5 STUDENT USER MANUAL - University of Waterloo
Contents
1. under the Basic Settings tab set x Boundary Type Y Inlet and x Location Y inflow surface and under the Boundary Details tab set x Flow Regime Option Y Subsonic x Mass and Momentum Option Y Normal Speed x Normal Speed Y 3 V ms 1 Turbulence Option Y Intensity and Length Scale x Fractional Intensity 0 05 x Eddy Length Scale 0 0075 Y m and then click Ok to close the panel and create the new boundary object Notice that incoming arrows appear on the inlet surface in the Viewer window and the boundary object is listed in the Object Se lector panel Clicking on Boundary INFLOW SURFACE object in the Object Selector panel causes the inflow surface to be outlined in red in the Viewer window e Click the Boundary Condition button to open the definition panel Fill in Name OUTFLOW SURFACE and click OK In the Edit Boundary Outflow surface in Domain Ductbend panel under the Basic Settings tab set x Boundary Type Y Outlet and x Location Y outflow surface and under the Boundary Details tab set x Flow Regime Option Y Subsonic Mass and Momentum Option Y Average Stat2. s Type of Run Y Full and s Run Mode Y Serial and then click Start Run After a few minutes execution should begin Diagnostics will scroll on the terminal output panel and the equation RMS residuals will be plotted as a function of time step After the first few time steps the residuals should fall monotonically Execution should stop after 22 time steps Close the Solver Manager 9 4 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 res file for post processing e create a vector plot on one of the symmetry boundary planes e save an image of the plot e create a vorticity variable e create a fringe plot of the vorticity field e create a line and export the velocity data along the line e export the inner wall pressure and wall shear stress distribution e probe the velocity field and e save the visualization state August 5 2004 9 4 Post processing 46 To accomplish these tasks click the CFX Post and then tab in the CF X 5 Launcher e Choose File Load Results and select the ductbend_001 res results file e Choose Create Vector accept Name Vector 1 and click OK define a vector object and open an edit panel In the panel set Locations Y FRO
3. General Options sub panel set Location Y Assembly 1 since this rolldown list allows mul tiple selections you will need to highlight DUCTBEND and then click on the panel outside of list Domain Type Y Fluid Domain Fluids List Y Water nominal x O Particle Tracking off Reference Pressure 1 Y atm x Buoyancy Option Y Non Buoyant and x Domain Motion Option Y Stationary on the Fluid Models sub panel set x Heat Transfer Model Option Y None Turbulence Model Option Y k Epsilon Turbulent Wall Functions Option Y Scalable x Reaction or Combustion Model Option Y None and Thermal Radiation Model Option Y None on the Initialization sub panel ensure that O Domain Initializa tion is off and then click Ok to close the panel Notice that the Domain Ductbend is now listed in the Object Selector panel If you double click on this object in the list then the Edit panel will reappear August 5 2004 9 2 Pre processing 42 e Click the Boundary Condition button to open a definition panel Fill in Name INFLOW SURFACE and click OK In the Edit Boundary Inflow surface in Domain Ductbend panel
4. choose File Quit Clean up by deleting the CFX5_intro directory and closing the CFX 5 Launcher panel August 5 2004 3 The Problem 7 0 075m ih 225 kg s Figure 1 Geometry of short radius duct bend 3 The Problem Consider the analysis problem of estimating the pressure drop of flow through the short radius duct bend shown in Figure 1 The duct bend has a width of 1 m and is made of galvanized steel with an average surface roughness height of 0 10 mm Water flows through the bend with a mass flow rate of m 225 kgs7 t 4 The CFD Model Specification 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 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 2 The CFD simulation code is fully three dimensional so even though we are primarily interested in flow in
5. View e The surfaces faces of the solid are labeled as regions for ease of attaching the boundary conditions To attach a region entity at the inflow see Figure 4 1 Right mouse click over Regions in the Tree View and select Insert Y Composite 2D Region to create a new region entity 2 Right mouse click over the new region entity Composite 2D Region 1 and choose Rename to edit the region name to inflow surface 3 In the Details View Parameters tab section click on Location Y None In the Model View put the mouse cursor over the inlet surface area and left mouse click The inlet surface area should turn green 4 In the Details View click on Location Y Apply e Repeat for the outflow surface Notice that if you left mouse button click on inflow surface or the outflow surface in the Tree View that the resulting region in the Model View turns green e Create a region entity attached to the inner wall This region is com posed of three primitive surfaces To select a set of surfaces or faces hold the Ctrl key down while clicking on the component surfaces in the Model View e Repeat for the outer wall e For the front surface use the Z view of the Model View when selecting the surface Notice that there are two parallel planes in the lower left corner of the model view The front most of these planes should be outlined in red e Repeat for the back surface except
6. a point near the centre of the line Then move the cursor down Click to complete a dimension which is labelled H3 e In the Details View change the value of H3 to 0 075 m 5 Repeat the procedure used for the entrance inner wall to draw the exit inner wall a Draw a horizontal line in the upper right XY quadrant near the end point of the inner wall bend b Set the length of the line to 0 20 m with the General tool from the Dimensions toolbox August 5 2004 9 1 Geometry and Mesh Generation 36 c Join the exit inner wall to the inner wall bend with the Coincident tool from the Constraints toolbox Draw the outer entrance wall with the Line tool Start at the outer left end point of the inflow edge look for the P constraint symbol and draw a vertical line that is coincident C with the X axis Draw the outer bend wall with the Arc by Center tool Put the center at the origin make the start point at approximately 20 above the X axis in the upper left quadrant and make the end point coincident C with the Y axis Use the Coincident constraint tool to join the start point of the arc to the end point of the outer entrance wall Draw the outer exit wall with the Line tool Draw a horizontal H line coincident C with the Y axis above its final desired location Use the Coincident const
7. directory folder when setting the filename with the Windows file prompt window In the Tree View right mouse click on Preview and select Insert Preview Group To select a location for the group in the Details View put the cursor in the the Model View notice a box symbol on the cursor and create a box that surrounds the complete model by clicking the upper left corner and dragging to the lower right corner In the Tree View select Generate Options and set Show Inflated Front Y Yes in the Details View In the Tree View right mouse click on Preview Group 1 and select Gen erate This Surface Mesh An Overall Progress window should open in the lower left corner and after a short time you should see a mesh of triangles on the surfaces of the solid There will be some gaps on the front and back surfaces near the inner and outer walls where the inflated boundary meshing is used Click the Generate a volume mesh for this problem icon on the top row of icons buttons Again a progress window will open in the lower left corner When this window closes go to the Tree View and select Errors to ensure that no errors are reported in the Details View To close this phase select File Exit and choose Save all highlighted items August 5 2004 9 2 Pre processing 39 9 2 Pre processing The first CFD phase is preprocessing In this phase the complete CFD model mesh fluids flow processes bound
8. ratio turbulent to molecular viscosity ratio L U are specified Typical turbulence length scales are 5 to 10 of the width of the domain through which the mass flow occurs Outlet an outlet region is a surface over which mass leaves the flow domain For each element face on an outlet region one of the following must be specified e fluid velocity speed and direction e mass flow rate or e static pressure 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 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
9. this time after selecting the front surface as seen in the Model View select the rear of the two parallel planes in the lower left corner 9 1 3 Mesh Generation e In the Tree View expand the Spacing entity e Select the Default Body Spacing entity to open the Body Spacing De tails View Set Maximum Spacing m 0 0075 August 5 2004 Geometry and Mesh Generation 38 Right mouse click on the Periodicity entity and select Insert Periodic Pairs to create a periodic pair of meshing surfaces in the Z direction In the Details View click Location 1 None select the front surface in the Model View and click on Apply Repeat to select the back surface for Location 2 Remember to use the location planes in the lower left corner of the Model View In the Tree View right mouse click on Inflation and select Insert Inflated Boundary to create an Inflated Boundary entity Select the three sur faces of the inner wall in the Model View for the Location and set Maximum Thickness m 0 0075 Repeat to create an Inflated Boundary of Maximum Thickness m 0 0075 on the outer wall In the Tree View select Options to see the mesh options in the the De tails View Set Surface Meshing Y Advancing Front and set GTM Filename Ductbend_mesh1 gtm Make sure that you are in the cor rect
