Best Practices: Volume Meshing Kynan Maley

Transcription

1 Best Practices: Volume Meshing Kynan Maley

2 Volume Meshing Volume meshing is the basic tool that allows the creation of the space discretization needed to solve most of the CAE equations for: CFD Stress Analysis Heat transfer Electro-Chemistry Magneto Hydro Dynamics...

3 Pipeline Meshing Pipeline meshing allows you to: Change geometry, mesh type, refinements, location/number of prism layers, etc. Automatically update the mesh and map the old solution Rapidly evaluate multiple designs Solution mapping Make changes to the geometry and mesh while retaining your solution Physics is independent of mesh Wrapper Remesher Polyhedral Mesher Trim Cell Mesher Prism Layer Surface Preparation / Meshing Volume Mesh Generation 3

4 Volume Meshing in STAR-CCM+ General Purpose 3D Meshers: Polyhedral Trimmer Tet Mesher General purpose meshers on special geometries tend to produce non optimal meshes in terms of: Cell count Quality

5 Volume Meshing in STAR-CCM+ Examples of special geometries: Thin objects/parts Extruded parts Long or curved pipes/ducts Specialized (2.5D) Meshers: Prism Layer Mesher Extruder Thin Mesher Generalized Cylinder Mesher Advancing Layer Mesher

6 Volume Meshing in STAR-CCM+ 2.5D meshing is a synthetic definition of those meshing techniques that exploit the fact that certain special geometries have a general mesh in 2 dimensions while in the 3 rd dimension the mesh has some form of simplification: Extruded in a predetermined direction Extruded along the local normal direction Swept along a 3d curve or axis

7 Volume Meshing in STAR-CCM+ Full volume meshing pipeline is parallel poly mesher Reducing memory and wall time

8 Volume Meshing Golden rule of volume meshing in STAR-CCM+ Volume meshers have requirements for the input surface: Closed Manifold Non-intersecting Often cell quality issues in the volume mesh can be tracked down to face quality issues in the surface mesh Recommendation is to use the Surface Remesher always prior to volume meshing (with same size settings)

9 Volume Meshing Surface Mesh Quality of CAD determines path to closed, manifold, non intersecting surface The surface wrapper is used for the worst quality CAD Also useful for de-featuring your model Other methods exist to fix minor CAD issues

10 General Purpose Mesher Polyhedral - Trimmer Polyhedral mesher General purpose, reliable, robust Capable of multi-region conformal meshing Suitable for Conjugate Heat Transfer simulations Trimmer Fast and high quality Anisotropic refinement Perfect for large domains such as: Cars in wind tunnels Airplanes Ships Trains

11 General Purpose Mesher Trimmer Mesher Rule of thumb here is to use a trimmed mesh for cases that have large cartesian aligned flow directions Also useful when Trimmer Wake Refinement is needed (refinement follows shape of boundary) Can be done in a local coordinate system, allowing alignment with flow direction

12 General Purpose Mesher Refinement using Volumetric Controls In many situations it is desirable to have the possibility to accurately control the mesh size High gradient zones Shocks High error zones This can be accomplished by placing appropriate Volumetric Controls

13 General Purpose Mesher Refinement using Volumetric Controls Volumetric Controls allow a number of refinement types: Surface Mesh Volume Mesh Isotropic Anisotropic (Trimmer) Prism Layer Mesh

14 General Purpose Mesher Refinement using Boundaries and Feature Curves Surface mesh size can be set at individual Boundaries Feature Curves The volume mesh size is related to the surface size and growth rate

15 General Purpose Mesher Polyhedral Refinement Level Refinement levels provide a quick way to globally refine a polyhedral mesh Activated within the Polyhedral Mesher model settings One of two refinement levels selected in Reference Values Level 1: Splits each polyhedral cell into 6 or 7 new cells Level 2: Splits each polyhedral cell into 40 to 50 new cells Prism Layer unchanged

16 Volume Mesh Volume Ratio Avoid huge jumps in volume ratio, it will cause issues Keep the ratio as small as possible Prism layers can help improve blending from near wall to far field

