Appendix A Mesh Quality
ANSYS Meshing Application Introduction Overview Mesh Quality Metrics in ANSYS Meshing
Skewness Aspect Ratio Worst Element Mesh Quality Considerations for the FLUENT Solver General Considerations Impact of Mesh Quality on the Solution Mesh Quality Considerations for the CFX Solver Factors Affecting Mesh Quality CAD Issues Mesh Resolution and Distribution Meshing Method Inflation Strategies to Improve Mesh Quality CAD Cleanup Virtual Topology Pinch Controls Sensible Mesh Sizings and Inflation Settings General Recommendations Workshop A.1 Virtual Topology for an Auto Manifold Workshop A.2 FLUENT and CFX Mesh Quality Metrics 2 Mesh Quality Metrics in ANSYS Meshing
Mesh Metrics are available underMesh Options to set and reviewmesh metric information and toevaluate mesh quality Different physics and differentsolvers have different requirementsfor mesh quality Mesh metrics available in ANSYS Meshing include: Element Quality Aspect Ratio Jacobian Ration Warping Factor Parallel Deviation Maximum Corner Angle Skewness 3 Mesh Quality Metrics Skewness Two methods for determining skewness:
Based on the Equilateral Volume deviation: Skewness = Applies only to triangles and tetrahedra Default method for tris and tets Based on the deviation from a Normalized Angle deviation: Whereis the equiangular face/cell (60 for tets and tris, and 90 for quads and hexas) Applies to all cell and face shapes Used for prisms and pyramids optimal (equilateral) cell actual cell circumsphere Verify with the developers this Perfect Worst 4 Mesh Quality Metrics Aspect Ratio
Aspect for generic triangles and quads is a function of the ratio of longest side to the shortest side of the reconstructed quadrangles (see User Guide for details) Equal to 1 (ideal) for an equilateral triangle or a square aspect ratio = high-aspect-ratio quad aspect ratio = high-aspect-ratio triangle Please update or remove this page. 5 Mesh Quality Statistics in ANSYS Meshing
The min, max, averaged and standard deviation for the selected mesh metric are shown for the surface mesh (after Preview Surface Mesh generation) and for the volume mesh (after Preview Inflation layer or Generate Mesh generation) The worst elements can be highlighted using the Show Worst Elements under the Mesh object in the Tree Outline 6 Mesh Quality Considerations for FLUENT
FLUENT requires high quality mesh to avoid numerical diffusion Several Mesh Quality Metrics are involved in order to quantify the quality, however the skewness is the primary metric The aspect ratio and cell size change mesh metrics are also very important In worst scenarios and depending on the solver used (density based or pressure based) FLUENT can tolerate poor mesh quality. However some applications may require higher mesh quality, resolution and good mesh distribution The location of poor quality elements helps determine their effect Some overall mesh quality metrics may be obtained in Ansys Meshingunder the Statistics object Additional mesh quality metrics may be retrieved in FLUENT GUI under Mesh/Info/Quality from the menu, or using the TUI commands mesh/quality 7 Mesh Quality Requirements for FLUENT
The most important mesh metrics for Fluent are: Skewness Aspect Ratio Cell Size Change (not implemented in Ansys Meshing) For all/most applications: For Skewness: For Hexa, Tri and Quad: it should be less than 0.8 For tetrahedra: it should be less than 0.9 For Aspect Ratio: It should be less than 40, but this depends on the flow characteristics More than 50 may be tolerated at the inflation layers For Cell Size Change: It should be between 1 and 2. Poor mesh quality maylead to inaccuratesolution and/or slowconvergence Some applications mayrequire even lowerskewness than the suggested value 8 Skewness and the Fluent Solver
High skewness values are not recommended Generally try to keep maximum skewness of volume mesh < However this value is strongly related to type of physics and the location of the cell FLUENT reports negative cell volumes if volume mesh contains degenerate cells. Classification of the mesh quality metrics based on skewness: * In some circumstances the pressure based solver in Fluent can handle meshes containing a small percentage of cells with skewness ~0.98. * Excellentvery good good acceptable bad Inacceptable* 9 Impact of the Mesh Quality on the Solution
Example (max,avg)CSKEW=(0.912,0.291) (max,avg)CAR=(62.731,7.402) Mesh 1 VzMIN-90ft/min VzMAX600ft/min Large cell size change (max,avg)CSKEW=(0.801,0.287) (max,avg)CAR=(8.153,1.298) Mesh 2 VzMIN-100ft/min VzMAX400ft/min 10 Mesh Quality Considerations for CFX
