 2 Terminology Throughout this document the term thermal model is used to represent the geometry based definition of no
42. h resolution is increased etc This fact means that coincident nodes are available to merge with those on the edges of the FDM surfaces The finished model is depicted in Figure 2 6 The default mesher resolution results in an 8x8 grid of element lengths along the insides of the flange region with 9 nodes at the endpoints of each equally spaced segment This means the side panels represented by TD FDM rectangles can be chosen to use a node resolution of 9 using edge nodes such that the nodes along the inner periphery of the flange are coincident with the nodes at the bottom edges of the side panels they can be merged Those shared nodes will participate both as members of finite elements and as locations within a finite difference grid The number of nodes representing the height of the box need not be 9 as long as all sides use the same Creating CAD Geometry 2 9 value such that nodes can also be merged along the vertical edges The top panel s resolution is then chosen such that its nodes also match those of the top most edges of the side panels such that those nodes are also coincident and can be merged Once all the merging has taking place any adjustments to the mesh density or to the resolution of any of the side panels will require a complete adjustment of other panels to match as well as a repeat of the merge operations Figure 2 6 AutoCAD and Thermal Desktop Finished Box Model Nodes Visible If the air cooled fins
43. he face is a three or four sided polygon TDMesh also supports extrude and revolve operations for regions arbitrary surfaces and polygon meshes Again the final output may consist of only surface elements or only solid elements or both However because of the nature of the extrusion or revolution pro cess the surface elements are not always triangular and in fact they are often quadrilaterals on side surfaces and the solid elements are not always tetrahedrons instead they are often pentahedrons Furthermore unlike meshing an arbitrary solid using TDMesh certain op tions are available when extruding or revolving the ability to selectively generate and customize key surfaces This capability facilitates application of boundary conditions e g radiation convection and model reduction via the exploitation of symmetry In the following discussion a part is a CAD surface or solid that is the basis of the mesh to be generated A preview mesh or simply preview is the geometric mesh made from and corresponding to the part The geometric mesh is called a preview because it is simply a set of points and lines and is not yet a thermal model The user may examine the preview to be sure it meets the needs of the thermal analysis If acceptable the preview can be used as the basis of generating a TD finite element model FEM mesh If the part is modified and or meshing parameters are modified e g finer resolution is chosen t
44. hed by TDMesh Or if the complex object is successfully meshed the resulting model might be slow to solve and perhaps cumbersome to use e g when assigning boundary conditions because of the enormous number of elements Therefore removing any details not required by the thermal analysis small holes fillets chamfers etc and using a simplified starting point e g a thickened or extruded mid plane instead ofa thin solid will be well worth the effort TDMesh is not the starting point it should be invoked after appropriate preparations have been made as emphasized in other sections of this document For arbitrary surface and solid geometry TDMesh creates triangular surface elements and tetrahedral solid elements More advanced techniques than such tri tet meshes are available in dedicated third party mesh preprocessors For these reason and for cases where TDMesh fails to accommodate a geometric detail that cannot be adequately simpli fied it should be noted that some meshing needs will be best met by third party mesh preprocessors In these situations the analyst should refer to the section on importing finite element nodes and elements Section 5 Finite Element Models on page 5 1 TDMesh Section 6 2 will produce triangular elements on an surface e g AutoCAD region or surface An AutoCAD region is a planar surface that is constructed out of bound ing wireframes The bounding wireframes may be straight lines arcs or sp
45. hen the preview and the FEM mesh may be easily regenerated 6 1 Final Preparation for Meshing Section 2 described how to build CAD parts in AutoCAD Inventor and other software Section 3 described how to use imported CAD parts and Section 4 describes how to prepare parts before or after importing such that sensible and efficient thermal models are possible 1 Atthe very least the reader should be familiar with creating AutoCAD regions and solids and with the Union Sub traction and Intersect commands before continuing 6 2 Meshing This subsection therefore does not describe geometric preparations since they are cov ered in prior sections Instead the preparations for efficient display of meshes are listed Meshes tend to be geometrically complex to draw and so tips for accelerating their display and refresh rates are well worth noting First consider turning off the visibility of Thermal Desktop nodes by unchecking the TD RC Nodes button on the Graphics Visibility tab of the Preferences option as de scribed in the main Thermal Desktop User s Manual The locations at which nodes exist will be obvious as the vertices of the resulting mesh and rarely will the user need to select and edit FEM mesh nodes directly Second consider turing off AutoCAD s selection preview By default when the cursor passes over a mesh it will be highlighted as a prompt However highlighting forces the redraw of every line in the mesh which
46. hermal Desktop solid then the functionality of those will most likely benefit the thermal analysis 2 4 Example Cases This section uses specially designed examples to identify important modeling techniques and to underline key decisions The AutoCAD drawing files and Inventor part files corre sponding to these examples are available for inspection 2 4 1 Example Electronic Eguipment Enclosure A box containing the switch control for a railway is to be stationed outside and will be sealed against the environment to minimize maintenance despite the dissipations of the internal electronics When placed in a location in the northern hemisphere the north side of the box will be the coldest since it will always be shaded This fact will be exploited to keep the electronics inside from overheating in summer Selective use of paint will be made on each side plus arack of vertical cooling fins not shown will be attached on the northern side The box is constructed by stamping a single sheet of aluminum as shown in Figure 2 3 A gasket under the bottom flange forms a seal with a foundation plate not shown This gasket is a thermal insulator A thermal model of the box is desired that can predict the solar heat loading and infrared and convective heat rejection including estimation of the gradients within the conductive surface 5 Future versions of Thermal Desktop are expected to eliminate this difficulty in the same way that the cente