17 Volume Mesh Volume Ratio - Trimmer The volume ratio for trimmed cells is influenced through different Growth Rate values On continuum level Trimmer > Properties: Template mesh growth rate Reference Values > Template Growth rate > Properties: Default Growth rate

18 Volume Mesh Volume Ratio - Trimmer On continuum, boundary and interface level Boundary growth rate It controls the rate of size changes between cells adjacent to surfaces and cells in the core

19 Volume Mesh Volume Ratio - Trimmer Example Template Growth Rate options Boundary Growth rate None Fast Very Slow Default Growth rate

20 Volume Mesh Prism Layer Mesher What are prism cells? A polyhedral base, a copy of it at top and rectangular sides connecting both Where are prism cells used? Wall Prism Layer (turbulence, heat transfer) Extruder Thin Mesher Advancing Layer Mesher

21 Wall Prism Layers Generation of Prism Layer Mesh The Prism Layer thickness is subtracted from the boundary Offset surface A core mesh is created The Prism Mesh is extruded to the boundary

22 Wall Prism Layers Locations of Prism Mesh Where are Prism Layers generated? Only at boundaries of type Wall Why is no Prism Layer created at my fluid-solid interface? Although the boundaries forming an interface are often of type Wall, being an interface overrules this setting: At an interface no prism layers will be generated as default

23 Wall Prism Layers Introduction to Properties Options With the recent releases of STAR-CCM+ the creation of boundary layers has been further improved Today I will show you some of the Model Properties with which to influence the prism mesh in narrow passages Gap Fill Percentage Minimum Thickness Percentage Layer Reduction Percentage

24 Wall Prism Layers Default 25% Default 10% Default 50% 10% 25% 85%

25 Wall Prism Layers Expert Settings

26 Additional Mesher Extruder The extruder meshing model performs an additional volume meshing step once the core mesh has been generated The model can be activated for any of the core mesh types and enabled for any boundary Care should be taken however that the extrusion volume will not interfere with the existing mesh by intersecting it in any way Generates prism cells which extends the confines of the starting surface Can use any coordinate system: Cartesian Cylindrical Spherical

27 Additional Mesher Extruder Extruder Mesher Options Frozen Boundaries Frozen Boundaries On Frozen Boundaries Off

28 Additional Mesher Extruder Example 1 Inflate the computational domain in all directions One possibility is to change the Part on the Geometry level Another is to extrude the outer boundaries without the Frozen Boundaries option

29 Additional Mesher Extruder Example 2 L 3L-8L

30 Additional Mesher Generalized Cylinder Generates an Extruded mesh along lengths of a part considered a cylinder Automatic cylinder detection

31 Advanced Mesher Thin Mesher The thin meshing model allows thin regions in the geometry to have a prismatic type volume mesh Reason is to improve the overall cell quality and reduce the cell count when compared to an equivalent tetrahedral or polyhedral type core mesh When very thin structures cannot be modeled using baffles their thickness must be modeled with a minimum of 3 cells through the thickness

32 Advanced Mesher Example - Thin Mesher

33 Advanced Mesher Advancing Layer The advancing layer produces prismatic cell layers near wall boundaries Extruding the cells from the surface into the region volume allows for a thicker layer with a more uniform distribution than the prism layer mesher

34 Advanced Mesher Advancing Layer Advancing Layer Options Two options for Stretching Function Several options for Stretching Mode To influence the advancing layer mesh at convex corners, refine the Feature Curve at this edge

35 Advanced Mesher Advancing Layer Example - Shuttle

36 Directed Mesher Directed meshing is a method for creating swept meshes from a 2D starting surface mesh The starting surface mesh may either be created by: Patching the surface and creating quadrahedral elements Using the surface of an existing volume mesh The surface is then swept along a path described by the CAD geometry This results in a high quality structured mesh Engine Powertrain Electric Machines

37 Overset Mesher Overset Meshing: A background mesh enclosing the whole solution domain Separate meshes enclosing each body The regions overlap, and flow-field information is passed between them No need to remesh during motion or after moving geometry!

38 Overset Mesher

39 Overset Mesher

40 Questions Any Questions?