Mesh quality requirements are somewhat different for the CFX solver than for the FLUENT solver due to the difference in the solver structure for the two codes Fluent uses a a cell-centered scheme, in which the fluid flow variables are allocated at the center of the computational cell, and the mesh-element is the same as the solver-element CFX employs a vertex-centered scheme for which the fluid flow variables are stored at the cell vertex, and the solver-element or control volume is a dual of the mesh-element. This means that the vertex of the mesh-element is the center of the solver-element Please complete this slide 11 Mesh Quality Considerations for CFX
The CFX solver calculates 3 important measures of mesh quality at the start of a run and updates them each time the mesh is deformed Mesh Orthogonality Aspect Ratio Expansion Factor | Mesh Statistics | Domain Name: Air Duct Minimum Orthogonality Angle [degrees] = ok Maximum Aspect Ratio = OK Maximum Mesh Expansion Factor = ! Domain Name: Water Pipe Minimum Orthogonality Angle [degrees] = ok Maximum Aspect Ratio = OK Maximum Mesh Expansion Factor = ! Global Mesh Quality Statistics : Good (OK) Acceptable (ok) Questionable (!) Mesh Orthogonality in CFX
Orthogonality measures alignment of: ip-face normal vector, n, & node-to-node vector, s. Orthogonality Factor = ns, >1/3 desirable Orthogonality Angle = 90-acos(ns), >20 desirable Are these different than Max/Min Face Angles in CFD Post? YES! Face angles correspond to angles between edges One can have an acceptable Face Angle and an unacceptable Orthogonality Angle if an element is skewed in two directions Mesh Expansion Factor in CFX
Expansion factor measures howpoorly the nodal position correspondsto the control volume centroid Mesh Expansion Factor ratio of largest to smallest element volumes surrounding a node, Browse. Select the file Auto-Manifold.agdb Named Selections Next, make sure that Named Selections will be brought into Meshing: Right-click on cell A2 and then select Properties Ensure Named Selections is checked, and the Named Selection Key is blank Close the Properties window Edit the Mesh Edit the Mesh (cell A3)
The Meshing window will open Start by suppressing the fluid region and meshing the solid: Select the Body selection icon from the toolbar Select the inner fluid region, so that it is highlighted in green, and then right-click and select Suppress Body Mesh Settings Select Mesh from the Outline tree
In the Details view set the Physics Preference to CFD The assumption here is that heat transfer will be solved in the solid region using a CFD solver Expand the Sizing section in the Details view and set: Span Angle Center = Medium Min Size = 1.0 mm Max Face Size = 10.0 mm Max Tet Size = 10.0 mm Right-click on Mesh in the Outline tree and select Preview Surface Mesh Since the body is not sweepable, the Patch Conforming method will be applied by default Examine the Mesh The Patch Conforming method meshes each individual surface.This produces a poor quality mesh on some surfaces in this geometry.Examine the surface mesh and look for regions of poor mesh quality.By switching between Geometry and Mesh in the Outline tree relate regions of poor mesh quality to the underlying surface geometry.Some examples are shown here: Adding Virtual Topology
Virtual Topology allows you to merge adjacent surfaces, removing undesirable surface geometry feature and producing a higher quality mesh Right-click on Model (A3) in the Outline tree and select Insert > Virtual Topology A Virtual Topology entry is added to the Outline tree In the Details view note that the Behaviour is set to Low Right-click on Virtual Topology in the Outline tree and select Generate Virtual Cells This automatically creates virtual cells using a Low merging strategy.Medium and High strategies are likely to result in more faces being merged into virtual cells Virtual Topology When Virtual Topology is selected in the Outline tree the viewer shows all virtual cells that have been created Examine the new surface geometry and note that most of the problematic faces have been merged to produce a cleaner surface geometry In the Details view change the Behaviour to Medium Right-click on Virtual Topology in the Outline tree and select Generate Virtual Cells Note that more faces have been merged into virtual cells Try generating virtual cells using the High option for Behaviour This does not work as well for this geometry as shown to the right Switch back to the Medium option and generate the virtual cells again Examine Improved Mesh Re-create the surface mesh and examine the regions that previously showed poor mesh quality You should find that the surface mesh has been greatly improved There are still some regions where the mesh quality could be improved.The arrows below shows one of these locations. If you zoom in and examine the geometry here you will find a kink at the edge of the surface Adding Virtual Cells Manually
