Appendix B CFX-Mesh

1. Appendix BCFX-Mesh ANSYS MeshingApplication Introduction

2. Introduction • CFX-Mesh provides automated mesh generation • Unstructured triangular surface mesh generation • Volume mesh is created from the surface mesh • Tetrahedral/prismatic/pyramidal volume mesh generation • Extruded meshes can also be created • If quad faces exist on the extruded face due to inflation, hex elements will be created for those quad elements • Advancing Front and Inflation (AFI)

3. Element Types • Valid element types for the CFX-Solver include tetrahedra, prisms (wedges), pyramids, and hexes

4. Accessing CFX Mesh • Launch ANSYS Workbench • Double click on Mesh under component systems. • This will create a ‘Mesh component’ in the Project Schematic area. • Right click on select “Import Geometry” and click on “Browse…” to load a previously created geometry or click “New Geometry” to open DesignModeler and create a new geometry. • Once the geometry is loaded double click on to open the meshing application.

5. Accessing CFX Mesh • In the Project environment go to Tools > Options… • Select “Geometry Import” • Select Analysis Type as “3D” • Ensure “Solid Bodies” is chosen • Only solid bodies are relevant to CFX Mesh • Check the “Named Selection” box to get Design Modeler defined named selections • Set the filtering prefix (keep the “Filtering Prefixes” box blank to import all named selections regardless of prefix)

6. Accessing CFX Mesh • Right-click on Mesh and Insert Method • Select the Body of interest • Now edit the method and choose CFX Mesh • Then right Click on resulting CFX-Mesh Method and Choose Edit in CFX-Mesh

7. Accessing CFX Mesh • Note that the meshing environment is now modified

8. Geometry Requirements • Geometry used for meshing in CFX-Mesh must consist of one or more Solid Bodies • In CFX-Mesh, the body will have the units specified in DM • Surface Bodies and Line Bodies are not supported in CFX-Mesh • On import of certain file formats, Workbench will convert sets of surfaces which fully enclose to a volume into Solid Bodies (see DM documentation for details) • Solid Bodies must not overlap each other • Where Solid Bodies in a multi-body part touch, they must have common faces • Bodies which are Frozen in DM will appear in CFX-Mesh and can be meshed • To exclude a Solid Body from meshing, you can either suppress/delete it in DM, or suppress it in CFX-Mesh

9. Geometry Requirements • Example 1: • When Solid Bodies in a multi-body part touch, they must have common faces • If two bodies contact as shown, the face at the end of pipe is not one of the faces of the cylinder, CFX-Mesh will fail in generating mesh if the two bodies are in a single part • How to meet CFX-Mesh topology requirements? wrong

10. Geometry Requirements • To make a common face between pipe and cylinder, the cylinder needs to have the cylinder face that touches the pipe split into two: one face is the circular face which matches the end of the pipe, the other face is the remaining annular ring. • This can be done with an Imprint Face body operation in DM. right

11. Geometry Requirements • Example 2: • When part of pipe intrudes into the cylinder, part of the side surface is external to the cylinder, while the other part is internal to it. • What should the two bodies look like if they are in a single part?

12. Geometry Requirements • Again, the rule to remember is that adjacent solid bodies in a multi-body part must meet at shared faces • If two bodies contact as shown, where the side surface of the pipe is a single face, it does not match up with cylindrical cut-out in the bottom cylinder and CFX-Mesh will fail in generating mesh. Single Side Surface CylindricalCut-Out wrong

13. Geometry Requirements • To make sure the pipe and cylinder meet at a shared face, the pipe needs to have its side face segmented into the part that’s external to the bottom cylinder and the part that’s internal to it (via an Imprint Face body operation in DM). External Side Surface Internal Side Surface CylindricalCut-Out right

14. Other Geometry Requirements • The CFX-Mesh Help provides many useful examples of what can and cannot be handled in CFX-Mesh, and some ways around difficult geometries, including: • Bodies Joined by a Common Face • Bodies Touching at a Face • Body with a Hole • Body with an Enclosed Body • Bodies with an Enclosed Body and a Hole • Body with an Enclosed Body Touching the Face • Non-Manifold Geometry • Closed Faces (ie. Cylinders) • Thin Surface Topology • Poorly Parameterized Surfaces • Degenerate Geometry