10. with 5Note that some buttons are optional You must click each required button in sequence and choose Ok even if you accept all default inputs August 5 2004 9 2 Pre processing 40 the called Import Mesh Select the Mesh Format Y CFX 5 GTM file Import Mesh button at the top left which will open a panel and fill in File Ductbend_mesh1 gtm Click OK to import the file and close the Import Mesh panel A wireframe of the ductbend model should appear in the Viewer window In the Mesh Selector panel you should see the Assembly 1 object listed e Select the Open Region Selector icon on the right of the Mesh Se lector panel to check the region names In the Region Selector panel click on the inflow surface You should see the surface highlighted in red in the Viewer If neces sary click the Highlighting icon on the top row of the Viewer panel on Repeat for the other regions listed Expand the inner wall region to see the three primitive face that make up the region Each primitive face has a name of the form Fz By Pz face x of body y of part z where y and z are often 1 One of the region names will not be imported into CFX 5 as ex pected Instead that surface will be attached to the Default 2D Re gion If you would like to correct this click Create a new object icon on the right of the Regi
11. Bend Example 9 1 Geometry and Mesh Generation 9 1 1 9 1 2 9 1 3 Geometry Development Region Identification 2 0 2000 Mesh Generation 0 2 000 ee ee 10 10 11 13 13 14 18 19 19 19 21 22 22 23 23 23 25 25 28 28 30 30 CONTENTS ii 9 2 Pre processing 9 3 Solver Manager 9 4 Post processing 9 5 Clean Up August 5 2004 1 Introduction 1 1 Introduction In these notes the basic steps in a CFD solution will be illustrated using the professional software packages CAD2Mesh Version 2 1 and CFX 5 Version 5 6 trademarks of ANSYS These notes include an introductory tutorial and a mini user s guide They are not meant to replace a detailed user s guide For full information on these packages refer to the on line help doc umentation provided with the software These notes include sections on Getting Started Instructions for a short computer session in which the software graphical user interfaces GUIs are introduced The Problem A description of the example problem The CFD Specification A complete description of the CFD model im plemented in the software Software Components A description of the concepts and operation in volved in the five software components Design Modeller CF X Mesh CFX Pre CFX Solver and CFX Post and Commands for the Example Problem A complete step by step list of instructions for solving the mo
12. CFD Model Specification 9 Specification of the Boundary Conditions The boundary conditions that model the interaction of the surroundings with the solution domain are e uniform velocity of 3 ms and uniform turbulence properties of turbulence intensity of 5 and turbulence eddy length scales of 0 0075 m 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 front and back surfaces to ensure that the simulated flow is two dimensional The above provides a mathematically complete description of the CFD model In the next four sections information will be provided on the use of the software that can implement the CFD steps of geometry modelling mesh generation physical modelling solver operation and visualization The actual software commands to use for this example problem are given in Section 9 August 5 2004 5 CAD2Mesh Geometry and Grid Generation 10 5 CAD2Mesh Geometry and Grid Generation In the first step of the CFD computer modelling the solution domain is cre ated in a digital form and then subdivided into a large number of small finite elements or volumes Common finite element types shapes include tetra hedral prismatic and hexahedral T
13. ME 566 Computational Fluid Dynamics for Fluids Engineering Design CFX 5 STUDENT USER MANUAL G D Stubley Mechanical Engineering Department University of Waterloo Copyright 2004 by G D Stubley Contents 1 2 Introduction Getting Started 241 Windows XPY NEXUS 2358 wip see he ee eS Eee SS 2 2 Introduction to CAD2Mesh 048 2 3 Introduction to CFX Post 0 0 000000048 The Problem The CFD Model Specification CAD2Mesh Geometry and Grid Generation Basic Concepts and Definitions Geometry Creation 2 a Mesh Generation aooaa 5 1 5 2 5 3 5 3 1 5 3 2 REGIONS a L Ae hoes eee eal oe ee ete ee ER E Mesh Features 0 00 0000 bee eee CFX Pre Physical Modelling C Domain 4 64 Ge Bene a Aa aS wo A SOS we we ede ae 6 2 6 3 6 4 6 5 6 1 1 6 1 2 6 1 3 6 1 4 Fl ds List 3 s poanie va Sk ana A ee a a ii Boundary 3 5004 a ae Gite a E aia Ae a Domain Models oaa a a a Fluid Models aoaaa a Initialization somn es Ha ik A hoe Ee R a Output Control nu ee Hes ee Hee ea aE ie Ped ee Sa Simulation Type 20000022 eee Solver Control eia RR a Bo OR A ee CFX Solver Manager Solver Operation 7 1 Monitoring the Solver Run 4 CFX Post Visualization and Analysis of Results Bal y OD YCCUS iy eh Ae at ee Sk a et een lk ott ce 8 2 Tools 853 Controls ess ney Be te eae he SE Brie oh a A os Commands for Duct
14. NT SURFACE Variable Y Velocity Hybrid on Projection Y None Reduction 2 plots vector at every other mesh point O Visibility on to and click Apply The vector plot should appear in the Viewer window and the vector object is listed in the Object Selector panel In the Object Selector list turn O DEFAULT LEGEND on and Wireframe on Orthographic projection will work best for two dimensional views Notice that if you double click on an object in the Object Selector list then an edit panel opens up for that object e Choose File Print and in the Print panel set File vectorplot png and Format Y PNG followed by Print and then Cancel The plot will printed to the file vectorplot png in your working directory You can import this file into other documents e Choose Tools Variables to open the Variable Editor panel Click the New button on the right of the list box Enter Name Vorticity and click Ok Fill in Expression Velocity v Gradient X Velocity u Gradient Y and click Apply e Choose Create Contour accept Name Contour 1 and click OK to define a fringe contour plot object and open an edit panel In
15. all and inner wall 7 Boundary Details tab set and click OK In the Edit Boundary x Wall Influence on Flow Option Y No x x Wall Roughness Option Y 0001 Yim Wall Velocity off Slip Rough Wa iI x Roughness Height 0 and then click Ok object Click the Fill in Name Boundary Condition OUTER WALL and click OK I Outer wall in Domain Ductbend panel to close the panel and create the new boundary button to open the definition panel n the Edit Boundary August 5 2004 9 2 Pre processing 44 under the Basic Settings tab set x Boundary Type Y Wall and x Location Y outer wall and under the Wall Influence on Flow Option Y No Slip Boundary Details tab set x O Wall Velocity off x Wall Roughness Option Y Rough Wall x Roughness Height 0 0001 Y m and then click Ok object to close the panel and create the new boundary Click on the Global Initialization button to open the initialization panel You can accept Y Automatic initialization for all properties and click Ok Click on the Solver Control butto
16. ary conditions etc is defined and saved in a hierarchical database After opening CFX Pre the commands listed below will accomplish the following steps e import the mesh gtm file set simulation type to steady state establish a fluid with nominal properties of water specify the region through which the fluid will flow the fluid nom inally water and the physical models fluid flow no heat transfer turbulence standard k e model with scalable wall functions set up and attach the rough wall boundary condition the inlet bound ary condition the outlet boundary condition and the symmetry bound ary conditions set the global initial conditions set the solver controls for discretization scheme time step type and convergence criteria set the output variable list and e write the complete CFD model definition to a def file To accomplish these steps execute the following commands Open the CFX 5 6 Launcher Start Programs Mechanical Engineering CFX 5 6 set the working directory to N ductbend From the CFX 5 Launcher click the CFX Pre tab Choose File New Simulation which will open a panel called New Simulation File in this panel fill in the database name File name ductbend cfx and click on Save to close After a short wait watch the lower right corner to see the Completion Meter follow the second row of buttons from the top beginning