47. icon on the toolbar and then select the part to be meshed This action invokes the creation of a new Mesh Controller object The Mesh Controller pro vides access for later editing and further mesh construction steps that can be deferred if needed The Mesh Controller dialog will appear Figure 6 1 TD FEM Mesh Controller A xj M Mesh Preview Geometric enerate Preview Preview Mesh Control Set Label Geometric Mesh Status Preview Status Display Preferences Does not exist M TD FEM Mesh Properties Element Edit Node Properties Grey for Properties 2D parts Co ntrol Edit Surface Properties Edit Solid Properties TD FEM Mesh Management FEM Mes h Generate TD FEM Mesh From Preview FEMM h Control es Status Does not exist Delete TD FEM Mesh Release from Controller Figure 6 1 Thermal Desktop Mesh Controller surface no mesh yet generated Selecting Generate Preview invokes the following form Figure 6 2 which con trols the generation of the geometric mesh or preview The dialog is shown for a mesh controller used on a 2D part The options for adjusting the mesh resolution are covered in Section 6 2 2 Mesh Generation Parameters Element Size Mesh 9 Fraction of Max Dimension 0 1 Resolution O Absolute Size 39 370078740157 in Control Max Turning Anale 45 Grey for 2D parts Generate Preview Cancel
48. ies and FEM mesh have all been created the user can revisit earlier decisions without losing all of their work As with any TD object node surface and solid properties can be altered at any time either at the element level or at the Mesh Controller level which will override any element level customiza tions The preview mesh itself can be revised as needed perhaps to increase or decrease mesh resolution While this step will necessarily require deletion and regeneration of the FEM mesh and therefore any element level customizations any node surface or solid properties that have been defined using the Mesh Controller will be preserved such that a new FEM mesh can be immediately generated using the revised preview mesh However please also note that certain boundary conditions such as heat loads contactors etc will be lost if the FEM mesh is deleted or regenerated Modifying the underlying part surface or solid will require that the preview be regen erated and this in turn requires that the FEM mesh be regenerated as well The Mesh Controller manages the regeneration of the preview the deletion of the previous FEM mesh and the creation of the new FEM mesh with the previously used node surface and solid properties making such modifications easy to accomplish 6 4 Meshing 6 2 1 Generating a Preview Mesh Controller To construct a mesh of a planar region surface or solid type tdmesh at the Command Prompt or select the
49. l schemes etc be assumed axially symmetric Is the flame applied evenly Is the unit tilted such that the water level is no longer symmetric Is the autoclave situated in a row with sister units such that environments are not symmetric Is the rack of instruments to be inserted also axi symmetric Depending on the answers to these questions a full 3D model might be required whether created in AutoCAD assuming the design is not still changing or Inventor preferable if vessel design changes are still being investigated On the other hand if symmetry can be assumed then a 2D mesh of the cross section can be extruded with identical nodes at each circumferential slice Section 6 4 resulting in several orders of magnitude increase in run times In fact it is probably a good idea to create the axisymmetric model even if the full 3D model is required because the multiple orders of magnitude speed increase can be exploited to run optimizations sensitivity and uncertainty analyses correlations to test data etc 11 Software salespersons are surely tempted to present such plug and play capabilities as the standard taking engineering decisions out of the loop via automation 2 12 Creating CAD Geometry Therefore both types of models will be constructed to illustrate the differences in ap proach and usage The 3D model will be constructed in Inventor and the axi symmetric model will be constructed in AutoCAD since a 2D surface is th
50. lid part the model size grows even faster 1 F This means a FEM model of a solid object meshed with F 0 01 will be roughly three orders of magnitude times larger than one meshed with F 0 1 The solution costs for radiation calculations and for transient thermal analyses will rise even faster requiring a total growth of perhaps six orders of magnitude in solution times Small fractions should be used sparingly and values below 0 01 should be avoided if possible Often what is important to thermal analyses is a mesh fine enough to adequately represent total volume and surface area and meshes fine enough to resolve detailed internal temperature gradients are often a luxury rather than a necessity 6 8 Meshing Figure 6 6 Example Surface Most Coarse Possible Fraction 1 0 MA ja ZA VA 2 SOSA AO A 1093 ESA A AS ALAS INIA NORRIS ACERT REAKTIV ED ARSE ESE SIO OY SPREE M SSE ERASERS NA NAASE IK RSN AS Figure 6 7 Example Surface Fine Resolution Fraction 0 03 In fact if the fractional or absolute control parameter is too small the mesher itself may take an inordinate amount of CPU time to compute the preview mesh Jf a too small reso lution parameter is entered accidently use the escape key ESC to abort mesh construction To illustrate the effects of the Max Turning Angle control a circular hole will be cut in the surface presented in Figure 6 4 resu
51. lines Regions may be modified by boolean operations such as subtracting a circle from a region to create a hole An AutoCAD surface can be a simple or complex surface created by lofting extruding or revolving an open wireframe or by exploding a solid AutoCAD solids can be created in many ways including boolean operations lofting extruding and revolving surfaces or Meshing 6 1 closed wireframes the starting vertex is coincident with the ending vertex When TDMesh is applied to an arbitrary solid 3D object the user will have the choice of generating only triangular surface elements or generating only tetrahedral solid elements or both TDMesh may also be applied to AutoCAD polygon meshes An AutoCAD polygon mesh is a collection of vertices and faces used for geometric display Polygon mesh faces are either three or four sided polygons An AutoCAD polygon mesh can be specified man ually by creating vertices and faces by extruding or revolving curves or by fitting a surface between four boundary curves e g the AutoCAD command edgesurf AutoCAD polygon meshes are the original way surfaces were geometrically represented in AutoCAD before the recent introduction of advanced NURB based surface and solid modeling When TD Mesh is applied to polygon meshes the FEM mesh generated matches the vertices and faces of the polygon mesh no further subdivision is performed Triangular or quadrilateral ele ments are created depending on if t
