TASCflow c STUDENT USER MANUAL
Contents
1. choose New to open New Boundary Condition window In this window choose Type SOUTFLOW accept Name 3_OUTFLOW and click Ok Choose Attachment oSpecified Region To fill in the region name choose Select which opens the Region Manager window In this window choose OUTLET from the list and click ZOk With oFluids on choose Type 1 4 Commands for Duct Bend Example 37 oPressure Pressure Value 0 0 Pressure Type oStatic and fill in Applied As oConstant With oTurbulence on no in formation is needed Click Apply to accept this boundary condition and its attachment and choose New to open New Boundary Condition window In this window choose Type oSYMMETRY accept Name 4 SYMMETRY and click Ok Choose Attachment oSpecified Region To fill in the region name choose Select which opens the Region Manager window In this window choose SYMMET1 turn oMultiple Select on then choose SYMMET2 from the list and click Ok No further information is required for this boundary con dition Click Apply to accept this boundary condition and its attachment with the 1 Wa11 boundary condition highlighted choose Display Only the walls of the flow domain should be highlighted in red e choose PreProcess Check PreProcess to open2. 4 4 4 U Mom 6 76 2 1E 03 1 2E 02 12 11 2 3 0E 02 OK V Mom 7 54 2 9E 03 1 8E 02 12 21 2 2 7E 02 OK W Mom 160 81 7 6E 05 6 8E 04 13 22 3 3 5E 01 ok P Mass 0 36 1 4E 03 2 0E 02 14 12 2 9 1 2 8E 02 OK Table 1 2 Typical convergence diagnostics operates in a batch mode it does not take much of the user s time The operation of the solver should be monitored and facilities are provided for this The initial guesses for the velocity pressure turbulent kinetic energy and dissipation rate nodal values will not necessarily satisfy the discrete algebraic equations for each node If the initial nodal values are substituted into the discrete equations there will be an imbalance in each equation which is known as the equation residual As the nodal values change to approach the final solution the residuals for each nodal equation should decrease Table 1 2 shows typical solver diagnostic output listing the residual reduction properties for the first few time steps iterations of a solver run For each field variable equation set the following information is output each time step RMS Res the root mean square of the nodal normalized residuals Max Res the maximum nodal normalized residual in the flow domain September 3 2002 26 CFX TASCflow STUDENT USER MANUAL September 3 2002 Max Locati
3. e choose Create Mesh Seed Two Way Bias turn oNum Elems and L2 L1 on fillin Number 15 fillin L2 L1 1 5 fillin Curve List Surface 4 3 and click E Apply e choose Create Mesh Seed Uniform turn oNumber of Elements on fill in Number 2 fill in Curve List Surface 9 1 and click Apply e choose Create Mesh Surface turn oEnsure Structured Mesh on fill in Surface List Surface 1 3 Solid 1 1 click in box and then move box cursor to upper left of viewing screen left click and drag mouse to create a large box which completely encloses the solid body model to pick all surfaces and click Apply e go to the tool bar and click Hide labels on to make the image clear rotate the model to check that the mesh looks correct Turn oAnalysis on and then carry out the following actions in the Analysis on window e turn oReblock in VOLMESM on e turn oCalculate Mesh Quality off e turn oVisualise Mesh Quality off fill in Solids for VOLMSH Solid 1 3 and click Apply a box warning that equivalencing tolerance is being reset may appear You can click OK and ignore the warning After a short wait CF X Build will shut down Using Windows Explorer you should see a file in your working directory with the name build4 geo 99999 where the number at the end is generated automatically by the system and changes each time VOLM
4. Figure 1 7 Steps in the development of a surface mesh As shown in Figure 1 7 mesh development follows the steps Placement of Nodes on Edges nodes corners of elements are placed along the edges of each surface in the solution domain September 3 2002 16 CFX TASCflow STUDENT USER MANUAL Development of Surface Mesh interpolation is used to create a surface mesh of quadrilateral Quad4 elements such that the element nodes coincide with the specified edge nodes and Development of Volume Mesh interpolation is used to create a volume mesh of hexahedral Hex8 elements such that the element faces coin cide with the surface mesh quadrilateral elements Figure 1 8 Mesh paths for multi block surface The CFX Build meshing program IsoMesh uses mesh paths a group of topological parallel edges to determine the placement of nodes on surface edges Figure 1 8 shows a two dimensional multi block surface with the mesh paths labelled A G Since the mesh is structured each edge along a mesh path must have the same number of elements and nodes The placement of nodes along each edge of a mesh path is determined by the following in order of priority 1 mesh seed or user specified node placement 2 carrying a mesh seed along the mesh path or September 3 2002 1 3 The Solution 17 3 using the Global Edge Length to establish the number of elements of unifor
5. choose Create 2D Patch fillin Patch Name wallin fill in Surface Solid 1 2 2 2 3 2 and click amp Apply Surface Mesh Seeding The commands listed in the next paragraph will develop the structured mesh for the interior of the solid body model These commands build the mesh by seeding a uniform mesh distribution with 10 elements along the inlet axis 8 elements along the bend axis and 10 elements along the outlet axis seeding a non uniform mesh distribution between the duct walls with 15 elements that are smaller near the walls seeding two mesh elements between the two symmetry surfaces calculating surface meshes on all of the surfaces and calculating the volume mesh in the interior of the solid model Turn oMesh on and then carry out the following actions in the Mesh window turn oAuto Execute off choose Create Mesh Seed Uniform turn oNumber of Elements on fill in Number 10 fill in Curve List Surface 9 2 and click Apply notice the little yellow circles that appear September 3 2002 34 CFX TASCflow STUDENT USER MANUAL e choose Create Mesh Seed Uniform turn oNumber of Elements on fill in Number 8 fill in Curve List Surface 10 2 and click Apply e choose Create Mesh Seed Uniform turn oNumber of Elements on fill in Number 10 fill in Curve List Surface 11 2 and click Apply
6. e there is a Calculator capability that can be used to make the initial fields non uniform i e to vary over the flow domain The solution fields calculated by CFX TASCflow are stored in the rso file The last step of setting the initial conditions will be to write the initial fields out to the rso file 1 3 4 Solver Operation Setting Solver Parameters The operation of the equation set solver in CFX TASCflow is controlled by a large set of parameters Many of these parameters are given default values and need not be changed by the beginning user However some features should be explicitly set by the user for each run 1 the timestep for the flow evolution 2 the pressure offset Section 1 3 5 September 3 2002 24 CFX TASCflow STUDENT USER MANUAL September 3 2002 3 the choice of discretization scheme and 4 the selection of output fields Even though the focus is on steady flow simulations transient evolution is used in the iterative solution algorithm in effect each iteration is treated as a step forward in time The choice of timestep for the flow evolution plays a big role in establishing the rate of convergence Good results are usually obtained when the timestep is set to approximately 30 of the average residence time or cycle time of a fluid parcel in the flow domain This residence time is referred to as the global time scale In incompressible flow fields the actual pressure level do
7. Apply to make the first surface notice that a green box with a green label 1 appears on the screen and that the upper corners of the box have aqua blue point labels e choose Create Surface Revolve items in the Geometry window check that oAuto Execute is off fillin Axis f 0 1 0 1 0 0 1 0 1 1 0 fill in Total Angle 90 fill in Curve List Surface 1 2 1 2 is the 2 curve of the 1 surface and click Apply second surface notice that to make the e choose Create Surface Extrude items in the Geometry window check that oAuto Execute is off fillin Translation Vector lt 0 2000 gt fill in Curve List Surface 2 2 and click Apply to make the third surface e choose Create Solid Extrude items in the Geometry window check that oAuto Execute is off fill in Translation Vector lt 000 1 gt fill in Surface List Surface 1 3 put cursor over Surface 1 and click and then move to Surface 2 and Surface 3 holding the shift key down while clicking to pick a list of surfaces and click Apply to make the three solids notice that a blue wireframe defines the edges of the solids and the Solid labels are in blue Boundary Surface Identification To make the implementation of boundary conditions easier when using CF X TASCflow we will give names to the boundary surfaces of the solid body model In particular