17. cept Name define a polyline object In the edit panel use Method Y Polyline 1 with Boundary List Y INNER WA LL and Location Y and click OK to Boundary Intersection Plane 1 and then click Apply This creates a line the follows the inner wall You can follow the steps for export along a line to export the values of the pressure total pressure and wall shear stress along this line into the data file wall dat Choose Create Point accept Name Point 1 and click fine a point object Use Method Y XYZ to 0 10 0 04 0 before clicking Ap OK to de and initialize the point ply Choose Tools Calculator to open the Function Calculator panel Use Function Y probe Loca tion Y Point 1 Variable Y Velocity u Gradient X to get a list of all possible variables before clicking on the Note Use the m Calculate August 5 2004 9 5 Clean Up 48 button The result with units appears in the results box Move the point around to probe other regions in the flow s Choose File Save State and enter tutoriali cst for the file name to save all of the information associated with the visualization and post processing objects you have created in this session You can load this state file File Load State to r
18. ces such as an airfoil in a large flow domain it might be desirable to set the triangular mesh length scale smaller than the default body spacing In this case a new Face Spacing can August 5 2004 5 3 Mesh Generation 15 be defined and assigned to the airfoil surface Besides setting the triangular element length scale the following properties must be set for the new face spacing Radius of Influence The distance from the region that has a tetrahedral mesh length scale equal to that of the surface triangular elements Expansion Factor The rate at which the tetrahedral mesh length scale increases outside the radius of influence This value con trols how smoothly the mesh length scale increases from the face region to the default body spacing far from from the face e For complex surfaces the face spacing type should set so that the geometry of the surface is well represented by the mesh relative error or angular resolution Controls are used to locally decrease the mesh length scale in the region around a point line or triangular plane surface The spacing in the vicinity of a control is set by three factors Length Scale fixes the size of the tetrahedral mesh elements Radius of Influence sets the distance from the control that has a mesh of the specified length scale and Expansion Factor controls how smoothly the mesh length scale in creases to the default body spacing far from the control For line and triangl
19. cing Front starts at the edges of the sur face and is a similar algorithm to the volume meshing algorithm described above Since it creates regular meshes on simple rect angular type surfaces it is the recommended algorithm August 5 2004 6 CFX Pre Physical Modelling 18 6 CFX Pre Physical Modelling CFX Pre is a program that builds up a database for storing all of the infor mation geometry mesh physics and numerical methods that is required by the equation solver The contents of the database is written to a def definition file at the end of the CFX Pre session The database is organized as a hierarchy of objects Each object in the hierarchy is composed of sub objects and parameters There are two main objects Flow and Library The Flow object holds all of the data on the flow model and the Library object holds the property data on a set of common fluids The major components of the Flow object are organized in the following hierarchy e Flow Domain x Fluids List x Boundary x Domain Models Domain Motion Reference Pressure x Fluid Models Heat Transfer Model Turbulence Model Turbulent Wall Functions Initialization Output Control Simulation Type Solver Control x Advection Scheme x Convergence Control x Convergence Criteria CFX Pre has functions to create new objects in the hierarchy and to edit existing objects through edit panels For most objects the edit panel pr
20. d e CFX Post for visualizing the results and calculating secondary flow properties The user interfaces for CFX Pre and CF X Post are similar To get a feel for the interfaces we will use CFX Post to look at a simulation which has already been solved the flow through a ductbend 1 From the ME566 homepage click on the link to CFX 5_intro archive file Save the downloaded file in your directory Use WinZip Start Programs Accessories WinZip 8 1 to extract the file ductbend res from the archive into your directory 2 Open the CFX 5 6 Launcher panel from Start Programs Engineering CFX 5 6 3 Enter Working Directory N CFX 5_intro and let CFX 5 6 automat ically create the new directory for you 4 Click the CFX Post tab 3CFX Build is no longer a supported component August 5 2004 2 3 Introduction to CF X Post 5 5 Choose File Load Results and select the File name ductbend res All of the pertinent CFD model data mesh flow attributes and boundary condition information for this problem is stored in this file 6 There are three major areas on the screen Command menus and but tons at the top selector and edit panels on the right and Viewer window A wireframe model of the geometry should be in the Viewer The view can be changed by holding down a mouse button while mov ing the mouse around holding down the left button rotates the object by following the mouse
21. d e dissipation rate of turbulent kinetic energy CFX Pre provides a default algorithm for calculating initial values based on interpolating boundary condition information into the interior of the domain This default algorithm is adequate for many simulations The initial conditions can significantly impact the efficiency of the iter ative solution algorithm If there are condition difficulties then values or expressions with CEL can be used to provide intial conditions that e match the initial conditions to the dominant inlet boundary conditions and e align the flow roughly with the major flow paths from inlet regions to outlet regions 6 3 Output Control For many cases especially those with a strong emphasis on fluid mechanics it is necessary to output additional fields to the res output data file For example it is often worthwhile to output the turbulent stress fields through out the flow domain wall shear stresses on all boundary walls and gradient operations applied to all primary solution variables 6 4 Simulation Type The numerical formulations for steady and transient flows differ slightly The focus is on steady flow simulations however transient evolution with no transient accuracy is used in the iterative solution algorithm Each iteration is treated as a step forward in time 6 5 Solver Control The numerical methods operation used in the equation set solver are largely fixed however some aspects of the num
22. del problem The following font format conventions are used to indicate the various commands that should be invoked Menu Sub Menu Sub Sub Menu Item chosen from the menu hierarchy at the top of a main panel or window Button Tab Command Option activated by clicking on a button or tab Link description Click on the description to move by a link to the next step page Name value Enter the value in the named box Name Y selection Choose the selection s from the named list Name Panel or window name Name On off switch box and Name On off switch circle radio button Many of the features available in these software packages will not be explored in introductory CFD courses August 5 2004 2 Getting Started 2 Getting Started This working session has two purposes 1 to ensure that your Windows XP NEXUS operating system is opera tional and 2 to introduce the look and feel of the software 2 1 Windows XP NEXUS The CFD software is available on the workstations in the Fulcrum lab E2 1313 WEDGE lab E2 1302B and GAFF CPH 2367 The workstations use the Windows XP operating system on Waterloo NEXUS 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 a
23. directly with these primitive entities Therefore there is a facility for creating and August 5 2004 5 3 Mesh Generation 14 Anisotropic Triangular Element Isotropic Triangular Element Tetrahedral Element Triangular Prism Element Figure 3 Shape of common two and three dimensional elements naming composite 2D surface regions When the mesher is initialized all of the primitive 2D surfaces are assigned to the Default 2D Region As surfaces are assigned to new user defined regions they are removed from the Default 2D Region However the Default 2D Region must contain at least one surface These region names and the their corresponding surface meshes are passed on to CFX Pre 5 3 2 Mesh Features The mesh is composed of two dimensional triangular elements on the surfaces and tetrahedral and prism elements in the body of the solid Figure 3 shows the element shapes The properties of the mesh are controlled by the settings of the following features Default Body Spacing Set the maximum length scale of the tetrahedral elements throughout the volume of the body Some of the actual tetra hedral elements may be smaller due to the action of other mesh features or in order to fit the tetrahedral elements into the body shape Default Face Spacing Set the length scale of the triangular elements on the surfaces e For simple meshes it is sufficient to set Face Spacing Type Y Volume Spacing e For surfa