52. ll 3D solid layers TD_FEM3D so that only surfaces are visible for a coated 3D model AutoCAD groups can also be used to help perform selection operations Meshing 6 15 6 2 6 Editing the Mesh using the Mesh Controller Once a Mesh Controller has been created by generating a preview it can be accessed by selecting anywhere on the preview including the ID for example the 1D53 as shown at the right in Figure 6 5 and editing the object or by locating the Mesh Controller in the Model Browser List gt Meshers The Mesh Controller will be saved along with the drawing even if no FEM mesh yet exists such that this task can be completed later Also as described in prior sections any of the modeling decisions e g mesh resolution material properties etc can be revisited at any time using an existing Mesh Controller However some operations will force the deletion of the FEM mesh e g changing preview mesh resolution which in turn might require some customizations e g assignment of heaters convection etc to be repeated 6 2 7 Moving the Mesh To move using the AutoCAD Move command a mesh created using TDMesh the best practice is to move the part and the Mesh Controller together The nodes of a TDMesh cannot be moved away from the Mesh Controller much like a node cannot be seperated from its TD surface If the Mesh Controller is moved alone then if the mesh is regenerated the mesh will be realigned with the part
53. load or radiation connection on the bottom representing the burner etc The number of SINDA nodes required in the full 3D model Figure 2 1 0 4 is about 1450 whereas the number in the axisymmetric model is only about 50 By itself this results in approximately 40 fold decrease in solution cost per time step or per steady state iteration In transients the time constant of each node in the axisymmetric model is also much larger 14 Overlapping views of the surfaces and solids cause the mottled appearance of this figure but this defect of graphics makes it easy to see the elements including the small elements that are formed at the recess formed by the copper disk Creating CAD Geometry 2 15 than that of the nodes in the full 3D model In other words few timesteps are required in the axisymmetric model resulting in a highly nonlinear decrease in total run times hundreds of simplified what if cases can be run using the axisymmetric model in the same time it takes to make a single full 3D transient run Figure 2 10 3D Mesh of De featured Autoclave 2 4 3 Example Milled Flow Channels A cold plate is formed by milling coolant channels into a relatively thick plate Figure 2 11 with simple manifolding provided by a set of reliefs milled into the plate where the channels meet The cold plate will be topped by a flat plate not shown Because of the thickness of the plate 0 5 units compared to the depth of the channels
54. lting in the following default mesh Figure 6 8 Although some of the turning angles around the hole are smaller than the default value of 45 degrees especially as needed to encompass the thin section above the circle the maximum turning angle is visible at the right section of the circle s circumference The turning angle is the angle that a straight line segment turns into the next straight line segment as the segments pave a 1D curve Meshing 6 9 Figure 6 8 Example Surface with Hole Default Resolution Max Turning Angle 45 To better visualize the effects of this resolution parameter Figure 6 9 shows the effects of both increasing and decreasing the Max Turning Angle parameter from the default value Nara SLI Y ROA lt PY POD SBE AL SEDO KRAAG Figure 6 9 Example Surface with Hole Max Angle 90 Left and 22 5 Right L Another option for controling mesh resolution is adding break points to edges An edge that is defined by three points will be subdivided by the mesher into more elements than an edge defined by two points Likewise a face that is subdivided will be meshed differently than a face that is not even if the subdivisions are in the same plane Faces of solid geometry may be subdivided using the imprint command Modify gt Solid Editing gt Imprint Edges To ensure nodes are created along a certain line or curve on a solids face create the line or
55. ly created elements that will be generated from the preview mesh 6 2 4 2 Generating FEM Mesh from Preview Generate TD FEM Mesh from Preview button will create Thermal Desktop nodes and elements from the preview mesh These are the network objects that Thermal Desktop will use for thermal calculations Note that these FEM objects will be placed on a new layer whose name is based on the object ID and label of the Mesh Controller Surfaces are placed on a layer named TDFEM 2D label id and solids are placed on TDFEM 3D label id where label is the optional user assigned label Section 6 2 5 1 and id is the TD designated Mesh Controller ID e g 714 or 5C4 Section 6 2 5 2 describes how to use these layers to easily control the visibility of each component of a TD mesh Each TD mesh will also be placed into a unique AutoCAD group named according to the same scheme used for layers Up to three such groups can exist for a single Mesh Controller one for the nodes 0D one for the surface elements 2D and one for the solid elements 3D as described in more detail in Section 6 2 5 2 If the label is changed corresponding layer and group names are automatically updated Meshing 6 13 Delete TD FEM MESH provides a means of deleting a FEM mesh though this step is not necessary in order to regenerate a mesh deletion of the FEM mesh is performed auto matically if the preview mesh is regenerated Release f
56. ly approxi mate curved surfaces As described in the previous section surfaces may be created by extruding revolving lofting or sweeping open wireframes Surfaces may also be extracted from solids by ex ploding the solid Regions are created either by using the REGION command and selecting a planar closed wireframe or using the BOUNDARY command and selecting an interior point of the region The BOUNDARY command will determine all boundaries using any coplanar lines and curves including wireframes surfaces and other regions AutoCAD can also be used to create meshed surfaces from multiple lines Using the Draw gt Modeling gt Meshes menu lines can be used to create revolved meshes ruled meshes edge meshes and tabulated meshes The meshes are described by a matrix of M by N points that define the verticies of the planar faces The values of M and N are defined by the AutoCAD variables SURFTABI and SURFTAB2 Boolean operations can be performed on regions As an example the process of making a plate with a hole in it will be described First create a rectangle and a coplanar circle coplanar will be the default if the UCS is not changed in between creation of the two objects Use the region command to convert both the rectangle and the circle into regions Then type subtract or Modify gt Solids Editing gt Subtract choosing the rectangle first as the object to subtract from and then the circle as the object to be subtr