15. Geometry Update • Geometry Update allows for quick modification of geometry and mesh regeneration. • Updates geometry while retaining most or all CFX-Mesh settings. • Updates from CAD systems in plug-in mode is faster and cleaner. • If importing in reader mode, then you must save the new geometry into the appropriate file before updating. • Most geometry updates work with the minimum required modification to your mesh settings • Depends on the complexity of the changes made to the geometry, the CAD format and the method of import • Look at status symbols on each entity at the end of the update. If there are problems, you should correct them before regenerating the mesh. or Right-click on Geometry in Tree View Status Symbols Error OK

16. Geometry Checking • Geometry Checking checks for the presence of certain undesirable features in faces and edges which can cause poor mesh quality or failure of the mesher. • Can be accessed from the Go menu, by right-clicking on Geometry in Tree View, or in the top right toolbar. • Note: the “lock” symbol means the item must remain. • Results of check can be viewed under Errors in Tree View • If a Warning or Error refers to a specific curve or face, it will be highlighted when selected • Last warning gives a summary of the checks

17. Geometry Checking • Failed check does not necessarily result in poor mesh • Worth checking the mesh on any faces which failed the checks • Doesn’t check for ALL problems which can be present, just a few specific problems: • Sliver Edge Checking • Sliver Face Checking • Parameterization Face Checking • Verify Options determine values which generate errors

18. Sliver Edge Checking • Looks for short edges in the geometry • Short edges can produce a mesh which is over-refined in regions near the short edges • To remove short edges, change the default from NO to YES for Remove Short Edges in Fix Options • You can change the tolerance used for the check by using Verify Options

19. Sliver Face Checking • Computes a ratio of perimeter length to area for each face • Faces with a high sliver factor can result in a poor quality surface mesh • You can change the limit used for the check by using Verify Options • Default of 25 is usually sensible • Each face identified will be highlighted when the individual warning message is selected

20. Parameterization Face Checking • Provides guidance on the parameterization of the surfaces • Each potentially poorly parameterized surface will be highlighted when the individual warning message is selected

21. Virtual Topology • By Default CFX-Mesh resolves every edge using a minimum of 3 vertices and meshes every face • Results is large mesh when there are many short edges and narrow faces in the CAD data • May not require a fine mesh in these areas for CFD • If proper mesh controls are not used in these areas, the resulting mesh may be of poor quality or the mesher might fail • Virtual Topology allows users to combine faces and edges into Virtual Faces and Virtual Edges. Can be addedin CFX-Mesh or the Meshing Application. 78 Surfaces represent car body

22. A single virtual surface Virtual Topology • CFX-Mesh only sees the combined Virtual Face or Virtual Edge • Mesher does not resolve the constituent faces or edges, giving higher quality mesh with the required refinement • Does NOT modify the underlying CAD • Virtual topology will be discussed in more detail later

23. Geometry Display • To change the appearance of your geometry, left-click on Geometry in the Tree View • Transparency (%) • 100% means completely transparent • 0% means completely opaque • Shine (%) • Controls how much light is reflected by the faces of the mesh • 0% gives lowest reflection and looks matt • 100% gives highest reflection and looks very bright Transparency can be very useful for selecting hidden surfaces since there isn’t a wireframe view

24. Composite 2D Regions • You can use 2D Regions to give meaningful names to parts of the geometry which may comprise many faces • Composite Regions can be used for: • Specifying Locations in CFX-Mesh • Defining Boundary Conditions in CFX-Pre • Default and additional Regions available • To create a new Region, right-click on Regions and choose Insert>Composite 2D Region • No primitive 2D Region can be assigned to more than one Composite 2D Region

25. Composite 2D Regions • Named Selections are imported from DM as Composite Regions • Select Named Selection under Default Geometry Options on the Project tab before proceeding to CFX-Mesh. • Can be set as default option in the Geometry Import options panel • Composite 2D Regions can be hidden!! • Removes the constituent faces from the viewer • Hidden faces cannot be selected

26. 2D Regions and Faces • Where two Solid Bodies meet at a common face: • There is just one face present in the geometry • There are two 2D Regions • Each meshing feature that requires you to specify a location has its own rules about 2D Regions on the same face • Ex. Face Spacing: Do not try to apply different Face Spacings to 2D Regions which are the two sides of a common face (surface mesh is generated on the common face, not 2D Regions) • Ex. Inflated Boundary: You can have different settings for the two different 2D Regions which make up a common face

27. 2D Regions and Faces • Use attached Selection Rectangles to select individual 2D Regions easily and accurately • CFX-Mesh will not allow you to select locations for meshing features which break the rules given for each feature

28. Saving the Volume Mesh • Two formats • .CMDB file • Contains mesh and mesh settings • Larger file which takes longer to generate for large meshes • .GTM file • Suitable for import directly into CFX-PRE • Access Options from the Tools Menu • The right panel will show various CFX options.