8. or Hybrid c v values for interior nodes and boundary surface values for control volumes adjacent to the boundary surfaces September 3 2002 1 3 The Solution 29 Calculator The PostProcessor includes a Calculator which can be used to carry out math calculations These math calculations can be applied to scalar fields or parameters single value objects Some examples are o UZT TIA 2 yY will set each nodal value of the u velocity component to x 2y where x and y are the local Cartesian coordinates of the node e ubar aurg u will calculate the average value of the u velocity component from the mesh node values in the active region Note that ubar is a single value parameter The Calculator will also evaluate and display a single numerical expression This is useful for seeing the value of a parameter like ubar that has been calculated ACL Parameter Manager The ACL Parameter Manager tool shows the complete list of parameters which are defined and in use and the parameter values There is also the capability to change parameter values Macro Manager Some visualization and calculation operations are quite complex so there is a tool the Macro Manager for storing a set of commands These commands are in the AEA Command Language ACL which is documented in the On line help Unfortunately the Macro Manager does not have the capability to save GUI operations and play them back A Macro for calculating pressur
9. ASCII files and to provide output to the terminal screen output files and a graphics window While you will find the GUI reasonably intuitive there will be less intuitive steps and nomenclature that is leftover from the command line text input design of the underlying codes The actual CFD analysis begins by using CFX TASCflow to import the geo file created for the solid body model in CFX Build The mesh and geom etry information is translated to CFX TASCflow s native geometry database and stored in the grd file The next phases involve setting the relevant input data to establish the flow conditions boundary conditions etc Flow Zones and Attributes Two types of flow zones can be set up Fluid Zone this default zone has fluid flowing through it You must set up the properties of the fluid and the fluid processes for this zone and Solid Zone these zones can be established to create objects which block off the fluid zone Three types of objects can be created Inactive a solid block with no fluid flow or conductive heat transfer CHT a solid block with conductive heat transfer but no fluid flow and Porous porous mesh solid screens porous plug etc with highly constricted fluid flow For the Fluid Zone the following physical processes and sub processes will be relevant for a beginning user of CFD Fluid Flow activates the solution of the momentum and mass conserva tion equations The following sub processes may be re
10. Apply to accept the above choose PreProcess Boundary Conditions to open the Boundary Conditions Manager window In this window issue the following com mands choose New to open New Boundary Condition window In this window choose Type oWALL accept Name 1 WALL and click Ok Choose Attachment oGeneral Default With Fluids on choose Surface Type oRough and fillin Roughness Height 0 0001 With Turbulence on notice that no informa tion is required Click Apply to accept this boundary condition and its attachment choose New to open New Boundary Condition window In this window choose Type oINFLOW accept Name 2 INFLOW and click Ok Choose Attachment Specified Region To fill in the region name choose Select which opens the Region Manager window In this window choose INLET from the list the inlet plane will be shown in red on the wireframe display and click Ok Notice that Region INLET is now filled in With Fluids on choose Type oVelocity Vector Based on Normal oNormal and fill in Speed 3 0 With Turbulence on fill in Intensity 0 03 and then select oFraction of Inlet Length Scale with oMed to specify a turbulence length scale that is 10 of the inlet length scale Click EApply to accept this boundary condition and its attachment
11. In cases where the coordinate values must be entered use the following conventions Point x y z the blanks between values can also be commas Vector lt x y z gt and List Object1 Object2 i e 110 111 specifies an axis in the z direction through the point 1 1 in the x y plane Boundary and Object Identification CFX Build has the ability to assign meaningful names to two dimensional surfaces and three dimensional volumes These named surfaces and volumes are known as Patches in CFX Build While this operation is not truly nec essary it is easy to use CFX Build s mouse picking capability to identify surfaces that will later have boundary conditions attached to them and to identify solid zones in the interior of the solution domain CFX TASCflow will convert these patch names to region names when the CFX Build geom etry file is imported For historical reasons CFX Build has a very small number of patch naming options 1 3 The Solution 15 2D Patch start with INLET OUTLET PRESS SYMMET WALL CNDBDY BLKBDY or USER2D 3D Patch start with SOLID SOLCON POROUS or USER3D plus Suffix add a suffix to make each patch name distinctive and unique Surface Mesh Seeding The development of optimal meshes for CFD analysis is still an art which you can develop with practise These notes are restricted to considering the steps CFX Build goes through to establish a mesh for a multi block region
12. Parameters which are given default values are not stored in the prm file but they may be viewed by scrolling through the out file which is created by the solver Monitoring Solver Operation The solution of the algebraic equation set is the component of the code operation which takes the most computer time Fortunately because it 1 3 The Solution 25 TIME STEP 1 SIMULATION TIME 1 00E 01 CPU 9 53E 01 Equation Rate RMS Res Max Res Max Location Linear Solution 4 4 4 4 4 4 TKE 0 00 3 6E 03 2 3E 02 14 19 2 6 1 4 4E 03 OK EPS 0 00 2 7E 02 2 0E 01 14 20 1 5 7 7 6E 04 OK 4 4 4 4 4 U Mom 0 00 3 2E 04 2 8E 03 13 19 2 2 4E 01 ok V Mom 0 00 3 9E 04 8 6E 03 13 21 2 3 4E 01 ok W Mom 0 00 1 3E 06 6 7E 06 1 21 3 3 6E 01 ok P Mass 0 00 3 9E 03 5 9E 02 4 14 20 2 9 1 1 5E 02 OK Deleted existing backup RSO file rso bak TIME STEP 2 SIMULATION TIME 2 00E 01 CPU 9 79E 01 Equation Rate RMS Res Max Res Max Location Linear Solution 4 4 4 4 4 4 TKE 2 47 9 0E 03 5 8E 02 18 25 2 6 0 7 8E 04 OK EPS 0 60 1 6E 02 1 1E 01 13 19 1 5 8 6 4E 05 OK 4
13. the command line prompt Issu ing the command write rsf rsf_fields i j k x y z tau wall p 16 1 29 2 will create an rsf containing the opening line RSF 29 I J K X Y Z TAU_WALL P followed by a table of values giving the index numbers locations wall shear stress and pressure and wall shear for each node along the cen treline of the inner wall The contents of this file can be imported September 3 2002 40 CFX TASCflow STUDENT USER MANUAL September 3 2002 into a conventional spreadsheet program and further post processed in that environment 1 4 3 Clean Up The last step is to remove unnecessary files created by CFX Build and CFX TASCflow This step is necessary to ensure that you do not exceed your disk quota At the end of each session delete all files except db jou e name lun grd gci prm bcf and rso If you no longer need your results but would like to be able to replicate them then you should delete all files except e db jou e name lun gci prm and bcf After removing all unnecessary files use the WinZip utility to compress the contents of your directory This file can be used to recreate a mesh by choosing the CFX Build command File Utilities Rebuild
14. the commands given in the next paragraph will need make it easy to set boundary conditions e on the inlet surface e on the outlet surface e on the two symmetry planes and e on the outer and inner duct bend walls Turn Patches on and then carry out the following actions in the Patches window e turn Auto Execute off e choose Create 2D Patch fillin Patch Name inlet fill in Surface of the inlet surface and click Apply Solid 1 3 pick with the mouse by placing the cursor near the centroid gt 1 4 Commands for Duct Bend Example 33 e choose Create 2D Patch fillin Patch Name outlet fill in Surface Solid 3 4 notice that 3 4 is surface 4 of solid 3 and click Apply gt choose Create 2D Patch fill in Patch Name symmet1 fill in Surface Solid 1 6 2 6 3 6 Use the shift key plus left click to pick a list of surfaces with the mouse check that the correct surfaces are highlighted in brown If you have wrong surface in your list you can remove it by putting the cursor near its centroid and using the right mouse click and click Apply choose Create 2D Patch fill in Patch Name symmet2 fill in Surface Surface 1 3 and click Apply choose Create 2D Patch fill in Patch Name wallout fill in Surface Solid 1 1 2 1 3 1 and click amp Apply and