57. manuals tutorials showme features and access to various online resources Autodesk University www autodesk com au An annual user conference is hosted with extensive training opportunities CD based Training Accelerated Productivity www trainingtutorial com offers a Sol id Modeling self guided training course for a modest fee that explains the basics of creating Inventor parts 2 3 Summary of AutoCAD vs Inventor AutoCAD s heritage as a drafting tool means that it focuses on 2D objects and repre sentations Nonetheless for simple 3D objects e g a plate with a hole in it there are advantages to using it rather than Inventor Thermal Desktop is only available inside of AutoCAD and not Inventor Inventor provides powerful tools for generating and updating complex 3D parts but of course any parts generated in Inventor must be exported to AutoCAD usually as SAT files and that means any assignment of mesh and thermal properties must be repeated if the import is repeated In summary 1 If the desired object is a surface use AutoCAD Surfaces suffice in many cases for thin objects and are in many ways superior to sol ids since they are computationally more efficient see for example Section 2 4 1 Recall that a virtual thickness insulation etc can be added to Thermal Desktop sur faces and that surfaces can be extruded rotated etc into 3D objects or meshes and symmetry options might then be exploited to furth
58. mputational ef ficiency where such symmetry is applicable 6 22 Meshing Index A ACIS 4 4 4 6 import 3 2 AutoCAD commands ACISIN 3 2 BOUNDARY 2 2 delete face 4 2 explode 2 2 2 4 3 2 4 1 IGESIN 3 1 imprint 6 10 region 2 2 STEPIN 3 1 subtract 2 2 AutoCAD geometry creating 2 1 curves 2 2 6 2 mesh 2 2 meshing 6 1 6 22 region 2 2 2 3 6 1 6 3 6 5 6 18 6 20 surface 6 3 surfaces 2 2 2 5 wireframe 6 1 AutoCAD Mechanical Inventor Link 4 4 AutoCAD vs Inventor 2 5 B boolean operations 2 2 2 3 C commands AutoCAD ACISIN 3 2 BOUNDARY 2 2 delete face 4 2 explode 2 2 2 4 3 2 4 1 IGESIN 3 1 imprint 6 10 region 2 2 STEPIN 3 1 subtract 2 2 Thermal Desktop tdmesh 6 5 curve 2 2 6 2 See wireframe D delete face Inventor 4 4 DWG import 3 2 E explode 2 2 2 4 3 2 4 1 G geometry AutoCAD wireframes 2 1 creating 2 1 2 20 simplifying 4 1 4 6 snapping Thermal Desktop object to 4 6 I IGES import 3 1 import FE model as graphics 5 2 as nodes and elements 5 1 geometry 4 4 ACIS 3 2 DWG 3 2 IGES 3 1 STEP AP203 3 1 Inventor tools delete face 4 4 Inventor Link 4 4 M mapping results to FE model 5 2 mesh attach to articulator 6 16 AutoCAD geometry 2 2 controlling resolution 6 6 controlling visibility 6 15 copy 6 16 display preferences 6 14 editing 6 16 generating 6 12 move 6 16 viewing 6 14 mesh controller 2 6 6 4 6 6 6 8 6 12 6 16 controlling visibility 6 1
59. n the following mesh Figure 6 5 Figure 6 5 Example Surface Default Resolution Fraction 0 1 A Mesh Controller has been created by generating this preview mesh with the tag visible to the right of the surface 1D53 in this case This controller can be relabeled using the Set Label button in Figure 6 1 as described in Section 6 2 5 1 The Mesh Controller can be accessed in the Model Browser or edited directly in the drawing as can any other TD object Note that the size of the thin protrusion at the right of the part has not dictated the size of the mesh although smaller than average elements might be required to deal with the sharp corner and the thin region itself Instead the 10 fraction has been applied to the longest length The most coarse mesh that can be created for this object with a fraction of 100 1 0 is shown in Figure 6 6 Even though the fraction calls for one element along the longest length other internal parameters override this in order to capture detail and provide a smooth progression of element sizes Each edge will have at least one element Figure 6 7 shows a mesh that is more finely resolved than the default a fractional resolution of 0 03 3 Note that the number of nodes that will be generated rises approx imately with the square of the number of elements along the edge In other words the model size will be proportional to 1 F where F is the fraction employed For a so
60. ne lump 2 A group of faces that define an internal hollow space 4 4 Working with CAD Geometry Table 4 1 Tool Vendor Capabilities Pro Engineer PTC e Simplified representations e Mid planing Suggested paths to Thermal Desktop e Pro Engineer gt Inventor gt ACIS e Granite gt Inventor gt ACIS e ACIS gt Inventor gt ACIS e STEP FEMAP Siemens e Meshing e Mid plane meshing Suggested paths to Thermal Desktop e FEMAP Neutral e NASTRAN mesh e ACIS e Parasolid gt Inventor gt ACIS PATRAN MSC e Meshing Suggested paths to Thermal Desktop e NASTRAN mesh SolidWorks Dassault Systemes Suggested paths to Thermal Desktop SolidWorks Corp lt ACIS e Parasolid gt Inventor gt ACIS SpaceClaim SpaceClaim Corp History free model manipulations Suggested paths to Thermal Desktop e ACIS KeyCreator Kubotek e History free model manipulations Realyze Suggested paths to Thermal Desktop e ACIS Working with CAD Geometry 4 5 Table 4 1 Tool Vendor Capabilities IronCAD IronCAD e History free model manipulations Suggested paths to Thermal Desktop e ACIS NX Siemens Suggested paths to Thermal Desktop e Parasolid gt Inventor gt ACIS e FEMAP gt FEMAP Neutral a C amp R Technologies experience has shown that ACIS from Pro Engineer should be healed within Inventor to provide a usable geometry b History free model manipulations allow the user to model feature
61. nes 45 degrees are drawn from the from centers of the inside circles forming parts of the racetrack These lines can be used to crate the slanted channels walls via the Offset command These new lines and Extended to the manifolds and to the channel walls the four lines created in the last paragraph with any excess Trimmed To complete Sketch2 2D fillets inner radius channel width outer radius are applied to the flow channels where they curve A profile of manifold race tracks are extruded into the plate cut which consumes Sketch2 into feature Extrusion2 Sharing Sketch2 and cutting the regions formed by the channels into plate using a similar operation yields feature Extrusion3 The through holes are created last at the center points of the previous sketch s circles Sketch 3 and Sketch4 consumed by Holel and Hole2 respectively The finished part is shown in Figure 2 11 When saved as a SAT file in can be imported into AutoCAD and meshed by Thermal Desktop resulting in an identical model to that shown in Figure 2 12 However if any dimensions change the Inventor part can be quickly updated and re exported to AutoCAD whereas the AutoCAD drawn part Section 2 4 3 1 is largely indelible Also slivers are rare for adequately dimensioned and constrained parts built in Inventor and exported to AutoCAD in SAT format Slivers can occur using AutoCAD construction techniques as mentioned in the previous section but are most likel