24. dow so that the solver execution can be monitored When execution is complete the final results are written to the res file In addition all of the information pertinent to the operation of the solver is output to the out file including e the CPU memory or storage requirements August 5 2004 7 1 Monitoring the Solver Run 27 e the physical flow model e grid summary e estimate of the global length speed and time scales based on the initial fields e the convergence diagnostics e estimate of the global length speed and time scales based on the final fields e the fluxes of all conserved quantities through the boundary surfaces these should balance to 0 01 of the maximum fluxes and e the computational time required to obtain the solution August 5 2004 8 CFX Post Visualization and Analysis of Results 28 8 CFX Post Visualization and Analysis of Re sults 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 CF X Post has capabilities for visualizing the results in graphics objects for using calculation tools and for controlling the post processing state 8 1 Objects Three types of objects can be created geometrical flow visualization and viewer augmentations Geometrical objects include Point a point in the domain Often used to probe flow properties
25. e The operation of the solver should however be monitored and facilities are provided for this Table 3 shows typical solver diagnostic output listing the residual re duction 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 Rate the convergence rate RMS res current time step Rate RMS res previous time step which should typically be 0 95 or less RMS Res the root mean square of the nodal normalized residuals Max Res the maximum nodal normalized residual in the flow domain and Linear Solution after each equation set is linearized an estimate of the solution of the resulting linear equation set is obtained and statistics on this solution are 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 August 5 2004 7 1 Monitoring the Solver Run 26 H Table 3 Typical convergence diagnostics OK residual reduction criteria met ok residual reduction criteria not met but converging F solution diverging residual increased dramatically and solver terminates due to floating point number overflow error Some of the above information is displayed graphically in the monitor win
26. e controls the spacing can be varied over the control ie from one end point of the line to the other end point Periodicity In cases where the flow is two dimensional it is desirable to have a small number of mesh vertices in the cross stream direction This can be achieved by setting the thickness of the solid body to that of the default body spacing unfortunately this means that as the mesh size is reduced the solid body thickness should also be reduced To ensure that the numerical solution is truly two dimensional it is necessary to have an identical surface mesh on both of the flow bound ing surfaces in the cross stream direction This is achieved by defining Periodic Pairs Each periodic pair has two surfaces and is either ap plied to two dimensional planar Periodic Type Y Translational or axisymmetric Periodic Type Y Rotational flow domains Inflation In boundary layer regions adjacent to solid walls it is often desir able to make a very small mesh size in the direction normal to the wall in order to resolve the large velocity shear strain rates If tetrahedral August 5 2004 5 3 Mesh Generation 16 meshes are used in this region there will either be a large number of very small elements with equal spacing in all directions i e isotropic elements with vetex angles close to 60 or very thin squashed ele ments These choices are either inefficient or inaccurate A better eleme
27. ecreate these objects and images in later sessions This facility allows easy comparison of results between simulations e Close CFX Post 9 5 Clean Up The last step is to remove unnecessary files created by CFX 5 This step is necessary to ensure that you do not exceed your disk quota At the end of each session delete all files except e agdb cmdat cmdb wbdb def and _ res files If you no longer need your results but would like to be able to replicate them then you should delete all files except e def files After removing all unnecessary files use the WinZip utility to compress the contents of your directory August 5 2004
28. election filter turned on 2 Freeze the solid body to stop the Boolean merge or remove operations Tools Freeze This will form a new solid body as a component of a new part and 3 Select all solids and choose Tools Form New Part When the solid model is completed an agdb file is created and saved in order to store the geometry database 5 3 Mesh Generation The mesh generation phase can be broken down into the following steps 1 Read in the agdb file with the solid body geometry database NOTE that this step is not repeated In other words when a change is made to the geometry it is necessary to delete the old mesh database and then open a new mesh database 2 Name the surfaces as regions for ease of applying boundary conditions 3 Set the properties of the mesh 4 Cover the surfaces of the solid body with a surface mesh of triangular elements and 5 Fill the interior of the solid body with a volume mesh of tetrahedral elements that are based on the surface triangular mesh and that grow inwards from the surface mesh A gtm file containing all of the mesh information and region information is written at the end of this step The following comments and guidelines are for generating meshes for two dimensional flow simulation in relatively simple rectangular geometries 5 3 1 Regions The geometry database contains a list of primitive faces and edges that are formed in the generation processes It is often cumbersome to work
29. erical methods must be explicitly set by the user 1 the choice of discretization scheme 2 the time step size for the flow evolution and 3 the criteria for stopping the iterative process August 5 2004 6 5 Solver Control 24 The variation of velocity pressure etc between the mesh points ele ment nodes has to be approximated to form the discrete equations These approximations are classified as the discretization scheme The discretiza tion scheme for approximating advective transport flows listed in order of increasing accuracy options are e Upwind a constant profile between nodes e Specified Blend Factor a blend of upwind and high resolution and e High Resolution a linear profile between nodes In choosing a discretization scheme accuracy is obviously an important consideration Increasing the accuracy of the discretization often slows con vergence sometimes to the extent that the solution algorithm does not converge The choice of time step size for the flow evolution plays a big role in establishing the rate of convergence Good results are usually obtained when the physical time step size 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 The initial guesses for the velocity pressure turbulent kinetic energy and dissipation rate nodal values will not necessarily satisfy the disc
30. ew The point of rotation within the roll circle can be changed by clicking the left button while the cursor is on cylinder surface this may take some experimentation A small red sphere indicates the centre of rotation 7 The Tree View on the left shows the geometric entities that were used to generate the cylinder Expand the 1 Part 1 Body entity and click on Solid to see some properties of the cylinder in the Details View The cylinder was generated from two entities a Sketch1 which can be found in the XYPlane entity Click on Sketch1 to highlight the circle that the cylinder is based upon with yellow August 5 2004 2 3 Introduction to CF X Post 4 b Extrudel Click on the Extrudel entity and look in the Details View to see that the cylinder was generated by extruding the circle Sketch1 5 m in the normal direction 8 On line help is available in web page format similar to other Win dows programs Choose Help Help Topics to open the ANSYS Work bench Documentation Search for keywords rotation modes and se lect CFX Mesh 2 1 Help to find more information on changing the view 9 Choose File Exit and select Save all highlighted items to save your project and exit 10 Clean up by deleting the CAD2Mesh_intro directory 2 3 Introduction to CF X Post CFX 5 has three major components e CFX Pre for modelling the physics e CFX 5 Solver Manager for controlling the numerical computations an
31. f and back on to remove and then replace the scale legend Close the Vector panel with the X button in the upper right corner You can get an edit panel back by double clicking on the object in the Object Selector list August 5 2004 2 3 Introduction to CF X Post 10 11 To visualize the pressure field create a contour plot Remove the vector plot click O Vector 1 off in the Object Selector Choose Create Contour from the top menu row In the New Contour panel accept Name Contour 1 and click OK In the Contour Contour 1 edit panel set e Locations Y Symmet1 e Variable Y Pressure s Hybrid on e Visibility on and click Apply The fringe plot should appear in the Viewer window Go to the Render tab in the Contour edit panel turn O Draw Faces off and click Apply to see a line contour plot of the pressure field Does this pressure field make sense to you On line help is available in pdf file format Help Context sensitive help is also available Position the mouse pointer in the Object Se lector panel and press lt F1 gt to bring up the help page for that panel This should give a sense of the operation of the CF X 5 post processor Feel free to experiment with other object types and scalar fields When you have finished