15. the number of elements plus the ratio of the lengths of the first and middle element edges or the lengths of the first and middle element edges After the surfaces of the solid body model have been meshed the in terior can be meshed with hexahedral elements The CFX Build program VOLMESH is used to accomplish this task After setting this task up in the oAnalysis phase i e e ensure that oReblock is on This optimizes the block structure by try ing to combine many small blocks into one or more larger grid blocks A fewer number of grid blocks will make post processing easier and e setting the geometry scale factors usually left at their default values of 1 0 if all inputs are in metric The scaling factor in a particular coordinate direction multiplies all distances in that coordinate direc tion on output i e if the original distance is 100 mm and the scaling factor is 0 001 m mm then the output distance is 0 1 m September 3 2002 18 CFX TASCflow STUDENT USER MANUAL CFX Build will close and start the batch program VOLMESH executing When VOLMESH is completed a geo file will be created This file can be imported into CFX TASCflow for CFD processing 1 3 3 Pre processing CFX TASCflow runs with a Graphical User Interface GUI that provides a thin shell interface for a set of codes which carry out the CFD analysis The codes were all originally designed to operate from the UNIX command line to take text input from
16. ME 566 Computational Fluid Dynamics for Fluids Engineering Design CFX TASCflow c STUDENT USER MANUAL G D Stubley Mechanical Engineering Department University of Waterloo Copyright 1998 1999 2001 by G D Stubley Contents 1 CFX TASCflow c STUDENT USER MANUAL La Getting Started uo scr o eh A Goa ae gh ahs a ae E 1 1 1 Introduction to Windows 2000 1 2 Initialization 4 92 so As e a 1 1 3 Introduction to CFX Build 1 1 4 Introduction to CFX TASCflow 12 The Problem er c doe se he ee we i LI The SOlUtI N 2 4 essea sii a aE 1 3 1 The Duct Bend Model 1 3 2 Grid Generation 0 1 3 3 IPTE PTOCESSIO sopia aoken Boe bah BR AAR Ae E 1 3 4 Solver Operation 1 3 5 Post Processing 000 1 4 Commands for Duct Bend Example LAIL CRX Build 2222464245745 444346 E 1 42 CFX TASCflow 0 1 4 3 Clean Up se scd 634624458 555 64 44 44 111 iv CONTENTS September 3 2002 Chapter 1 CFX TASCflow STUDENT USER MANUAL In these notes the basic elements of a CFD solution will be illustrated using two professional software packages CFX Build Version 4 3 and CFX TASCflow Version 2 10 These notes can be viewed as an introductory tutorial on the two software packages and as such are a mini user s guide They are not meant to be or to replace a detailed user s guide For full infor mation o
17. SH creates a geo file 1 42 CFX TASCflow This brings us to the CFD phases of the solution There are three principal CFD phases September 3 2002 1 4 Commands for Duct Bend Example 35 Pre processing input information to get ready to calculate the velocity and pressure fields Solver execute the solver to calculate the field variables for the mesh and Post processing visualize and analyse the results e Start the CFX TASCflow Launcher click on Start Programs Mechanical Engineering CFX TACSflow CFX TASCflow 02 10 00 Set the working directory to N ductbend and then click the CFX TASCflow button Pre processing After the CFX TASCflow window opens up the commands listed in the next paragraph will accomplish the following steps import the CFX Build geo file and convert it to the grd file format specify the region through which the fluid will flow the physical models fluid flow no heat transfer turbulence standard k e model with wall functions and fluid properties water with p 1000kgs and u 0 001Nsm set up a rough wall boundary condition as the default condition set up and attach the inlet boundary condition the outlet boundary condition and the symmetry boundary conditions check pre processing write the pre processing information to the bcf file and generate an initial guess for the flow fields a uniform speed of 3ms with the velocity aligned with the grid lines th
18. The Solution 23 Initial conditions are usually simple assumptions that are assumed to apply uniformly i e to be constant over the complete flow domain For the velocity vector it is possible to set uniform values of e the 3 velocity vector components Uniform Cartesian or e fluid speed and grid direction Uniform Grid Aligned In the latter case the velocity vector is calculated for each mesh point by assuming that the velocity vector is parallel to the specified grid line at the mesh point is in the direction of increasing grid index and has the specified speed Pressure is set to the specified value at each mesh point While it is possible to specify values of the turbulent kinetic energy k and its dissipation rate e 1t is often difficult to intuitively establish appro priate guesses for these variables especially the dissipation rate However it is possible to establish initial values for k and e from estimates of the in tensity of the turbulence and the length scale of the turbulence as described in the discussion on inflow boundary conditions The choice of initial conditions can significantly impact efficiency of the iterative solution algorithm The following guidelines will help ensure that the initial conditions are reasonable e try to match the initial conditions to the dominant inflow boundary conditions e usually a grid aligned initial velocity field will promote convergence to the final solution fields
19. at follow the duct axis uniform pressure of 0 0Pa uniform turbulence intensity of 3 and uniform turbulence eddy length scale that is 10 of the inlet length scale To accomplish these steps execute the following commands choose File Import CFX 4 which will open a window called Import CFX 4 To select the filename turn 3 Dimensional Grid on select Browse to open a file browsing window from which you can select the geo file generated by CF X Build Highlight the geo filename with a left click in the right frame and click Ok The Filename should contain the geo filename Click Ok to import the file and close the Import CFX 4 window September 3 2002 36 CFX TASCflow STUDENT USER MANUAL September 3 2002 e choose File Load PreProcessing e choose PreProcess Zones amp Attributes to open the Zone Manager window Make sure that 3D Region lt default gt is filled in oFluid Flow is on and oHeat Transfer is off In the Fluid Subprocesses pane check that all sub processes are off except Turbulent which should be on Open the Turbulence Model Setup window by choos ing Turbulence and check that ok e Models and oDefault Model are on and that oHigh Re Models with oLog Law Standard near wall models are on Click Ok to close Open Material Properties window by choosing Materials We will use a custom fluid and fill in Density 1000 0 and Viscosity 0 001 Click ZOk to close Click
20. ave all of the present format information for the vector plot and the xy graph This facility will allow for the recreation of images with new results for easy comparison between simulations e choose PostProcess Scalar Field Manager select U from the scalar field list and choose Display With Region blank use the ZStart and ZMore buttons to scroll through the data to find the U velocity component at the node 5 5 2 Use ZCancel to close the Scalar Field Display and Manager panels e in the Visualization Object Manager panel turn Visibility off the graph should disappear select New choose Object Type Fringe Plot and click Ok e in the Fringe Plot panel choose 2D Region SYMMET1 and Scalar PTOTAL turn oFaces on and Lines off and click ZApply You will need to reset the viewport orientation to Z in the Viewport Manger window Viewer Veiwport Options The fringe plot should appear e choose File Post Processing State Restore and accept the File name viz state to restore the original formatting that existed before the fringe plot was created e choose Tools Command Line to open the Command Line inter face an interface that allows keyboard entry of post processing com mands In the interface window enter the command help rsf_info Information on using rsf files to export data will be displayed Hit Enter key until you return to
21. ck structured mesh Structured Mesh Each block in the model is filled with a set of hexahe dral elements or bricks possibly with non orthogonal and non parallel faces This set of elements is known as the solid or volume mesh The elements in the mesh are arranged in a structured or ordered manner In a structured mesh there are three independent coordinate indices i j k which uniquely specify the topological location of an element in the mesh similar to the way that three independent coordinate val ues x y z specify a point in Cartesian space Figure 1 3 shows an example of a structured two dimensional mesh and Figure 1 4 shows an example of an unstructured mesh Each mesh is fitted to the body shape of its block Figure 1 5 If it helps you can imagine that each mesh starts out as regular cube of rubber bricks and is then stretched and twisted until it fits inside its irregularly shaped block Joining Blocks In this introduction to CFD the rules for joining blocks 6 3Advanced techniques like grid embedding and grid attachment may allow some un structured meshes to be used with a CFD solver designed for structured meshes September 3 2002 10 CFX TASCflow STUDENT USER MANUAL Figure 1 4 A single block unstructured mesh are very simple At the join between two mesh blocks e the two surface faces that make the join must be identical in size shap