62. on the north side of the enclosure needed to be modeled then they could be included as a separate part assuming they are bonded such that a contact conduc tance is appropriate between the two parts If strong temperature gradients occurred in the north side as a result of these fins perhaps if the box were made of steel instead of aluminum for example this might force an increase in the nodal resolution of that north side rectangle especially as needed to calculate the contact effects correctly Increasing the resolution of this one side however would force a proportionate increase in the resolution of all adjacent 8 Strictly speaking locating the nodes at the edges of the elements represents an extra approximation as the mass of those half cells have been mathematically cantilevered In other words this is a very modest violation of a formal finite difference method and avoids other much worse approximations 2 10 Creating CAD Geometry plates in order to maintain coincident nodes such that they can be merged Additional mesh controls are planned for future versions of Thermal Desktop that would help eliminate the need for such a global escalation of model density but for now such considerations must be taken into account during model construction 2 4 2 Example Autoclave An large industrial autoclave is designed to be heated by a gas burner on the bottom flaring at the top to provide a wider mouth as needed to accept a r
63. rence for building Thermal Desktop surfaces For more information on IGES translation select the Help button on the IGESIN trans lation form 3 3 ACIS AutoCAD is capable of importing geometry objects stored in SAT ASCII files using the ACISIN command or Insert gt ACIS file menu command ACISIN converts the model to a body object or to 3D solids and regions if the body is a true solid or a true region AutoCAD is limited to reading ACIS version 7 0 and earlier Once imported the geometry may be used to snap Thermal Desktop objects to the correct dimensions or may be meshed using TDMesh as described in the Thermal Desktop Ad vanced Modeling Guide in a seperate volume 3 4 DWG AutoCAD DWG files including those created from Thermal Desktop may be inserted into other drawing files using the Insert gt Block menu command The resulting dialog box provides the means to specify the insertion point and define scaling factors for the file or block being inserted If the explode option is not selected on the Insert block form the user must type EXPLODE or select Modify gt Explode to separate the block into its individual entities 3 2 Importing CAD Geometry 4 Working with CAD Geometry 4 1 Simplifying CAD Geometry Simplifying geometry is probably the most important step of advanced modeling With out simplification an automated mesher such as TDMesh Section 6 on page 6 1 can focus or fail on a section of the model that ha
64. rline of a FIOCAD pipe can be re assigned without losing the pipe data 6 Since the focus is on geometric model construction the drawing files do not represent full Thermal Desktop models but rather starting points from which a full thermal fluid model could be constructed 2 6 Creating CAD Geometry Figure 2 3 Stamped Aluminum Box for Switching Electronics If a CAD drawing of the box were available as shown in Figure 2 3 or if it could be made quickly by the thermal analyst using Inventor then it could be imported into AutoCAD and meshed For an imported drawing the bolt holes should be deleted or suppressed as thermally unnecessary detail and they should not be constructed by the thermal analyst using Inventor Also the fillets should be removed or suppressed since they are thermally negligible and will otherwise cause increased mesh density as the mesher struggles to maintain geometric fidelity Even with these corrections direct meshing of the de featured enclosure as a solid Figure 2 4 would still be inadvisable for more subtle reasons explained next Treating thin objects as solids instead of surfaces can cause undesirable mesh densities when the part is meshed An exaggeration of this effect is presented in Figure 2 5 In the top part of the figure the mesh generated to represent a thin solid plate contains many small elements at the edges the small characteristic dimension of that thickness has caused the local cell size to
65. rlying geometry thus providing the benefits of the Thermal Desktop surfaces true curvature for radiation calculations parameterization etc while verifying an accurate representation of the geometry 4 6 Working with CAD Geometry 5 Finite Element Models Many times thermal analysts must work with structural analysts in order to allow tem perature dependent stress calculations to be made Finite elements models may be imported in one of two ways as graphics or as nodes and elements Both methods are available using the Thermal gt Import gt menu option where the represented the finite element file format If temperature results will be mapped back to the structural model the structural model can be imported as graphics and the mapping option set by using the Post Processing Data Mapper Detailed instructions for importing models are provided in the Thermal Desk top User s Manual in another volume 5 1 Importing FE Model as Nodes and Elements If the user wishes to simply import the thermal model from a finite element preprocessor the finite element model should be imported as a thermal model nodes and elements This may be the process if a third party preprocessor such as FEMAP were used to gen erate the thermal model The implications of this choice are The thermal model is already built The thermal model resolution is determined upstream of Thermal Desktop e Curved surfaces are faceted and radiation will no