32. flow domain name the surfaces faces of the solid geometry to ease boundary condition specification specify the properties of the mesh in the interior of the flow do main and close to solid walls preview the surface mesh to check for anomalies and create the geometry file with volume mesh information August 5 2004 9 1 Geometry and Mesh Generation 33 fer i i i Na Selection Meter n n 4 Figure 4 Wireframe of the final sketch of the duct bend flow path 9 1 1 Geometry Development To begin e Open ANSYS Workbench from Start Programs Engineering ANSYS CFX CAD2Mesh 2 1 To open a new project 1 Follow the Create a new project link 2 Turn Nothing will build this project from scratch on 3 Set Name Ductbend 4 Set Location N using Browse if necessary August 5 2004 9 1 Geometry and Mesh Generation 34 In the Tree View select the XYPlane entity and then click the icon to create the Sketch1 entity as a component of the XYPlane Set O Create a subfolder based on project name on you should create a new folder for each project geometry Follow the Open the Project Page link 7 Check that Length unit Y Meters is selected and 8 Choose the Create DesignModeler geometry task New Sketch To start the sketching select the Sketchin
33. g tab To draw in the 2D sketch of the duct flow path Figure 4 1 Select the Sketching tab and 2 Click on the Z coordinate of the triad in the lower right corner of the Model View Select the Draw toolbox and 1 With the Arc by Center tool sketch the inner wall bend shape a Place the cursor over the origin watch for the P constraint symbol and left mouse button click b Move the cursor to the left along the X axis With the C constraint visible click the left mouse button to put the start point of the arc on the X axis c Sweep the cursor clockwise until the C constraint appears at the Y axis Click the left mouse button 4 Switch to the Dimensions toolbox to size the inner wall bend radius a Select the Radius tool b Select a point on the arc Then move the cursor to the inside of the arc near the origin Click to complete a dimension which is labelled R1 c In the Details View notice that R1 is shown under the Di mensions title d Change the value of R1 to 0 025 m Notice the arc radius changes automatically Switch back to the Draw toolbox to sketch the inner entrance wall Notice that the drawing instruction steps are provided in the lower left corner August 5 2004 9 1 Geometry and Mesh Generation 35 a With the Line tool selected place the cursor in
34. he faces where two solids join can be thin surfaces Thin Surface A thin solid body in a flow like a guide vane or baffle can be modelled as an infinitely thin surface with no slip walls on both sides August 5 2004 5 2 Geometry Creation 11 Units To keep things simple and to minimize errors use metric units throughout Advanced Concepts See the Geometry section of the CF X Mesh 2 1 Help for further information on geometry modelling requirements To de velop improved skill follow the tutorials given in CFX Mesh 2 1 Help Tutorials 5 2 Geometry Creation The basic procedure for creating a three dimensional solid geometry is to make a 2D sketch of an enclosed area possibly with holes on a flat plane The resulting 2D sketch is a profile which is swept through space to create a 3D solid feature This process can be repeated to either remove portions of the 3D solid or to add portions to the solid Each sketch is made on a Plane e There are three default planes XYPlane XZPlane and YZPlane which coincide with the three planes of the Cartesian coordinate sys tem e Each plane has a local X Y coordinate system and normal vector the plane s local Z axis e New planes can be defined based on existing planes faces point and edge point and normal direction three points origin local X axis and another point in plane and coordinates of the origin and normal and e Plane transforms such as translations and rotation
35. hese notes present a basic methodology for developing simple geometries for tetrahedral meshes CAD2Mesh To develop expertise for more complex geometries work through the sequence of tutorials in given in CFX Mesh 2 1 Help 5 1 Basic Concepts and Definitions Vertex Occupies a point in space Often other geometric entities like edges connect at vertices Edge A curve in space An open edge has beginning and end vertices at distinct points in space A straight line segment is an open edge A closed edge has beginning and end vertices at the same point is space A circle is a closed edge Face An enclosed surface The surface area inside a circle is a planar face and the outer shell of a sphere is a non planar face An open face has all of its edges at different locations in space A rectangle makes an open face A closed face has two edges at the same location in space The cylindrical surface of a pipe is a closed face Solid The basic unit of three dimensional geometry modelling e is a space completely enclosed in three dimensions by a set of faces volume e the surface faces of the solid are the the external surface of the flow domain and e holes in the solid represent physical solid bodies in the flow do main such as airfoils etc Part One or more solids that form a flow domain Multiple Solids May be used in each part e the solid volumes cannot overlap e the solids must join at common surfaces or faces and e t
36. holding down the left button and the lt ctrl gt key simultaneously 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 using buttons at the top of the Viewer window Orthographic projection will work best for two dimensional views Notice that the Wireframe object is listed in the Object Selector panel on the right Click the selection box O Wireframe to remove the wire frame view Turn the view back on and use Z and Orthographic for the view 7 Now look at some results this is what makes CFD colourful Each visualization is created by defining a new visualization object and then editing the properties of the object To make a vector plot choose Cre ate Vector from the top menu row or use the corresponding button on the second row In the New Vector panel accept Name Vector 1 and click OK to open a Vector edit panel In the edit panel set e Locations Y Symmet1 e Variable Y Velocity Hybrid on e Projection Y None e U Visibility on and click Apply The vector plot should appear in the Viewer window and the vector object should be listed in the Object Selector panel In the Object Selector list turn O0 DEFAULT LEGEND of
37. hybrid value and August 5 2004 8 1 Objects 29 the second value is the conservative value For a node on a solid wall the hybrid velocity will be zero and the conservative velocity will typically be non zero Common flow visualization objects include Wireframe a singleton object automatically created to show the surface mesh on the flow domain The edge angle controls how much of the surface mesh is drawn An element edge is drawn if the angle between the two adjacent element faces is bigger than the edge angle For rectangular geometries and meshes an edge angle of 30 ensures that the outside edges of the domain are drawn and an edge angle of 0 ensures that all mesh edges are drawn Contour lines of a constant scalar value on a specified surface like el evation lines on a topological map The appearance of the plot is controlled by the rendering options For example with the smooth shading option turned on a fringe plot areas between contour lines are filled with colour is drawn and with no shading only the contour lines are drawn Vector the field of vectors in a region Streamline lines that are parallel to the local velocity vectors Typically streamlines are started on an inlet surface and will extend to an outlet surface and Particle Tracks If particle paths an advanced feature were calculated as part of the simulation then these paths can be drawn Again see the on line help for further information
38. ic Pressure x Relative Pressure 0 Y Pa and then click Ok to close the panel and create the new boundary object August 5 2004 9 2 Pre processing 43 e Click the Boundary Condition button to open the definition panel Fill in Name FRONT SURFACE and click OK In the Edit Bound ary Front Surface in Domain Ductbend panel under the x Boundary Type Y x Location Y and then click object Basic Settings tab set Symmetry and front surface Ok to close the panel and create the new boundary e Click the Boundary Condition button to open the definition panel Fill in Name BACK SURFACE and click OK In the Edit Bound ary Back surface in Domain Ductbend panel under the Basic Settings tab set x Boundary Type Y Symmetry and x Location Y back surface and then click object e Click the Fill in Name Boundary Condition INNER WALL Ok to close the panel and create the new boundary button to open the definition panel Inner wall in Domain Ductbend panel under the x Boundary Type Y x Location Y and under the Basic Settings tab set W