29. Saving the Volume Mesh • You may choose to write .cmdb or .gtm or both • User Defined location for .gtm will produce a dialog box to save choose a location when you Generate the Volume Mesh

30. Length Scales • The process of setting an element length scale for CFX-Mesh can be viewed as a 3 step process • Default Body Spacing • Face Spacing • Other Mesh Controls (Point Spacing, Periodicity, Inflation, etc) • Smallest effecting length scale is chosen

31. Face Spacing • Face Spacing can be set to one of 4 types: • Angular resolution - curvature sensitive, discussed next • Relative error - curvature sensitive, discussed next • Constant - constant length scale, overriding the Body Spacing (must be less than Default Body Spacing) • Volume Spacing - uses the same spacing on the face as the Body Maximum Spacing

32. Face Spacing • Face spacings have a volumetric effect. The region over which theyact are determined by the following settings: • Radius of Influence: extent of the Face Spacing influence, after which it will expand according to the Expansion Factor • Expansion Factor: rate of expansion of mesh scale from surface to interior • Location: Faces where the Face Spacing values will be applied • Can be selected from the Model View or Tree View Regions • Unnecessary for Default Face Spacing

33. Curvature Sensitive Mesh • Angular Resolution [Degrees] • CFX-Mesh chooses edge length such that the set angle is subtended at the center of circle with radius equal to smallest radius of curvature • Default is 30 degrees (recommended 5 to 60 degrees) • Relative Error [Δr/r] • Deviation of mesh from surface as a fraction of local radius of curvature • Minimum edge length - lower bound on length scale • Maximum edge length - upper bound on length scale (default same as volume background scale)

34. Without surface curvature sensitive meshing Curvature Sensitive Mesh With surface curvature sensitive meshing

35. Edge Spacing • Edge Spacing specifies the mesh length scale on an edge (or edges) and in the volume adjacent to the selected edges • To create a new Edge Spacing, right-click on Spacing and choose Insert>Edge Spacing • Parameters and effect on mesh are the same as with Face Spacing

36. Edge Spacing

37. Mesh Controls • Mesh Controls are used to refine the surface and volume mesh in specific regions of your model • Location can be defined using any point on the model or by specifying coordinates • Can be located anywhere in the 3D space of model (inside, outside or on the edge) • 3 types of volumetric Controls are available: • Point • Line • Triangle • Remember: Face Spacing also available for volumetric control

38. Point Spacing • Each of the 3 volumetric Controls requires you to specify a Point Spacing for the control at appropriate points.Any number of mesh controls can reference the same point Spacing • Length Scale • For the mesh size where the Point Spacing is applied • Must be less than Body Spacing Max • Radius of Influence • Radial extent of the fixed local length scale influence • Expansion Factor • Geometric rate of increase of local element length scale beyond radius

39. Point Control • Point Control controls the mesh spacing in a spherical region • Point • Select either a vertex from the model or coordinates • Spacing • Select a Point Spacing which defines the attributes for the Point Control (Length Scale, Radius of Influence and Expansion Factor) • Figure to right shows a Point Control on a 1m cube with: • Length Scale=0.05m • Radius of Influence=0.2m • Expansion Factor=1.2

40. Line Control • Line Control controls the mesh spacing in a region defined by a cylindrical volume between 2 spheres • Point • Select either a vertex from the model or coordinates for both Points • Spacing Definitions • Uniform requires only one Spacing • Non Uniform requires a Spacing for each end • Spacing • Select a Point Spacing which defines the attributes for the Line Control (Length Scale, Radius of Influence and Expansion Factor) • Figure to right shows a Line Control on a 1m cube with: • Length Scale=0.05m • Radius of Influence=0.2m • Expansion Factor=1.2