66. rom Controller allows a generated FEM mesh to become uncoupled from its Mesh Controller This is an advanced option that should be used with caution The uncoupled FEM mesh can no longer be accessed through this or any other Mesh Controller it is now a stand alone set of FEM elements that must be edited separately An uncoupled mesh should be moved into a different layer and AutoCAD group since otherwise confusion or conflict can arise if the Mesh Controller is used to regenerate a new and independent FEM mesh One possible use is of this option is to employ the same object to create multiple meshes though it is better to start with multiple copies of the underlying part Another use is to freeze the configuration of a FEM mesh such that subsequent changes within the Mesh Controller do not affect it Nodes created on TD finite difference surfaces and solids cannot be moved or deleted directly they are under the control of the surface or solid Likewise with a mesh controller Nodes cannot be moved or deleted directly by the user they must be deleted using the controller or moved using the controller When moving a controller it is recommended to move the underlying part simultaneously Otherwise if the mesh is regenerated it will be placed back at the location of the part Individual nodes can be moved or deleted once the mesh is released from the controller 6 2 5 Viewing the Mesh TDMesh uses an underlying part CAD surface or solid a
67. rt of the Thermal Desktop with built in CAD tool suite For Thermal Desk top users with plug in versions with AutoCAD purchased separately upgrading to the built in version provides an economical way to get access to Inventor As later chapters will show Inventor can also be used to import and simplify CAD drawings from third party tools Creating CAD Geometry 2 1 Using the Draw gt Modeling menu these wireframes can be extruded revolved lofted or swept Ifa closed wireframe is used a solid will be created if an open wireframe is used a surface will be created Closed co planar wireframes including a set of co planar wireframes sharing end points may also be converted to regions using the region command in the text window or the Draw gt Region menu option before either being meshed or performing boolean operations Regions are two dimensional so wireframes must be coplanar Similar to regions are planar surfaces A planar surface may be created using the Draw gt Modeling gt Planar Surface menu selection The user is then queried for two points defining a ractangle s origin and diagonal or a closed loop planar wireframe 2 1 1 2 Surfaces Regions and Meshes AutoCAD has three main types of surfaces surfaces region and meshes Surfaces may be three dimensional objects with curvature Regions must be planar but may be used in boolean operation Meshes are comprised of planar faces and therefore can on
68. s and extra copies of various cut objects are retained for inspection on an invisible layer At each significant stage of construction a copy of the AutoCAD drawing file is saved and archived The purpose of this action is to provide a starting point should any significant dimensions change and a new plate must therefore be constructed This action underlines the central flaw in the AutoCAD approach for such a complex object changes in the plate will be difficult to accommodate without starting over Another difficulty in the AutoCAD approach is the degree of care that must be taken during construction such that slivers are not created that will cause a subsequent meshing task to either fail or to result in an inappropriately detailed mesh Section 6 2 3 In fact this is exactly what happened during the first attempts to construct this part The causes of the resulting slivers was not clear but appeared to be due to slight errors in the sweep operations 16 An extrusion is preferable since the extruded depth can be changed later without losing other construction steps 17 Neglecting these holes simplifies the resulting model with negligible loss of accuracy They have been added to this model for explanatory purposes not modeling purposes 2 18 Creating CAD Geometry Note 90 degree max turning angle is used to avoid excessive resolution at through holes and reliefs Figure 2 12 Meshed Cold Plate in AutoCAD 2 4 3 2 Creating th
69. s no bearing on the thermal results An extreme example but not uncommon would be a part with threads included in a bolt hole it can be argued that the bolt hole or at least its location may be necessary for a thermal analysis but the threads would have no effect The mesh that would be required to capture each thread face would be extreme if meshing was even successful and would unduly complicate the model 4 1 1 AutoCAD AutoCAD has some basic capabilities for modifying geometry for simplification If solids are present opening the Solid Editing toolbar will help otherwise most commands are found under the Modify and Modify gt Solid Editing menus For simplifying the model the following sections will describe some of the more common editing tools 4 1 1 1 Explode The Modify gt Explode menu option will break a solid into its faces The primary benefit of this is simplifying a solid to just the face so that the extrude or revolve mesher can be used Treating thin objects as solids instead of surfaces can cause undesirable mesh densities when the part is meshed An exaggeration of this effect is presented in Figure 4 1 In the top part of the figure the mesh generated to represent a thin solid plate contains many small elements at the edges the small characteristic dimension of that thickness has caused the local cell size to shrink at the edge In bottom part of Figure 4 1 a surface has been meshed instead perhaps after explodin
70. s such as holes fillets cham fers etc without knowledge of how the feature was created The tools with these capabilities have varying levels of feature recognition 4 2 Snapping TD Objects to CAD Geometry Each custom Thermal Desktop surface has a collection of grips please refer to the AutoCAD User s Guide for more information about editing with grips that are used to graphically modify the shape of the surface For example the cylinder has grips to change its location orientation height radius and start and end angles Using grips in combination with snap modes and the geometry calculator AutoCAD cal command enables easy and powerful geometry construction Any grip may be used to move a surface by toggling the grip mode to move mode by pressing the space bar or entering the letters mo at the command prompt Graphical objects in AutoCAD including Thermal Desktop objects define snap points for various locations on the objects Examples of object snaps are end point mid point center point intersection extension and node point Setting the running object snap mode to end mid center and node will automatically select these points during picking operations Holding down the lt Shift gt key and then holding down the right mouse button may be used to bring up a pop up menu to select a snap mode during picking operations Using the grip points with object snaps allows the user to match the Thermal Desktop objects to the unde