39. ing operations on scalar fields and Functions CEL functions for conventional mathematical operations and Tools Variables allows the calculation of new field variables New vari ables can be defined in terms of defined expressions or by entering new expressions directly into the variable definition For example cal culating local pressure coefficients based on the maximum speed in the domain backstep i e Cp eee max can be done with the expression Pressure 0 Pa 0 5 Density maxVal sqrt Velocity u 2 Velocity v 2 backstep 2 for a variable called Cp 8 3 Controls A set of controls can be used to save a particular post processing setup or state so that it can be repeated This is useful when comparing different simulations to see the impact of a design change on flow properties Common controls include Camera The image size orientation perspective etc in the Viewer win dow is associated with a Camera viewer imagine that the user moves a camera around in space to create a two dimensional image of a three dimensional object in the camera s viewer Camera views can be August 5 2004 8 3 Controls 31 saved deleted and refreshed with the buttons on the right top of the Viewer window Note that the camera does not determine which graphics objects are visible Session Provides a mechanism for recording and saving a series of opera tions This is useful for users interested in learning power progra
40. mming of CFX Post see on line help for more information State Saves all information views graphical objects expressions and vari able in a cst state file Loading a state file will automatically re generate all views objects expressions and variables from a previous CFX Post session August 5 2004 9 Commands for Duct Bend Example 32 9 Commands for Duct Bend Example As indicated in Section 1 the following conventions will be used to indicate the various commands that should be invoked Menu Sub Menu Sub Sub Menu Item chosen from the menu hierarchy at the top of a main panel or window Button Tab Command Option activated by clicking on a button or tab Link description Click on the description to move by a link to the next step page Name value Enter the value in the named box Name Y selection Choose the selection s from the named list Name Panel or window name Name On off switch box and Name On off switch circle radio button 9 1 Geometry and Mesh Generation The first phase uses CFX CAD2Mesh The commands listed below will accomplish the following steps e Create a solid body geometry that will represent the flow domain set up the project directory and page set up a plane for sketching on sketch in the flow path through the duct Figure 4 generate the solid body geometry e Generate a discrete mesh in the
41. n to open the panel Under the Basic Settings Advection Scheme Option Y Upwind Timescale Control Y Physical Timescale tab set 7 7 Physical Timescale 0 041 Y s 30 of the average residence time of a fluid parcel inside the flow domain Max No Iterations 50 Residual Type Y MAX Residual Target 1 0e 3 and then click Ok Click the Output Control button to open the panel To the right of the empty Results list box click the New button Choose Name Option Y Results 1 and click OK For Results 1 set Full 7 O Output Variable Operators on and choose Y All and O Output Boundary Flows on and choose Y All and then click Ok August 5 2004 0 3 Solver Manager 45 e Click the Write Definition File to open the panel Accept the de fault filename ductbend def and then click OK You should save changes when requested 0 3 Solver Manager The CFX 5 Solver Manager window will open after CFX Pre closes In the Define Run panel set e Definition File ductbend def NOTE If restarting a partially con verged run you would enter the name of the most current results file
42. nces 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 domain A standard boundary condition object includes a name a type a set of surfaces and a set of parameter values In CFX Post the boundary condition object name is used to refer to the set of surfaces on which the boundary condition is applied For this reason the habit of naming each boundary condition by the name of its surfaces as defined in CFX Build will be followed Boundary condition types include Inlet an inlet region is a surface over which mass enters the flow domain For each element face on an inlet region one of the following must be specified e fluid speed and direction either normal to the inflow face or in a particular direction in Cartesian coordinates August 5 2004 6 1 Domain 20 e mass flow rate and flow direction or e the total pressure 1 z2 Pota P rad Protal spec 2 and flow direction If the flow is turbulent then it is necessary to specify two properties of the turbulence Most commonly the intensity of the turbulence r Average of speed fluctuations Mean speed 3 and one additional property of the turbulence the length scale of the turbulence a representative average size of the turbulent eddies or eddy viscosity
43. nd Sketch Toolbox on the left a Details View at the bottom left and a Model View window Place the mouse cursor over one of the command buttons in the top row A brief de scription of the button s action should appear you may need to click in the ANSYS Workbench once to make it active Visit each button with the mouse cursor to see its action 4 One method of controlling the view is with the coordinate system triad in the lower right corner of Model View Click on the Z axis of the triad to see an end view of the cylinder Click on the cyan sphere to select the isometric view 5 Another method of controlling the view is with the mouse left button in conjunction with a mouse action selection From the upper row of buttons select the Pan action Holding the left mouse button down drag the mouse over the Model View to translate the view Select the Zoom action and repeat the mouse action to change the size of the view 6 Select the Rotate action Notice that a roll circle appear in the Model View With the mouse cursor outside the roll circle press and hold the left mouse button to rotate the view about an axis perpendic ular to the Model View window Repeat with the mouse cursor within the roll circle to rotate about the centre of the roll circle Move the cursor to either the left or right rectangle on the roll circle and hold the left button to rotate about the horizontal axis of the Model Vi
44. nd close windows and to manage disk space usage with tools like WinZip 2 2 Introduction to CAD2Mesh CAD2Mesh has two major components e DesignModeler for modelling geometry and e CFX Mesh for creating a mesh within the ANSYS Workbench environment To get a feel for the interface we will use DesignModeler to view a cylinder which has already been created 1 Use a web browser to visit the ME566 homepage www eng uwaterloo ca me566 Click on the link to CAD2Mesh intro file Save the downloaded file as cylinder agdb in your directory 2 Open ANSYS Workbench from Start Programs Engineering ANSYS CFX CAD2Mesh 2 1 To open the existing geometry project a Follow the Import a DesignModeler database link b Use the Browse below the Select an existing DesignModeler database panel area to find and select the file cylinder agdb c Set Name CAD2Mesh intro If you get an error message from the License Manager click OK and if the program shuts down restart it August 5 2004 2 2 Introduction to CAD2Mesh 3 d Set Location N using Browse if necessary e g Set O Create a subfolder based on project name on Follow the Open the Project Page link and No zad AR Re Choose the Open this geometry task 3 There are four major areas on the screen command menus and but tons at the top a Tree View a
45. nt shape in this region is a triangular prism Figure 3 based on the surface triangular mesh The basic shape of the prism element is independent of the height of the prism mesh length scale normal to the wall The layer of prism elements is an inflated boundary with Maximum Thickness that is often approximately the same as the default body mesh spacing or First Layer Thickness that is often set by the properties of the local turbulent boundary layer Other properties include Number of Inflated Layers specifies the number of prism elements across the thickness of the inflated layer and Expansion Factor specifies how the prism height increases with each inflated layer above the wall surface This factor must be between 1 05 and 1 35 The final inflated mesh cannot be previewed in CAD2Mesh its a vol ume mesh and CAD2Mesh only has facilities for previewing surface meshes However the overall thickness of the inflated boundary can be previewed CFX Pre and CF X Post have facilities for viewing the mesh in the inflated layers Stretch The default body mesh length scale is isotropic The vertex angles in the isotropic tetrahedral elements are close to 60 In geometries that are not roughly square in extent it may be desirable make the mesh length scale longer or shorter in one particular direction This is achieved by stretching the geometry in a given direction meshing the modified geometry with an isotropic mesh and then