22. d time scales based on the initial fields the convergence diagnostics estimate of the global length speed and time scales based on the final fields the fluxes of all conserved quantities through the boundary surfaces these should balance to 0 01 of the maximum fluxes and the computational time required to obtain the solution 1 3 The Solution 27 1 3 5 Post Processing To the typical user of CFD the generation of the velocity and pressure fields is not the most exciting part It is the ability to view the flow field that makes CFD such a powerful design tool CFX TASCflow has a suite of tools for obtaining useful visual and quantitative information from the results stored in the rso file The PostProcessor works with graphic objects and two type of quanti tative objects Scalar Fields numerical values associated with the mesh nodes i e each node has a unique numerical value for each defined scalar field and Parameters an object that takes on a single value Visualizer The Visualizer is used to generate visual views of the flow field and its properties The Visualizer creates an image by drawing a set of visualization objects The viewing window is to the left of your screen and in this window the following visualization objects can be drawn Wireframe an outline of the flow domain for a given sub region Grid Surface an internal or boundary surface of the grid Contour Plot lines of a constant scala
23. e and Vector a directed line segment with direction and magnitude Methods include Note that there are short cuts For example you can create a box shaped solid in one step September 3 2002 14 CFX TASCflow STUDENT USER MANUAL September 3 2002 Create Point XYZ specify the Cartesian coordinates of a point Create Curve Point specify two three or four points of a curve Create Curve Arc3Point an arc through the specified start mid dle and end points Create Curve Revolve sweep specified point about specified axis by a specified angle Create Curve XYZ a line with one point at the origin and the other at the end of the specified vector Create Surface Extrude sweep a specified curve along a specified vector Create Surface Revolve sweep a specified curve about a specified axis through a specified angle Create Surface XYZ a plane with one corner on the origin and its diagonally opposite corner at the end of the specified vector Create Solid Extrude sweep a specified surface along a specified vector Create Solid Revolve sweep a specified surface about a specified axis through a specified angle and Create Solid XYZ box with one corner on the origin and its diag onally opposite corner at the end of the specified vector Input Many of these methods require you to enter a point vector or axis i e a list of two points to set an axis In some cases these can be picked off the graphics screen
24. e and orientation and e the mesh distribution on the two surface faces that make the join must be identical Applying these two rules to the geometry shown in Figure 1 3 means that mesh block A must be broken up into 5 smaller blocks as shown in Figure 1 6 Units Both CFX Build and CFX TASCflow assume that you have used a consistent set of units i e metric British or your own invention To keep things simple and to minimize errors we will use metric units throughout These rules can be relaxed by using the oConstraints tools of CFX Build and the General Grid Interface GGI tools of CFX TASCflow September 3 2002 1 3 The Solution 11 Figure 1 5 A single block body fitted structured mesh Geometry Building Physical Entities The solid body model is built from the following pa rameterized physical entities Point a 0 dimensional point in Cartesian xyz space Curve a 1 dimensional line through space specified as a vector func tion of one independent variable or parameter joining points ei ther explicitly defined or implicitly created Surface a simple 2 dimensional surface in space specified as a vector function of two independent variables closed by four curves and four corner points Solid a simple 3 dimensional volume in space specified as a vector function of three independent variables closed by six surfaces and eight corner points Topolog
25. e additional parameters window Now turn on oWrite mass flow scalar fields oWrite surface area fields September 3 2002 38 CFX TASCflow STUDENT USER MANUAL September 3 2002 oWrite BC information oWrite turbulent Reynolds stresses and oWrite vorticity fields Click Ok to return to the main parameter window Click Ok to save and close window e choose Solve Solver Monitor and check that Serial oOne Computer Normal oSystem Single and Batch are on before clicking Start After a few minutes execution should begin Diagnostics will scroll on the terminal output pane and the equation residuals will be plotted as a function of timestep After the first few timesteps the residuals should fall monotonically Execution should stop after 20 timesteps Click Close Post processing The most interesting step is the analysis of the results To illustrate this step the commands listed in the next paragraph step through the following tasks e load the results from the rso file for post processing e create a vector plot on one of the symmetry boundary planes e save a postscript image of the plot e create a plot of the wall shear stress along the inner wall e look at a table of u velocity component values e create a fringe plot of the total stagnation pressure field and e create an ASCII file containing values of pressure and wall shear stress along the inner wall To accomplish these tasks e ch
26. e coefficients based on the maximum Vas speed in the flow domain i e Cp and displaying the fringe plot is Define MACRO PCOEFFICIENT Macro PCoefficient purge calc spd sqrt u xu vev ww calc spdmax max spd calc pcoeff p 0 0 0 5 spdmax spdmax define parameter draw_filled on fringe pcoeff 2 range local levels 11 plot EndMacro September 3 2002 30 CFX TASCflow STUDENT USER MANUAL Writing PreProcessing before exiting CFX TASCflow will save this macro in the gci file Other useful information like user defined parameters etc is also stored in the gci file 1 4 Commands for Duct Bend Example The following conventions will be used hereafter to indicate the various commands that should be invoked Menu Sub Menu Sub sub menu Item chosen from the menu hierar chy oRadio Button Option activated by clicking on a radio button Elcon Command Option activated by clicking on the icon and Box Name value Enter value in the box To begin set up a directory you will want to make a new directory every time you set up a new geometric model and run CFX Build to create the mesh for this problem e Create a folder ductbend on your N drive e Open the CFX 4 3 Launcher Start Programs Mechanical Engineering CF X 4 3 e set the working directory to ductbend and e click on the ECFX Build tab 1 4 1 CFX Build The first step is to set up CFX Build s session fil
27. eartbeat is green if input is possible blue if an interruptible operation is underway and red if an uninterruptible operation is underway d gt Below the diamond boxes is a tool bar featuring icons for other common tasks We will look at some of these later 6 The goal of the geometry section of CFX Build is to generate a volume called a solid by CFX Build for historical reasons the mesh generator was originally developed for stress analysis in solids Once the solid is defined it will be filled with a mesh and the fluid flow within that volume will be determined The geometry form has many options which permit the generation of solids in various ways Chapter 2 will go into some of the options in more detail 7 For now we will generate a simple brick solid Choose Solid from the Object menu and XYZ from the Method menu 8 There are various ways that the dimensions of the solid can be set We want to do so simply by setting the brick dimensions Click on the arrow Vector in the window beside the Geometry dialog box Then set the brick dimensions in the Vector Coordinates List box change the lt 1 1 1 gt to lt 3 2 1 gt Click on Apply A view of the brick from the 2z direction will appear 9 The view of the object can be changed in several ways On the tool bar there are icons for 10 preset views front view rear view isometric views etc Click on several of the ch
28. ection window similar to that of other Windows programs Choose the build_intro directory and click on the ZOK button to close the window You should now see your selected working directory displaced on the CFX 4 3 Launcher 4 Click the CFX Build tab on the Launcher Wait until the wide window along the top of the screen appears 5 There are four main components to this window 1 1 Getting Started 3 a The top menu bar File Group controls the global be haviour of CFX Build Most of them should be greyed out be cause there is nothing loaded Choose New from the File menu Click in New Database Name and enter a name of your choice Click on OK A dialog box entitled New Model Pref erences will appear click on OK This step should cause all items in the menu bar to be ungreyed b Below the menu bar are a set of diamond boxes Geometry Constraints which outline the steps typically used in gen erating a mesh for a CFD analysis For now we are only interested in the geometry section where the geometry bounding the region of fluid flow is defined Click on Geometry which brings up a form entitled Geometry to the right side of the screen c On the top right of the top menu window are a series of useful icons including refresh graphics reset graphics interrupt the current operation and undo the last operation The coloured box called the h