41. Triangle Control • Triangle Control controls the mesh spacing in a region defined by a prismatic volume between 3 spheres • Point • Select either a vertex from the model or coordinates for all 3 Points • Spacing Definitions • Uniform requires only one Spacing • Non Uniform requires a Spacing for each corner of the triangle • Spacing • Select a Point Spacing which defines the attributes for the Triangle Control (Length Scale, Radius of Influence and Expansion Factor) • Figure to right shows a Triangle Control on a 1m cube with: • Length Scale=0.05m • Radius of Influence=0.2m • Expansion Factor=1.2

42. Periodicity • Using Periodicity allows you to generate identical meshes for faces that will be specified as part of a periodic boundary condition in ANSYS CFX • The CFX Solver makes more accurate calculations when mesheson periodic pairs are identical (one-to-one) • Periodicity can be either Translation by a fixed vector or Rotation • Rules/Limitations: • Each face in the Location 1 face list must map to an equivalent face in the Location 2 face list • Multiple faces can be selected for each of Location 1 and Location 2, provided each face in the Location 1 face list maps onto a face in the Location 2 face list using the specified transformation • Inflation cannot be applied to a face which is part of a Periodic Pair • See the documentation for further details on Periodicity

43. Periodic Pairs • Periodic Pairs create identical meshes on the 2 locations selected • Location • Select face(s) either directly from the Model View or select a Composite 2D Region from the Tree View • All faces selected must be on the external boundary of the model and must not be included in an Inflated Boundary • Periodic Type • Rotational requires 2 points to define an axis, and possibly an Angle of Rotation • Points can be either a vertex from the model or coordinates • Translational requires no further input

44. Inflation • Inflation is the generation of prismatic element layers by “inflating” triangular surface elements • Purpose: • Prism elements more effectively and efficiently captures boundary layer effects • Node density near the wall is increased • Velocity profile is captured by the prism layer • Tetrahedral elements efficiently fill the volume region

45. Inflation • You can control the number, thickness and expansion rate of inflation layers • You can inflate from any surface or boundary condition, except those included in a Periodic Pair • Inflation layers can be viewed within CFX-Mesh

46. Inflation • Number of Inflated Layers • If First Layer Thickness Option is used, this is a maximum number of layers • If Total Thickness Option is used, this is the actual number of layers (unless layers are removed to improve mesh quality) • Expansion Factor • Each layer, moving away from the face, is one Expansion Factor thicker than the previous. • Number of Spreading Iterations • Advanced quality control, see documentation for details • Minimum Internal Angle • Advanced quality control, see documentation for details • Inflation Option • Total Thickness • First Layer Thickness

47. Inflation Option - Total Thickness • Total Thickness • The total thickness of the inflation is controlled by the: • Thickness Multiplier • Local element edge length • Determined by Face Spacing and Controls • Maximum Thickness • Set individually for each Inflated Boundary • Creates a less smooth transition from the inflated prism mesh elements to the tetrahedral mesh elements • The number of inflated layers is more constant, and you have some control over height of layers on face-by face basis

48. Inflation Option - Total Thickness • Process used for creating the layers of prisms when using the Total Thickness option is given below: • CFX-Mesh calculates the total thickness of the inflation layers as follows: • Multiply the Thickness Multiplier by the local element edge length • Where this is less than the specified Maximum Thickness, then this gives the total thickness of the layers • Where this is greater than the specified maximum Thickness, then the Maximum Thickness is taken to be the total thickness of the layers • Use the specified Number of Inflated Layers and Expansion Factor to calculate the height of each layer, given the total thickness that has just been calculated • Inflation thickness will not be constant over the inflated edge if the element edge length changes in the region of the inflation layer

49. Inflation Option - First Layer Thickness • First Layer Thickness • Does not control the overall height of the inflation layers • Prisms based on First Prism Height or y+,Expansion Factor and Number of Inflated Layers • Creates smoother transition from inflated prism mesh elements to the tetrahedral mesh elements • First Prism Height must be less than the Max Spacing under Body Spacing • You should examine the mesh to visualize the extent of the inflation and the quality of the transition from prisms to tetrahedral elements

50. First Prism Height = Reference Length * (Desired) y+ * 80 * Reynolds Number(-13/14) Inflation Option - First Layer Thickness • Define First Layer By y+ • Computes First Prism Height based on user inputs • Desired y+, Flow Reynolds Number and Reference Length Dy = LDy+ 80 Re(-13/14)