You can manually add Virtual Cells to improve the mesh further Pick the Face selection icon from the toolbar Orient the view approximately as shown below (note the X-Y axes) Check that Virtual Topology is selected from the Outline tree Select the four faces shown below, then right-click and select Insert > Virtual Cell 3 1 2 4 Examining Improved Mesh
Re-create the surface mesh and examine the region again You should find an improved surface mesh You can continue adding Virtual Cells as necessary In some cases the automatic virtual cell creation may merge faces that you do not want to merge.You can delete individual virtual cells by selecting the Virtual Face from below the Virtual Topology entry in the Outline tree and right-clicking to delete. Right-click on Mesh and select Generate Mesh to create the final solid mesh Viewing the Fluid Body The next step is to create the mesh for the fluid region In the Outline tree expand the Geometry > Part section Right-click on the first solid and select Hide Body to hide the solid region Right-click on the suppressed (second) solid and select Unsuppress Body With the second solid selected, in the Details view expand the Graphical Properties section and set the Transparency to 1 Adding Inflation Select Virtual Topology from the Outline tree
Virtual Cells have already been created on the fluid region from earlier Check that the automatic virtual cells look reasonable There should be no small surfaces remaining in the model The next step is to add inflation to the fluid walls Right-click on Mesh and select Insert > Inflation In the Geometry field you need to select the solid body corresponding to the fluid region from the Viewer then click Apply Once this has been selected click on No Selection in the Boundary field so that the Apply / Cancel buttons appear Creating the Fluid Mesh
Now select one of the faces from the model that is not an inlet or outlet Select Extend to Limits from the toolbar as shown: All the fluid walls should now be selected Click Apply in the Boundary field in the Details view To generate the final mesh right-click on Mesh and select Generate Mesh Checking the Mesh Quality
Expand the Statistics entry and set the Mesh Metric to Skewness.Note that the Max Skewness is within the acceptable range for the FLUENT solver. If you had generate the mesh without VT, the Max Skewness would have been considerably higher Without Virtual Cells Fluid Region Mesh NO VT VT Workshop A.2 FLUENT and CFX MeshQuality Metrics Goals This hands on tutorial will demonstrate how the Meshing Application in ANSYS is used to generate a CFD mesh for an internal flow domain The geometry represents portions of an aerospace valve region, decomposed into 3 bodies The goal is to produce a conformal hybrid CFD mesh including hex, pyramid, prism and tetrahedral elements including pinch controls and to examine mesh quality metrics for the Fluent and CFX solver preferences 46 Creating a Meshing System
Launch ANSYS Workbench from the START menu Click on Component Systems in the Toolbox on the LHS of the WB main panel Double click the Meshoption 47 Importing the Geometry
Right click (RMB) on the Geometry button and select Import Geometry (the question mark on the button goes away once a geometry file is imported) Import the Aero-Valve.agdb file from the tutorial folder Double click on the Mesh button in the Project Schematic to launch the Meshing Application 48 Geometry The original geometry is a Solid part and the Fluid region was extracted out in DesignModeler (DM). Other operations performed in DM; A parameter was defined for the position of the valve Some outlet ports were closed One multi-body part was created and a given the name Fluid and the material Fluid Individual bodies were re-named and Named Selection was used to define the Inlet and Outlet Fillets were added to some sharp corners to improve mesh quality 49 Meshing Options In the Meshing Options panelselect the following meshing options: Physics Preference CFD Mesh Method Automatic Click OK after you make the selection In Units, make surethe setting is mm Global Mesh Parameters
Set global Mesh control parameters: Click on Mesh to change settings Verify Defaults Physics Preference CFD Solver Preference Fluent or CFX Fluent is used initially, but results for the CFX setting are also presented Set Sizing parameters SetUse Advanced Size Function On: Curvature Set Curvature Normal Angle to 15 Set Min Size to 0.20 mm Maintain all other defaults Inflation and Pinch Parameters