71. s surface formed at the far end of the revolution in a plane containing the axis of revolution and side collectively refers to all other surfaces Selecting Generate Preview invokes the preview parameter options shown in Figure 6 19 This form is similar that of the full 3D mesher in terms of mesh resolution adjustments but it also contains fields for defining the number of degrees to revolve can be positive or negative and the number of layers to employ Layers refers to solid sections such that the number of copies of the base surface mesh including the original base surface mesh will be one plus the number of solid layers For revolutions equal to 360 degrees the base and end meshes are automatically merged to form a continuous part These two values can be redefined at any time but doing so will force a regeneration of the preview mesh and this will in turn cause the deletion of any FEM meshes that have been created based on those previews the same effects that are caused by changing mesh resolution within the base part The user can selectively omit the preview mesh for any or all of the three surfaces base end side associated with the revolved part This means the mesh revolver can create open shells e g sides only and other objects that cannot be created using the full TDMesh 3 The side includes the internal toroidal shapes in the example shown in Figure 6 18 internal surfaces are also con
72. s will be given priority of selection For assemblies options include component part feature faces and edges and sketch features For parts options include feature faces and edges and sketch features Parts faces edges and sketch features will be selectable for any part The other options will Working with CAD Geometry 4 3 only be available if the part or assembly was created within Inventor or the information was imported with the model For Inventor lt SHIFT gt or lt CTRL gt must be held down to select multiple objects 4 1 2 2 Delete Face To delete faces lumps or voids click the Delete Face tool on the Feature toolbar To maintain a solid object after deleting a face select the Heal option otherwise the remaining faces will form a surface As opposed to AutoCAD multiple surfaces can be selected and if congruent will be deleted as a single face In the example above the faces of the chamfer around the base plate can all be selected together and deleted at one time without having to delete any other surfaces If the part was created in Inventor the feature chamfer blend hole etc will be selectable and can be supressed or deleted 4 1 2 3 Access to Inventor Data from Thermal Desktop To use Inventor geometry data in Thermal Desktop the user must decide how the ge ometry will be used as scaffolding for the thermal model or as a basis for TDMesh For the former the user may export the geometry from Inventor u
73. sected or subtracted with other solids to create unusual shapes One particularly useful command once a solid is finished is explode Modify gt Ex plode This command replaces the solid with all the surfaces that it contains The explode command can be used to create unusual surfaces for example the curved edge of the solid depicted in Figure 2 2 2 1 2 Training Resources Help Menu Extensive resources are accessible via the Help menu in AutoCAD in cluding manuals tutorials showme features and access to various online resources Autodesk University www autodesk com au An annual user conference is hosted with extensive training opportunities CADTutor Website www cadtutor net A tutorial site with an extensive user forum 2 2 Autodesk Inventor 2 2 1 Capabilities Inventor is not intended for the creation of surfaces though solids can be imported into AutoCAD and then exploded Inventor s ability to create complex solids however greatly exceeds the AutoCAD capabilities including parametric modeling While a CAD adverse thermal analyst may be hesitant to take such a step it only takes a few days of training to gain access to powerful model building capabilities Furthermore learning a little Inventor enables the user to modify or defeature CAD drawings imported from third party packages such as Pro E or Solidworks as discussed in Section 4 In AutoCAD objects are created somewhat free form in 3D space
74. shrink at the edge In bottom part of Figure 2 5 the surface has been meshed instead perhaps after ex ploding the solid This 2D mesh can be assigned a virtual thickness within the TD surface edit form but if a geometrically faithful thickness is required perhaps for contact radiation leakage etc then the 2D mesh can be extruded Section 6 3 Nonplanar surfaces can also be extruded along local surface normals If the thickness is large enough or if the conduc tivity low enough several 2D mesh layers can be used If instead the thickness is small Creating CAD Geometry 2 7 Figure 2 4 Stamped Aluminum Box De featured but Still a Solid Model enough or conductivity high enough these layers can contain the same node numbers when the TDmesh Extrude option is used resulting in a model which is geometrically faithful but thermally approximate If a CAD model had been imported from say Pro E or Solidworks and the holes and fillets removed then a further step would still be necessary after importing into AutoCAD exploding the box to yield the surfaces on the solid These surfaces would then form the basis of the model using any of the techniques listed in the last paragraph For the current model of the electronics enclosure the use of relatively thick conductive aluminum means that there is no need to include temperature gradients in the thickness direction Also there is no need to represent the thickness geometrically
75. sidered sides Meshing 6 21 Mesh Extrusion Parameters E xj Base Surface Element Size Fraction of Max Dimension Absolute Size Max Turning Angle M Extrusion Subdivision 20 degrees Angular Sweep Equal Size Layers Parametric Layer List m Meshed FacesfInterior IV Generate Surface Mesh Preview at Base IV Generate Surface Mesh Preview at End of Extrusion IV Generate Surface Mesh Preview on Sides of Extrusion IV Generate Solid Mesh Preview cma Help Figure 6 19 TD Mesh Revolver Parameters Base end and side sur faces can be generated or omitted separately Ifa full 360 degree revolution is performed then the first and last surfaces are coincident Only one set of nodes will be generated at this location and the user should not activate the properties of the base and end surfaces e g radiation insulation only the side surface is now applicable The node ID s used for the base layer may be repeated for each copy of the base layer by checking the Repeat Base ID s for 2D Symmetry option on the Mesh Node Prop erties form as shown in Figure 6 17 The essentially creates a 3D representation of a 2D radially symmetric configuration Node and conductor data for each layer is combined together before being sent to SINDA FLUINT for analysis and is computationally a 2D radial symmetric problem inside of SINDA FLUINT This will improve co