29. elow the menu bar at the top of the screen turn oGeometry on and then carry out the following actions in the Geometry window first though go up to the tool bar and click Show labels choose Create Point XYZ items in the Geometry window check that oAuto Execute is off fill in Point Coordinate List 0 0 0 and click Apply to make the first point notice that a cyan aqua blue coloured 1 appears choose Create Point XYZ items in the Geometry window check that Auto Execute is off fill in Point Coordinate List 0 075 0 0 and click Apply to make the second point notice that two points appear in the graphics window with cyan labels choose Create Curve Point items in the Geometry window check that Auto Execute is off fill in Starting Point List Point 1 click in this box while it is empty move the cursor into the graphics window place the cursor box over Point 1 on the screen and left click mouse fill in Ending Point List Point 2 and click Apply to make the line notice that a yellow line with a yellow label 1 appears choose Create Surface Extrude items in the Geometry window check that oAuto Execute is off fillin Translation Vector lt 00 10 0 gt gt September 3 2002 32 CFX TASCflow c STUDENT USER MANUAL September 3 2002 fill in Curve List Curve 1 again use the mouse to pick the curve and click
30. en you have finished choose Quit from the File menu 12 Clean up by deleting the tascflow_intro directory and closing CFX TASCflow Launcher window 1 2 The Problem To illustrate the use of the CFD software tools consider the analysis problem of estimating the pressure drop across the short radius duct bend shown in Figure 1 1 The duct bend has a width of 1m and is made of galvanized steel with an average surface roughness height of 0 10mm Water flows through the bend with a mass flow rate of m 225kgs 1 3 The Solution 1 3 1 The Duct Bend Model The first phase in the CFD solution is a planning stage in which the complete CFD model of the duct bend is specified This specification includes 1 3 The Solution 7 pS Figure 1 2 Geometry of the duct bend model Physical Model Specification The steel walls of the bend and other duct pieces are assumed to be rigid and joints in the duct work are assumed to be smooth The galvanized steel is assumed to have a uniform surface roughness height The width of the bend is sufficient that the flow can be considered to be two dimensional Domain Geometry Specification To ensure that reasonable flow pat terns are simulated in the bend it is necessary to add short entrance and exit lengths of duct to simulate the actual flow through the bend when it is situated in a duct The domain geometry is shown in Fig ure 1 2 The CFD simulation code is fully t
31. eometry through the middle of the elbow appears Note that even though this problem is two dimensional it was set up as three dimensional because CFX TASCflow solves only three dimensional problems But because it is two dimensional we need only look at one plane in the z direction The view can be changed by holding down a mouse button while moving the mouse around holding down the left button rotates the object by following the mouse holding down the left button and the lt ctr1 gt key simulta neously rotates the object about a vector perpendicular to the screen the middle button scales it and the right button translates it Try to get a feel for these operations You may centre the view or choose a preset view by selecting Viewport Options from the Viewer menu 7 Now let s look at some of the results of the solution this is what makes CFD colourful Choose New from the Visualization Object Manager box Choose Vector Plot as the object type and click on Ok Set the Region to SYMMET_T Change the Type from direct to surface and click on Apply A vector plot of the velocity field should appear note that the vectors are coloured according to speed 8 Now let s visualize the pressure field Turn off the Visibility for the vector plot and click on New to create another visualization object Choose Fringe Plot for the object type A fringe plot
32. ers If you do not have a default printer in this folder click on the EAdd Printer icon to open the Wizard Choose oNetwork Printer oFind a printer in the Directory to open the Find Printers window Set the In POLARIS click Find Now select the Lever printer from the list of printers and click ZOK Click Finish to close the Wizard and close the Printers folder e Open the program Start Programs Internet Tools Exceed Exceed This will take a few minutes while configuration files are downloaded to your account Once the configuration files are downloaded the Ex ceed window will be open on your desktop Close this window e To maximize the screen area you may wish to hide the Windows taskbar when it is not in use Start Settings Taskbar amp Start Menu will open a window in which you can set Auto hide on Click on Apply and OK to save this setting 1 1 3 Introduction to CFX Build CFX Build is the program you will use to generate meshes for the CFX TASCflow solver In this section you will be introduced to the CFX Build user interface by generating and viewing a simple brick geometry Perform the following 1 Create a new folder build_intro on your N drive Note that CFX Build does not allow spaces in folder directory names 2 Open the CFX 4 3 Launcher by Start Programs Mechanical Engineering CFX 4 3 3 Set your working directory by clicking on the Browse button This opens a folder sel
33. es and some preferences e File New will open a window called New Database in this window fill in the database name e New Database Name ductbend and click on ZOK to close e After a short wait the window New Model Preferences will pop up Click on ZOK to close In geometries with very small length scales it is better to switch the Tolerance from oBased on Model to Default and then to set the Tolerance to a value significantly smaller than the smallest length scale in the Global Preferences window opened by choosing Preferences Global September 3 2002 1 4 Commands for Duct Bend Example 31 Geometry Development The commands given in the next paragraph will build the geometry for the solid body model of the flow domain by setting the point 0m 0m 0m as the left end of the inlet setting the point 0 075m 0m 0m as the right end of the inlet joining these two points to create a curve line extruding this line 0 10m in the y direction to create a rectangular surface sweeping the upper curve of this surface through 90 counterclockwise about the axis x 0 1m y 0 1m to create a surface with an arc shape extruding the right curve of this surface 0 20m in the x direction to create a second rectangular shape and finally extruding all three surfaces 0 10m in the z direction to create three solids which will define the solution domain On the row of radio button commands just b
34. es not play any role in establishing the flow field it is pressure differences which are im portant Therefore the pressure field can be changed by a constant value without changing the results This is known as the pressure offset and is usually zero The variation of velocity pressure etc between the mesh points has to be approximated to form the discrete equations These approximations are classified as the discretization scheme CFX TASCflow uses one of the following schemes listed in order of increasing accuracy e oUpwind e oMass Weighted e oModified Linear Profile or e oLinear Profile Skew The accuracy of any of these schemes can be increased by turning on oPhysical Advection Correction In choosing a discretization accuracy is obviously an important consideration However increasing the accuracy of the discretiza tion often slows convergence sometimes to the extent that the solution algorithm does not converge For many cases especially those with a strong emphasis on fluid me chanics it is necessary to output additional fields to the rso output data file For example it is often worthwhile to output the turbulent stress fields throughout the flow domain and the wall shear stresses on all boundary walls For fully three dimensional simulations care should be taken to only select those fields that are required or the rso file will be prohibitively large The parameters that you set are stored in the prm file