46. on Selector panel The Region Ed itor panel will open up where you set x Name back surface assuming that this is the missing sur face x Combination Y Alias x Dimension Y 2D x Select Default 2D Region to put into the Included Regions panel list and Click OK to close panel e Click the Steady State Transient button to open the Simulation Type panel Check that Option Y Steady State and then click Ok e Click Tools Material Editor to open up a edit panel in which a new material can be defined In the Material Editor panel fill in Name Water nominal Thermodynamic State Y Liquid Composition Y Pure August 5 2004 9 2 Pre processing Al Template Y Water at 25C expand Equation of State Fluid Type Y General Fluid Density 1000 Y Kam 3 expand Transport Properties Dynamic Viscosity 0 001 VW kgm 1s7 1 and then click Ok s Click the Domains button A small Define Domain panel will open Fill in Name Ductbend and click OK to define the domain object An Edit Domain Ductbend panel will open with several tabbed sub panels On the
47. on each of these objects and their generation Common Viewer augmentation objects include Legend a scale legend to associate property values with colours Text a piece of text Clip Plane a flat plane that is used to reduce the portion of the domain shown in the viewer and Instancing Transformation for repeating views to show full geometries when periodic or symmetry conditions have been used to model a portion of a domain August 5 2004 8 2 Tools 30 8 2 Tools Common post processing tools include File Export allows the export of portions of the results in a space separated tabular form This is useful for exporting data to programs like Excel for graphing etc File Print allows the export and printing of the graphical image in the viewer window Images saved in encapsulated postscript eps or portable network graphics png formats can be easily inserted into reports and presentations Tools Calculator is a powerful tool for carrying out a wide range of math ematical calculations including a range of integration and averaging calculations Tools Expressions allows the input of CEL expressions To learn the ex pression syntax right mouse click in the Definition box within the Expression Editor panel to get a list of possible variables expressions locators functions and constants that can be input into a new expres sion There are two type of functions Functions CFX Post func tions for perform
48. ovides guidance on the possible parameter settings August 5 2004 6 1 Domain 19 6 1 Domain 6 1 1 Fluids List A fluid or mixture of fluids in more complex multi phase flows has to be associated with each domain The fluid for a particular domain can be selected from the fluid library which has many common fluids The Tools Material Editor provides a mechanism for defining new fluids The properties of Fluid Type Y General Fluid can be general functions of temperature and pressure for liquids or gases The CFX Expression Language CEL is used to input formulae for specifying equations of state and other applications such as boundary variable profiles initialization and post processing CEL allows expressions with standard arithmetic oper ators mathematical functions standard CFX variables and user defined variables All values must have consistent units and variables in CEL ex pressions must result in consistent units Full details of CEL including the names of the standard CFX variables are included in the CFX 5 Reference Material Guide For low speed flows of gases and liquids it is adequate to use constant property fluids It is easiest to build up a new constant property fluid by using a fluid from the existing CFX fluid library as a template For example to define constant property air at 20 C air at 25 C is a good template 6 1 2 Boundary Throughout each domain mass and momentum conservation bala
49. raint tool to join this line to the end of the outer bend wall Use the Equal Length constraint tool to make the outer exit wall the same length as the inner exit wall and Draw a line from the end of the outer exit wall to the end of the inner exit wall to form the outflow edge Make sure that the end points are coincident P e Return to the Project page by clicking on the The sketch should now be an enclosed contour on the XYPlane e To create the three dimensional solid body 1 Switch to the Tree View by selecting the Modeling tab 2 Click on the Extrude button to create the Extrudel feature the Details View Check Base Object Sketch1 Select Operation Y Add Material Select Direction Y Normal Set FD1 Depth gt 0 0 0075 Select As Thin Surface Y No and Select Merge Topology Y Yes 3 Click on the Generate button to create a Solid Use the In iso metric view in the Model View to check that you have a three dimensional solid grey body Ductbend Project tab at the top left corner of the window August 5 2004 9 1 Geometry and Mesh Generation 37 9 1 2 Region Identification e Choose the Generate CFX Mesh DesignModeler Task Notice the three primary areas Model View Tree View and Details
50. rete 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 The iterative algorithm will stop when either the maximum number of iterations is reached or when the convergence criterion is reached whichever occurs first The convergence criterion is a convergence goal for either the maximum normalized residuals or the root mean square RMS of the normalized residuals Note that the residuals are normalized to have values near one at the start of the iterative process August 5 2004 7 CFX Solver Manager Solver Operation 7 CFX Solver Manager Solver Operation A solver run requires a definition file to define and initialize the run Ta ble 2 shows how def and res files can be used to define different runs Definition File Initial File Use def Start from simple initial fields res Continue solution for further convergence def res Restart from existing solution with new flow model Table 2 Input file combinations 7 1 Monitoring the Solver Run The solution of the algebraic equation set is the component of the code oper ation which takes the most computer time Fortunately because it operates in a batch mode it does not take much of the user s tim
51. returning the geometry along with the mesh to its original size This means that if the y direction is stretched by a factor of 0 25 without stretching in the other two directions then the mesh size in the y direction will be roughly 4 times that of the other directions Take care to ensure that the resulting tetrahedral elements do not get too squashed For this reason the stretch factors should be between 0 2 and 5 at the very most more moderate stretch factors are desirable Proximity flags set the behaviour of the mesh spacing when edges and surfaces become close together For simple rectangular geometries set Edge Proximity Y No and Surface Proximity Y No August 5 2004 5 3 Mesh Generation 17 Options are used for setting the output filename and for setting the algo rithms used for generating the volume and surface meshes Volume Meshing Y Advancing Front is the only algorithm allowed with this version of CFX Mesh The algorithm starts with a surface mesh and then builds a layer of tetrahedral elements over the surface based on the surface triangular elements This creates a new surface The process is repeated advancing the layers of tetrahedral elements into the interior of the volume Surface Meshing Y Delaunay is a fast algorithm for creating isotropic surface meshes Suited to complex surface geometries with small mesh spacing Surface Meshing Y Advan
52. s can be used to modify the base definition of the plane The creation of a sketch is similar to the creation of a drawing with modern computer drawing software e A sketch is a set of edges on a plane A plane can contain more than one sketch e The sketching toolbox contains tools for drawing a variety of common two dimensional shapes e Dimensions are used to set the lengths and angles of edges e Constraints are used to control how points and shapes are related in a sketch Common constraints include August 5 2004 5 2 Geometry Creation 12 Coincident C The selected point or end of edge is coincident with another shape For example the end point of a new line segment can be constrained to lie on the line extending from an existing line segment Note that the two line segments need not touch Coincident Point P The selected points are coincident in space Vertical V The line is parallel to the local plane s Y axis Horizontal H The line is parallel to the local plane s X axis Tangent T The line or arc is locally tangent to the existing line or arc Perpendicular L The line is perpendicular to the existing line and Parallel The line is parallel to the existing line As a sketch is drawn the symbols for each relevant constraint will appear If the mouse button is clicked while a constraint symbol is on the sketch then the constraint will be applied Note that near the X and Y axes it i
53. s often difficult to distinguish between coincident and coincident point constraints and e Auto Constraints are used to automatically connect points and edges For example if one edge of a square is increased in length the opposite edge length is also increased so that the shape remains rectangular Features are created from sketches by one of the following operations Extrude Sweep the sketch in a particular direction i e to make a bar Revolve Sweep the sketch through a revolution about a particular axis of rotation i e to make a wedge shape Sweep Sweep the sketch along a sketched path i e to make a curved bar and Skin Loft Join up a series of sketches or profiles to form the 3D feature like putting a skin over the frame of a wing Features are integrated into the existing active solid with one of the following Boolean operations Add Material Merge the new feature with the active solid Cut Material Remove the material of the new feature from the active solid Slice Material Remove a section from an active solid and August 5 2004 5 3 Mesh Generation 13 Imprint Face Break a face into two parts For example this will open a hole on a cylindrical pipe wall Sometimes it is necessary to use multiple solids in a single part These solids must share at least one common face This common face might be used to model a thin surface in the flow solver In this case 1 Select active solid with the body s