Set Inflation parameters Click drop-list for Use Automatic Tet Inflation and select Program Controlled, leave all others as default Set Maximum Layers to 4 Activate View Advanced Options Set Pinch control Set Pinch Tolerance = 0.15 mm Activate Generate on Refresh Set Mesh Metrics to Skewness ( for Fluent) Note: Program Controlled Inflation will add inflation on all boundaries that do not have assigned Name Selection. It does not add inflation to Fluid-Fluid interfaces Note: Smooth Transition provides a transition between the inflation layers and the tetrahedral mesh following the specified Growth Rate Note: Layer Compression is the default Collision Avoidancefor Fluent and Stair Stepping is default for CFX Note:When edge length or distance between vertices is less than the pinch tolerance, software will ignore the edge or remove extra vertex during meshing Note: Pinch Tolerance should besmaller than Size Function Min Size Pinch Controls Create Pinch control :
Right-Mouse-Button -click in the Tree (RMB (Tree)) Select Create Pinch Controls 10 Pinch Controls are created (Expand the Mesh button to list the pinch controls) Viewing Pinch Controls
View the Pinch Controls Ctrl Left-Mouse-Button Select the Pinch controls, these will be highlighted in the viewing window Sweep Method Assign Sweep Method to the inlet and outlet bodies:
Select Mesh button in Tree Select the bodies (as shown below) Set the Cursor Mode to Body Selection Left-Mouse-Button click (Select) one sweepable body Hold Ctrl key and select the second body InsertMethod Right-Mouse-Button -click in the graphics window (RMB (Window)) Insert - Method The Automatic Method form appears In the Automatic Method form Select Sweep from the pull-down menu Sweep Method Settings Set Sweep Method controls Src/Trg Selection;
Select Manual Source Click on the Source Selection Field This will activate the face picker Hold the Ctrl key and pick both the Inlet and the Outlet face Apply the Selection Additional Settings Set Free Face Mesh Type; All Quad Set Sweep Num Divs; 20 Set Sweep Bias Type;_________ Set Sweep Bias;4 Outlet Inlet Inflating the Sweep 2D-Inflation on swept bodies: Pick Faces;
Set the Cursor Mode to Face Selection Select the Inlet and Outlet faces (green) RMB (Window) Insert-Inflation Pick Edges Set the Cursor Mode to Edge selection Select four edges surrounding theinlet and outlet faces (marked in red) Apply the selection Inflation Settings Set Maximum Thickness: 3.0 mm Maintain all other options Initial Surface Mesh Surface-mesh the model:
Right-click on Mesh and select Preview Surface Mesh This will provide us with feedback about mesh quality and density The Advanced Size Function creates a very fine mesh in the swept bodies, We can reduce the size by specifying the edge intervals on the Inlet and Outlet Edge Sizing Scoped edge mesh on swept bodies:
Insert Scoped Edge Size ; Activate edge picker Pick the four edges surrounding theinlet and outlet faces Right-click Insert ->Sizing Set Parameters Change the Type Number of Divisions; 20 Change Behavior; Hard . Preview Inflation Check the inflation layer: (Optional)
Right-click on Mesh and select Preview Inflation View the mesh Statistics, mesh size and max skew is around and 0.92 respectively We are ready for volume meshing . Volume Mesh with Fluent Settings
Mesh the model: RMB (Tree) select Generate Mesh Again, check the Statistics for the total element count and Max Skewness which will be around and 0.92 respectively . Using a Section Plane to View Internal Mesh
Create a Section Plane: Click on the Z-Axis at the lower right corner to orient the model Click the Selection Plane icon Press and hold the left mouse button while moving along the indicated red arrow then release The position of the Section Plane can be adjusted by moving the slider bar Click on Show Whole Element Reselect the rotation button to adjust the view Viewing the Worst Elements
Rotate the geometry to view the mesh RMB (Tree) Show Worst Elements Note the location; far from the main flow field Tip: Select Wireframe from the View menu to help see the element CFX Solver Preference Using CFX Solver Preference (optional)
Change Solver Preference:CFX RMB (Tree) select Generate Mesh Note the higher Max Skewness for the CFX Solver settings 64 Checking the Quality in FEModeler
Check quality in FEModeler (optional) Meshing application RMB (Tree) Update Close Meshing Application Workbench 2 Drag-and-Drop FE Modeler on top of Mesh in the Project Schematic Double click on Model FEModeler RMB (Tree) Insert Mesh Metrics Mesh Metrics - Valve 4 NodeLinear Tetrahedron Set Mesh Metric Type: Aspect Ratio Max aspect ratio is less than 50 . The mesh is now complete
Saving the Project The mesh is now complete Select File > Close to close FEModeler In the WB panel select Update In the WB panel select File > Save Project As and give the project a name Exit from ANSYS Workbench by selecting File > Exit 66