35. h scales of 0 0075m i e 10 of the duct height across the inlet surface e uniform static pressure across the outlet surface e no slip conditions along the duct walls and the standard wall function treatment to resolve log law behaviour in the near wall region where the flow is not fully turbulent and e symmetry conditions on the top and bottom surfaces to ensure that the simulated flow is two dimensional The above provides a mathematically complete description of the CFD model In the rest of this section information on the use of the two software tools that can implement this model and obtain a simulation of the flow field in the model solution domain will be provided The actual software commands to use for this example problem are given in the last section of these notes 1 3 2 Grid Generation Multi Block Model A computer model of the solution domain is created and broken up into a set of finite elements This model e is composed of multiple blocks regularly or irregularly shaped boxes e defines the region through which the fluid flows e may have interior blocks through which the fluid cannot flow e may have a complex shape intake manifold intake valve cylin der geometry passage between radial compressor blades etc and e for historical reasons is often referred to as a solid body model 1 3 The Solution 9 Figure 1 3 A three blo
36. hree dimensional so even though we are primarily interested in flow in the plane shown in Figure 1 2 the geometry model must have a width into the page A slice of width 0 10m will suffice Specification of Fluid Properties For this application the fluid is water at STP which can be treated as a simple liquid with constant properties p 1000kgm y 0 001Nsm 7 Specification of Flow Models For this analysis it is reasonable to as sume the following flow features e steady incompressible flow e fully turbulent flow the Reynolds number is approximately 225 000 e the turbulent momentum stresses can be modelled with the stan dard k e model OU OU 1 1 September 3 2002 CFX TASCflow STUDENT USER MANUAL September 3 2002 where the turbulent viscosity 4 is proportional to the fluid den sity the velocity scale of the turbulent eddies and the length scale of the eddies The velocity and length scales of the turbulent ed dying motion are estimated from two field variables which are calculated as part of the model k the turbulent kinetic energy and e the rate at which k is dissipated by molecular viscous action Specification of the Boundary Conditions The boundary conditions that we will use to model the interaction of the surroundings with the model solution domain are e uniform velocity of 3ms and uniform turbulence properties of turbulence intensity of 3 and turbulence eddy lengt
37. ical Entities In addition to the physical entities that are created by the user the following topological entities entities which define a September 3 2002 12 CFX TASCflow STUDENT USER MANUAL Figure 1 6 Modified three block structured mesh to match block joining rules joining relationship between physical entities are automatically cre ated Vertex the endpoint of a curve or the corners of a surface or solid Edge the topological closing curve of a surface or solid Face the topological closing surface of a solid and Body a group of surfaces that form a closed volume Entity Hierarchy There is a hierarchy of the physical entities The gen eral strategy for creating simple solid blocks follows the hierarchy 1 define the two endpoints of a curve 2 create a curve joining the points 3 sweep the curve through space to create a surface and September 3 2002 1 3 The Solution 13 4 sweep the surface through space to create a solid In this process CFX Build will automatically define new points for the corners of every surface and solid define new curves for the clos ing curves of each surface and define new surfaces to make the clos ing surfaces of each solid If an entity has been explicitly defined it will have a unique cardinal identifier number e g Point 2 Curve 3 Surface 1 etc However if the entity has been automatically de fined
38. is basically the same as a contour plot but colours the areas between contour lines In the Fringe Interface section select the region to be SYMMET _T select the scalar to be P pressure Click on Apply a plot of the mesh having grid lines coloured according to pressure will appear A more colourful plot appears by turning off Lines and turning on Faces Does this pressure field make sense to you 9 On line help is available through the index in the help menu Context sensitive help is also available To access it click the right mouse button in the GUI panel to change the mouse pointer to a red question mark Then left click on the widget you need help with It may take a little while to load the help viewer The mouse pointer can be changed back to normal by right clicking again September 3 2002 CFX TASCflow STUDENT USER MANUAL September 3 2002 0 10m 0 025m m ie o Ml h 225 kg s Figure 1 1 Geometry of short radius duct bend 10 A search facility is also available for the on line help On the Nexus sys tem this is accessed at Start Programs Mechanical Engineering TASCflow CFX TASCflow Help Search Try to find help sec tions that refer to Sutherland a correlation for allowing variable vis cosity in gases 11 This should give a sense of the operation of the CFX TASCflow post processor Feel free to experiment with other object types and scalar fields Wh
39. iscosity ratio turbulent to molecular viscosity ration u u or the length scale of the turbulence a representative average size of the turbulent eddies Typical turbulence length scales are 5 to 10 of the width of the domain through which the mass flow occurs Outflow an outflow region is a surface over which mass leaves the flow domain For each element face on an outflow region either e fluid velocity speed and direction e mass flow rate or e static pressure must be specified A specified static pressure value can be set to a specific face applied as a constant over the outflow region or treated as the average over the outflow region No information is required to model the turbulence in the fluid flow at an outflow Opening a region where fluid can enter or leave the flow domain Pressure and flow direction must be specified for an opening region If the opening region will have fluid entering leaving close to normal to the faces i e a window opening then the specified pressure value is the total pressure on inflow faces and the static pressure on outflow faces a mixed type of pressure If the opening region will have fluid flow nearly tangent to the faces i e the far field flow over an airfoil surface then the specified pressure is a constant static pressure over the faces For turbulent flows the turbulence intensity must also be set Symmetry a region with no mass flow through the faces and with neg ligible
40. it will have an identifier number assigned in the hierarchal form Solid Surface Curve Point i e Solid 2 5 3 2 is Point 2 of Curve 3 of Surface 5 of Solid 2 Congruent Model If you stick to the simple types of physical entities and simple strategy outlined above then you will form a congruent model i e a model which can be meshed A congruent model is a model in which all surfaces and solids share common edges and vertices at their points of contact Care has to be taken when forming multi block solids to ensure that coincident points curves and surfaces are treated as single entities Following the recipe given above will ensure that two separate entities do not end up occupying the same physical space Geometry Grammar In the geometry building phase an Action is ap plied to an Object using a particular Method The Geometry form will automatically supply an appropriate list of Objects for a given Action and an appropriate list of Methods for a given Object Actions are Create create a new object Edit modify the properties of an existing object Show provide information about an existing object Transform create a new object by modifying an existing object and Delete delete the object from the model Objects are Point Curve Surface Solid plus Coord a coordinate system point of origin three coordinate axes and type rectangular cylindrical or spherical Plane a flat 2 dimensional surfac
41. levant Tracers activates the solution of a transport equation for a pas sive scalar i e simulates the advection and diffusion of a con served scalar quantity which is useful for visualizing different flow streams and Turbulent activates the turbulence models for estimating the tur bulent stresses September 3 2002 1 3 The Solution 19 Notice that there are advanced sub processes including particle track ing for modelling solid particle and liquid droplet motion com pressible for modelling transonic and supersonic flows of gases and rotating frame of reference for modelling flow inside turboma chinery Heat Transfer activates the solution of the thermal energy equation For many of these processes appropriate models must be chosen For example if turbulent flow is selected then a turbulence model and wall treatment must be chosen There are two two equation models the stan dard k e model with three variations Default recommended RNG and Kato Launder and the k w model with three variations Default recommended SST and Kato Launder There is also a second moment closure model which requires significantly more computing resources For the k e models there are two wall treatments of the transition region to laminar flow close to solid walls High Re recommended with two varia tions Log Law Standard recommended and Log Law Scalable and the Low Re requires a very fine grid in