54. the panel set Locations Y FRONT SURFACE August 5 2004 9 4 Post processing 47 Variable Y Vorticity Hybrid on O Visibility on and click Apply The fringe plot should appear in the Viewer window Choose Create Line accept Name a line object and open an edit panel Use Method Y Line 1 and click OK to define Two Points set Point 1 to 0 0 0 1 0 0 set Point 2 to 0 0 0 025 0 0 set the number of samples to 25 and click Apply to see the line make sure that the visibility of the contour plot etc is turned off Choose File Export to open the Export panel where you can set File uvelocity dat select Ctrl key plus click Velocity u X and Y from the Select Variable s list select Line 1 from the Locator list and click Save and Cancel to write the data to a file in a space separated format that can be imported into a conventional spread sheet program for plotting or further analysis Choose Create Plane accept Name fine a plane object and open an edit panel Use Method Plane 1 and click OK to de XY Plane with Z Ulm and click Apply set O Visibility off to avoid clutter in the view Choose Create Polyline ac
55. the lower left quadrant of the XY plane near the arc Click the left mouse button Move the mouse cursor down to create a vertical line Look for the V constraint symbol and click the left mouse button b Switch to the Dimensions toolbox to size the inner entrance wall length i Select the General tool ii Select a point near the centre of the line Then move the cursor to the right Click to complete a dimension which is labelled V2 iii In the Details View change the value of V2 to 0 10 m c To join the inner entrance wall and the inner wall bend switch to the Constraints toolbox i Select the Coincident tool ii Select the upper end of the entrance inner wall with a left mouse button click The square end marker should be yellow iii Select the square end marker of the arc that lies on the X axis with a left mouse button click The inner entrance wall should join the inner wall bend 4 To draw a line across the inflow entrance a Use the Line tool in the Draw toolbox b Place the cursor over the bottom end point of the entrance inner wall and notice that a P constraint symbol appears Left mouse button click to select this point and then move the cursor to the left and click while the H constraint symbol is visible c Use the General tool in the Dimensions toolbox d Select
56. the plane shown in Figure 2 the geometry model must have a width into the page A thin slice is August 5 2004 4 The CFD Model Specification 8 F 0 10m Figure 2 Geometry of the duct bend model used The width of the slice is set so that the slice is spanned by one discrete element see Section 5 3 2 For the present study the width of the slice is 0 0075 m the value used for the Default Body Mesh Spacing Specification of Simulation Type For this application a steady flow is appropriate Specification of Fluid Properties For this application the fluid is water which can be treated as a simple liquid with nominal constant prop erties p 1000 kgm 3 u 0 001 Pa x s Specification of Flow Models For this analysis it is reasonable to as sume the following flow features e 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 lt model OU OU 1 Tig m Fe 5 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 scales of the turbulent eddying motion are es timated from two field variables which are calculated as part of the model k the turbulent kinetic energy and lt the rate at which k is dissipated by molecular viscous action August 5 2004 4 The
57. tion k Epsilon the accepted state of the art turbulence model involves the solution of two transport equations Shear Stress Transport a variant of the k Epsilon model that pro vides a higher resolution solution in near wall regions and SSG Reynolds Stress a second moment closure model that explic itly solves transport equations for all six components of the turbu lent stress tensor and that requires significantly more computing resources than the two equation variations Turbulent Wall Functions are required to treat the transition to lami nar flow close to solid walls The wall treatments are tied to the turbu lence model choice The scalable wall function method used with the k Epsilon turbulence model is a variant of the standard wall function method The scalable wall function method automatically adjusts the near wall treatment with mesh pacing in the near wall region 6 2 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 For a turbulent flow sufficient information must be provided so that the following field values can be set initialized at each mesh point e velocity vector 3 components e fluid pressure August 5 2004 6 3 Output Control 23 e turbulent kinetic energy an
58. wall can be either smooth or rough Depending upon which August 5 2004 6 1 Domain 21 Inlet Sets Outlet 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 Common boundary condition combinations of these options are chosen suitable values must be input i e the size of the roughness elements etc Symmetry a region with no mass flow through the faces and with negligi ble shear stresses and negligible 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 advan tage of natural symmetry planes in the flow domain Since it is crucial that each surface element face have a boundary con dition attached to it CFX Pre automatically provides a default boundary condition for each domain Once all boundary surfaces have been attached to explicit boundary conditions the default boundary condition object is deleted This allows the user to identify surfaces which still require explict boundary conditions For the flow solver to successfully provide a simulated flow field the spec ified boundary conditions should be realizable i e they should correspond to conditions in a laboratory setup In particular ensure that the inlet and outlet boundar
59. within the domain Line a straight line between two points on the line Intermediate points on the line can be set at intesections with mesh faces referred to as a cut line or uniformly spread along the line referred to as a sample line Plane a flat plane in the domain Like a line object a plane object can be either a cut plane or a sample plane Isosurface the surface along which some scalar field property has a con stant value Polyline a piecewise continuous straight line between a series of points The line can be derived from the intersection of a boundary and an other geometrical object or as a contour line and User Surface a surface derived in a manner similar to a polyline object The definition of each of these objects involves choosing from a range of options for each atttribute including location line colour etc The object edit panels outline the possible options for each attribute of the object See the on line help for further information on each of these objects and their generation Flow visualization objects allows exploration of the velocity pressure temperature fields For each flow property calculated at a node there are two fields hybrid and conservative At interior nodes the two fields are identical For nodes on the wall there are two values for every flow property the value implied by the boundary condition and the average value in the sub element region around the node The first value is the
60. y conditions are consistent and that they take advantage of the known information Table 1 lists several common inlet outlet condi tion combinations along with the global flow quantity which is estimated as part of the solution for each combination 6 1 3 Domain Models In incompressible flow fields the actual pressure level does not play any role in establishing the flow field it is pressure differences which are important The solver calculates these pressure differences with respect to a reference pressure Solution fields are in relative pressure terms but absolute pres sure relative pressure plus reference pressure is used for equation of state calculations In turbomachinery applications it is convenient to analyse the flow in a rotating reference frame In this case the domain is in a rotating reference frame and its axis of rotation and rotation rate must be specified August 5 2004 6 2 Initialization 22 6 1 4 Fluid Models Heat Transfer Model options include None no temperature field is computed not an applicable option for ideal gases Isothermal a constant temperature field is used Thermal Energy a low speed neglecting kinetic energy effects form of the enthalpy conservation equation is computed to provide a temperature field and Total Energy a high speed form for conservation of energy including kinetic energy effects is computed Turbulence Model options include None laminar flow simula
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