76. sing ACIS format or using AutoCAD Mechanical link from AutoCAD to the Inventor file Linking will allow updates made in Inventor to be highlighted in AutoCAD but some limitations do exist such as scaling a Thermal Desktop model for new units does not scale the image of the linked Inventor file For use as a basis for TDMesh the Inventor file must be exported using ACIS not the only possible method but the preferred format When the ACIS format file is imported to AutoCAD the geometry will be available for meshing 4 1 3 Upstream Tools Table 4 1 on page 4 5 is a partial list of tools that could be used upstream of Thermal Desktop for simplifying geometry or simply translating from one format into another This list is based on datasheets and not experience This section is not comprehensive only an introduction to some tools that may be useful for simplifying geometry or possibly trans lating into formats suitable for importing into Thermal Desktop This list in not an endorse ment of any tools or products The product names used in this table are for identification purposes only All trademarks and registered trademarks are the property of their respective owners In the list the user will find some capabilities of the software and one or more suggested paths or supported file formats to move the data from the tool to Thermal Desktop 1 A group of faces that may contain some or no void A part or surface of a body should have at least o
77. t account for actual curvature e 1 1 node association between thermal and structural model this typically results in an overly defined and complex thermal model and is not necessary for mapping results The analyst must verify that elements and boundary conditions have been imported correctly Thermal Desktop will conduct tests on the imported elements for quality Any elements which are degenerate multiple nodes of the same element in the same location or of poor quality will be added to an AutoCAD group named BADELEMS Additionally if multiple elements are co located elements share the same node set two groups will be created one of the co located elements will be placed into DUPELEMS2 and all other co located ele ments will be placed in DUPELEMS The DUPELEMS group can be deleted to ensure only one element per node set The Model Browser can be used to list out AutoCAD groups so the user can check to see if these groups exist and to see what elements have been placed in them Finite Element Models 5 1 5 2 Importing FE Model as Graphics If the user wishes to generate the thermal model using Thermal Desktop surfaces the finite element model should be imported as graphics The implications of this choice are The analyst must build the thermal model e The analyst maintains control over model resolution e The analyst can choose between finite element and finite difference objects e The analyst must apply all boundary con
78. ts for omission when constructing a solid model or when suppressing features in an inherited CAD drawing Nonetheless the complexity of the design borders on the limits of AutoCAD solid construc tion techniques If the cross section of the cooker were rectangular with rounded corners e g shaped like a bread pan or an ingot mold then Inventor would be the preferred method For an axially symmetric 3D solid either tool could be used A recurring theme of this document is that the needs of the thermal model must flow upstream to influence the type of geometric model created This statement is in tended as a counterpoint to the idea that any geometric model can be turned into a sensible thermal model Such a notion of import and mesh is pervasive and appealing but mis leading Geometric models are built for machining CAM for validating interferences and tolerances for estimating mass and mechanical properties including perhaps structural and dynamic response etc but rarely are they built with the need for thermal fluid modeling in mind Eliminating features such as bolt holes fillets and chamfers is certainly one ex ample of how a faithful geometric model could otherwise result in an unwieldy thermal model But there are other more subtle ways in which the thermal modeling requirements can affect the geometric modeling approach For the case of the autoclave can the boundary conditions initial conditions sensor contro
79. with little consider ation to placing and dimensioning them Inventor takes a very different approach it is essentially a geometric programming environment This means that creating even a simple object might take a few more steps than in AutoCAD but as will be seen these extra steps enforce consistency and add the structure necessary to enable tremendous power For example to create a solid in Inventor a sketch must first be made on a work plane then this sketch must be consumed within an operation such as extrude or loft An update process is available a change to the underlying sketch will propagate to the higher 4 Caution If not done carefully and sometimes even with suitable care slivers and other defects Section 6 2 3 can arise For truly complex solids consider using Inventor instead 2 4 Creating CAD Geometry level operation e g extrude Such operations are nested and hierarchically ordered with dimensions applied along the way These dimensions can be entered parametrically like Thermal Desktop symbols or SINDA FLUINT registers Changes to these parameters prop agate through the part according to the dimensioning rules developed by the user during the construction of the part Examples of Inventor usage are provided in Section 2 4 and the corresponding files are available for inspection 2 2 2 Resources Help Menu Extensive resources are accessible via the Help menu in Inventor including
80. y to occur as a by product of CAD translators as explained in other Section of this manual 2 20 Creating CAD Geometry 3 Importing CAD Geometry 3 1 Step AP203 Importing of STEP information is significantly different than all the other Import and Export options The STEP translator is written by Autodesk and is available through Au toCAD Mechanical or Autodesk Mechanical Desktop bundled with Autodesk Inventor Suite Users who purchase CAD functionality along with Thermal Desktop have access to the STEP translator The STEP translator is accessed typing the command STEPIN in the command line The AutoCAD STEP Translator enables data exchange using STEP AP203 which is called Configuration Control of 3D Design Data For the purposes of Thermal Desktop AP203 provides B rep solids surfaces and wireframes STEP import is a way to get CAD data from another CAD system such as PRO ENGI NEER I deas CATIA UNIGRAPHICS and others The user must EXPORT a STEP file from those CAD systems The export command on most CAD systems will contain options for the user Generally it is best to try several different options to see what works best for the system being used once it is imported into AutoCAD Some of the options may produce so much information that the system may not be able to handle it in terms of memory and graphics limitations For more information on STEP translation select the Help button on the STEPIN trans lation form 3 2 IGES

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