42. m size length of longest edge in path Global Edge Length Number of elements For the example shown in Figure 1 8a mesh seeds are placed on edges Al D4 and F2 These mesh seeds are used to establish the mesh seeding for the A D and F edges respectively The remaining edges are seeded based on the Global Edge Length The Global Edge Length is used to establish the number of elements on edges B1 C4 the longest edge along the C path and El The surface mesh that results is shown in Figure 1 8b Meshing follows the Action Object Method syntax described in the geometry phase Typical actions are Create Delete and Show The relevant objects are Mesh Seed and Mesh For creating a mesh the only relevant method is IsoMesh Surface The common mesh seeding methods are Uniform Mesh Seed set a uniform placement by specifying either the number of elements or the edge length of each element One Way Bias Seed set node placement with either increasing or de creasing element edge length notice that the direction of each edge is shown as an blue arrow when this method is active The spacing is specified by setting either the number of elements plus the ratio of the lengths of the first and last element edges or the length of the first and last element edges and Two Way Bias Seed set node placement with a symmetric non uniform spacing i e decreasing to middle and then increasing to the end The spacing is specified by setting
43. n these software packages refer to the on line help documentation provided with both software packages 1 1 Getting Started This working session has 2 purposes 1 to introduce the Windows 2000 operating system and 2 to introduce the look and feel of CF X Build and CFX TASCflow 1 1 1 Introduction to Windows 2000 The CFD software is available on the workstations in the EERC LEVER lab E2 1302 The workstations in this lab use the Windows 2000 operating system which is very similar to the Windows 95 operating system used on the Waterloo Polaris system You should be familiar with techniques to create new folders or directories to delete files to move through the folder directory system with Windows Explorer to open programs through the Start menu on the Desktop toolbar to move resize and close windows and to manage disk space usage with tools like WinZip 14 is likely that many of the features available in these software packages will not be explored in introductory CFD courses CFX TASCflow STUDENT USER MANUAL September 3 2002 1 1 2 Initialization To have the system execute the CFD software properly you must configure a software package Exceed which will manage the windowing system used by the CFD software do the following for this first session only next time you log in these steps will not be necessary e Make sure you have a default printer set Open the Printers folder with Start Settings Print
44. oices and make sure the results are as you expect 2You probably have to double click in the dialog form box to be able to edit the values September 3 2002 CFX TASCflow STUDENT USER MANUAL September 3 2002 10 You can also use the mouse to rotate translate and scale zoom the object These choices are available on the left side of the tool bar Click on the translate button and move the brick around by holding down the middle mouse button while moving the mouse Try to get a feel also for the rotate buttons the rotate XY choice causes the object to rotate in a way which follows the mouse and the rotate Z choice causes the object to rotate about an axis perpendicular to the screen 11 Sooner or later you will need to access the help pages A general help with an guided tour and glossary is available from the top menu bar A context sensitive help is also available Position the mouse pointer in the geometry form and press lt F1 gt to bring up the help page for that form It may take a little while to load the viewer 12 A useful feature of CFX Build when generating more complex geome tries is the ability to show labels To the right of the preset view buttons is a button to show labels for the points and solids Next to the Show Labels button is another button to hide them 13 That s enough of CFX Build for now choose Quit from the File menu Close the CF X 4 3 Launcher window To
45. on its location Rate the convergence rate RMS res current time step Rate RMS res previous time step which should typically be 0 95 or less and Linear Solver after each equation set is linearized an estimate of the so lution of the resulting linear equation set is obtained and statistics on this solution is reported Work Units a measure of the effort required to obtain the solution estimate Residual Reduction the amount that the linear solver has reduced the RMS residual of the linear equation set and Status an indicator of the linear solver performance OK residual reduction criteria met ok residual reduction criteria not met but converging F solution diverging solver terminated residual increased dramatically and residual overflowed Some of the above information is displayed graphically in the monitor win dow so that the solver execution can be monitored When execution is complete the final results are written to the rso file The original contents of the rso are written over but a backup of the original rso can be created In addition all of the information pertinent to the operation of the solver is output to the out file including the CPU memory or storage requirements the files used for input and output an echo of the parameters explicitly set by the user the complete list of parameters grid flow attribute and boundary condition summaries estimate of the global length speed an
46. oose File Load PostProcessing Full RSO e inthe Visualization Object Manager panel choose New choose Object Type Vector Plot and click Ok e in the Vector Interface panel choose Region SYMMET1 you can do this by following Select choose Type Surface and click Apply The vector plot should appear e choose File Print to open up a Print panel In this panel choose Format Postscript fill in File Name vector ps and click Print The file vector ps should be created and can be viewed with the ghostview which is on the Waterloo Polaris workstations This file can be printed on the Lever lab printer To create an image that can be directly imported into a word processing document save the image in encapsulated postscript format eps 1 4 Commands for Duct Bend Example 39 e inthe Visualization Object Manager panel turn oVisibility off the vector plot should disappear select New choose Object Type XY Graph and click Ok e inthe XY Graph Interface panel choose 1D Region 16 1 29 2 land Scalar TAU_WALL and click Apply To scale the graph choose Viewer Viewport Options and turn oOrthographic on and select Y orientation Click ZOk to close the Viewport Manager window Click ZOk to view the scaled graph e choose File Post Processing State Save and accept the File name viz state to s
47. r value on a specified surface like elevation lines on a topological map Fringe Plot like a contour plot except the regions between the contours are filled in with colour Vector Plot the field of vectors on a nodal region or 2D surface XY Graph a plot of the variation of one scalar with respect to another scalar Streakline the lines followed by imaginary fluid parcels Relief Plot the 3D representation of a contour plot and Label a piece of text The generation of each of these visualization objects involves choosing a range of options including choice of scalar field appropriate region etc See the On line help for further information on each of these objects and their generation Once a graphical image has been created from a set of graphics objects it is often desirable to save the image so that it can be used in reports and presentations Images are saved by printing them from the File menu as either postscript or encapsulated postscript files September 3 2002 28 CFX TASCflow STUDENT USER MANUAL Region Manager Many of the components of CFX TASCflow such as the specification of boundary conditions require that a region of the flow domain be selected The Region Manager is the tool that is used to specify regions Nodal a portion of the flow domain mesh or Physical a physical plane line or point in the flow domain The structured mesh coordinate indices i j k are used to specify nodal
48. regions in the flow domain The general specification of a nodal region takes the form ib ie jb je kb ke gridname where the suffix b denotes beginning and e denotes end The following short forms are useful e for the main grid the gridname need not be specified e if a range is not specified for an index direction the complete range of that index is assumed i e 3 specifies the i 3 mesh plane e if the beginning index is not specified it is assumed to be 1 i e 3 5 2 8 is the same as 3 1 5 2 8 and e a mesh plane in the main grid can be specified by its index and value i e 13 is the same as 3 main While the Region Manager provides a GUI panel to fill in these ranges the resulting range will often be shown with this nomenclature and this nomenclature must be used when writing macros Scalar Field Manager The PostProcessor includes the Scalar Field Manager to e display and select from all of the scalar field nodal values calculated and used by the solver including grid locations x y and z velocity components u v and w pressure etc e to define new scalar fields i e to define normalized velocity compo nents e to create copies of existing scalar fields and e to display the numerical values of the mesh node scalar field quantities The display of scalar field values can be either as C V control volume solutions i e the solutions of the discrete conserva tion equation set
49. save disk space delete the folder build_intro 1 1 4 Introduction to CFX TASCflow CFX TASCflow is the program which performs the CFD analysis Unfortu nately it has a user interface which is somewhat different from CFX Build To get a feel for its interface we will look at a problem which has already been solved the flow through an elbow 1 Make a directory called tascflow_intro on your N drive 2 Use a web browser to visit the ME566 homepage www eng uwaterloo ca me566 Click on the link to tascflow_intro archive file Save the downloaded file in your directory Use WinZip Start Programs Accessories WinZip 7 0 to extract all the files from the archive into your directory You should now have a bef gci grd name 1lun and rso file 3 To start CFX TASCflow Launcher click on Start Programs Mechanical Engineering TASCflow CFX TASCflow 02 10 00 4 Set the working directory to N tascflow_intro and then click the CFX TASCflow button 5 The configuration mesh flow attributes and boundary condition in formation for this problem is stored in various files in your directory 1 1 Getting Started 5 and some are automatically read by CFX TASCflow In later assign ments you will go through the steps required to generate this informa tion yourself However the solution file must be read explicitly From the File menu choose Load Postprocess and then Full RSO 6 A wireframe model of the g
50. shear stresses and heat fluxes This condition is often used to simulate a two dimensional flow field with a three dimensional flow solver and to minimize mesh size requirements by taking advantage of natural symmetry planes in the flow domain Coalescence a boundary condition that can be applied to faces of zero area a situation that occurs in structured meshes when the width of the domain is pinched together Each boundary condition must be assigned to a set of surface element faces If all of the surface faces of the flow domain were named in the grid generation phase then it is relatively straightforward to attach the appropriate set of surface element faces for a particular boundary condition September 3 2002 22 CFX TASCflow STUDENT USER MANUAL September 3 2002 Inflow Sets Outflow Sets Solution Predicts velocity static pressure inflow static pressure total pressure velocity outflow pressure inflow velocity total pressure static pressure system mass flow Table 1 1 Common boundary condition combinations with the Region Manager Since it is crucial that each surface element face have a boundary condition attached to it it is recommended that one of the boundary conditions usually a wall boundary condition be set as the default condition If a face does not have a boundary condition explicitly attached to it then the default condition will be attached to it Using the Di
51. splay command on the default condition will show the faces attached to the default condition This will often help identify faces which are missing proper boundary conditions For the flow solver to successfully provide a simulated flow field the specified boundary conditions should be realizable i e they should cor respond to conditions in a laboratory setup In particular ensure that the inflow and outflow boundary conditions are consistent and that they take advantage of the known information Table 1 1 lists several common inflow outflow condition combinations along with the global flow quantity which is estimated as part of the solution for each combination The boundary condition specification steps ends with e acheck to ensure that the boundary conditions are properly specified and e then writing the complete boundary condition information to the bcf file Initialization The algebraic equation set that must be solved to find the velocity and pres sure at each mesh point is composed of nonlinear equations All strategies for solving nonlinear equation sets involve iteration which requires an initial guess for all solution variables Therefore for a turbulent flow sufficient information must be provided so that the following field values can be set initialized at each mesh point velocity vector 3 components fluid pressure turbulent kinetic energy and e dissipation rate of turbulent kinetic energy 1 3
52. the Check window Click Ok and if there are no errors the Check window will disappear If there are errors a message window will appear e choose File Write PreProcessing to save the preprocessing infor mation in a bcf file e choose PreProcess Initial Guess Generator to open the Initial Guess window Fill the following Velocity oUniform Grid Aligned Axis oJ Velocity Magnitude signed 3 0 and Pressure 0 0 Then choose Turbulence to open the Turbulence Initial Guess Window In this window select oMed Intensity to get a 5 initial turbulence level and then select oEddy Viscosity Ratio with oMed to set the initial effective viscous action of the turbulent eddies to be 10 times that due to molecular activity Click ZOk to accept these values Check that oWrite RSO is on then click Ok to write the initial solution fields into the rso file Solver Operation Before executing the fluid flow solver code it is necessary to set some pa rameters which control the operation of the code e choose Solve Solver Parameters and fill in oFluid Time Step 0 04 30 of the average residence time of a fluid parcel inside the solution domain Number of time steps 25 and Maximum residual 1 0e 03 and turn oUpwind Difference on in the discretization panel To add to the default output fields click on Additional Parameters and turn on ol O Control in th
53. the near wall region For Solid Zones also referred to as objects it is necessary to specify the mesh elements that are solid or blocked off to fluid flow If the zone has already been given a name in the CFX Build Patch phase then the Region Manager is used to select the named patch If a named patch does not exist then the Region Manager is used to define a new nodal region that can be selected Care has to be taken to ensure that objects are sufficiently separated to allow valid flow field solutions Objects cannot touch solely on a line or a point see Figure 1 9 They can touch along faces While it is valid to have a single element spanning the gap between two objects the flow field will not be resolved in this gap Therefore it is recommended that at least two elements span the gap between two objects Specification of Boundary Conditions Throughout the flow domain mass and momentum conservation balances are applied over each element These are universal relationships which will not distinguish one flow field from another To a large extent a particular flow field for a particular geometry is established by the boundary conditions on the surfaces of the flow domain It is crucial that these boundary conditions reflect a reasonable physical situation and that they are consistent i e a set of boundary conditions which set the net inflow mass flow rate to be greater than the net outflow mass flow rate is not consistent Boundary condi
54. tions are defined and then attached or applied to the element faces on each surface of the flow domain Typical boundary condi tions include The Region Manager is described Section 1 3 5 September 3 2002 20 CFX TASCflow STUDENT USER MANUAL Valid Objects Not Recommended Objects Invalid Objects Figure 1 9 Valid not recommended and invalid objects Wall a solid wall through which no mass can flow The wall can be sta tionary translating sliding or rotating If the flow field is turbulent then the wall can be either smooth or rough Depending upon which of these options are chosen suitable values must be input i e the size of the roughness elements etc Inflow an inflow region is a surface over which mass enters the flow domain For each element face on an inflow region either e fluid speed and direction either normal to the inflow face or in a particular direction in Cartesian coordinates e mass flow rate and flow direction or e the total pressure ee Pota P PV Protal spec 1 2 and flow direction September 3 2002 1 3 The Solution 21 must be specified If the flow is turbulent that it is necessary to specify the intensity of the turbulence I Average of speed fluctuations ES Mean speed and one additional property of the turbulence eddy v
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