Chapter 3 Meshing Methods for 3D Geometries ANSYS Meshing Application Introduction

Geometry Requirements Meshing Methods Overview Geometry Requirements Meshing Methods Tetrahedrons Patch Conforming Patch Independent (ICEM CFD Tetra) Swept Mesh Automatic MultiZone CFX-Mesh Workshop 3.1 Combining Sweep and Tetrahedral Methods for a Multibody Part Inflating Tetrahedral and Sweep Methods

Geometry Requirements All the 3D meshing methods require that the geometry consist of solid bodies If an imported geometry consists of surface bodies, additional steps would be required to convert it to a 3D solid if a 3D mesh is to be generated in the ANSYS Meshing Application (although surface bodies can be meshed with surface meshing algorithms)

Tetrahedral Meshes Advantages Disadvantages An arbitrary volume can always be filled with tetrahedra Can be generated quickly, automatically, and for complicated geometry Can be easily combined with curvature and proximity size functions to automatically refine the mesh in critical regions Can be combined with inflation to refine the mesh near solid walls (boundary layer resolution) Disadvantages Element and node counts are higher than for a hex mesh with a similar mesh density Generally not possible to align the cells with a flow direction Not well suited for thin solids or annuli due to non-isotropy of geometry and nature of element

Tetrahedral Algorithms Two different algorithms are available for generating tetrahedral meshes in the ANSYS Meshing Platform Patch Conforming: A surface mesh is generated first using a Delaunay or Advancing Front surface mesher which will, by default, respect all faces and edges in the geometry (note: some built-in defeaturing for features below the minimum size limit). The volume mesh is then created from the surface mesh via an algorithm based on TGRID Tetra. Patch Independent: Here a volume mesh is generated and projected to surfaces to yield the surface mesh. Faces and edges will not necessarily be respected unless loads or boundary conditions are scoped to them. This method is more tolerant of poor quality CAD. The patch independent algorithm is based on ICEM CFD Tetra. Both tetrahedral algorithms can be inflated for boundary layer resolution often required for CFD

Tetrahedral Meshing Common Parameters Minimum and Maximum Sizes Face and Body Sizes Advanced Size Functions (Curvature and/or Proximity) Growth Rate (gradual variation for CFD, avoid sudden jumps) Smoothing (helps achieve a more uniformly sized mesh) Statistics Mesh Metrics 6

Patch Conforming Tetrahedrons Right click on Mesh, insert a Method and Choose the bodies to which to apply the method. Set the Method to Tetrahedrons and the Algorithm to Patch Conforming Different parts can have different methods. A single part with multiple bodies can include a mix of patch conforming tetrahedrons and sweep methods and will still produce a conformal mesh (Workshop 3.1) The Patch Conforming method can be used in conjunction with Pinch Controls to help remove short edges. It also has built-in mesh defeaturing based on the minimum size Small Hole Faces in Close Proximity

Resolution of Circular Hole Faces (and edges) are respected Patch Conforming Tetrahedrons Example Resolution of Circular Hole Faces (and edges) are respected

Patch Independent Tetrahedrons Useful for CAD with many surface patches, sliver faces, short edges, poor surface parameterization, etc. With the Method to Tetrahedrons, set the Algorithm to Patch Independent Faces and edges will not necessarily be respected unless a load or named selection is scoped to them Note that there are additional settings concerning defeaturing as well as settings for curvature and proximity Faces in Close Proximity Small Hole 9

No Named Selections: Faces and Edges are not respected Patch Independent Tetrahedrons No Named Selections: Faces and Edges are not respected

Named Selections: Faces and Edges are respected Patch Independent Tetrahedrons Named Selections: Faces and Edges are respected

Inflating the Tetrahedral Method Inflation is scoped to bodies and defined for faces

Sweep Method Body must be sweepable Inflation can yield pure hex or prisms Manual or automatic source/target Normally single source to single target face, automatic thin model can be used for multiple faces with multiple elements through the thickness Right-click on Mesh: Show Sweepable Bodies 13

Sweep Method: Source/Target, Mesh Type

Sweep Method: Thin Model Useful when there are multiple faces as in the geometry shown below which has 3 source and target faces 1 2 3

Sweep Example with Bias in Sweep Direction Geometry with a single source and target face can be swept with a bias in the sweep direction 1 (Faces have been merged either in CAD or with VT)

Sweep with Inflation Inflation is scoped to a face with inflation specified on edges 1 (Faces have been merged either in CAD or with VT) Thin Model Sweeps cannot be inflated

Programmed Controlled Inflation Automatic Method The Automatic setting toggles between Tetrahedral (Patch Conforming) and Swept Meshing, depending upon whether the body is sweepable. Bodies in the same part will have a conformal mesh. Tetrahedron (Patch Conforming) Swept Tetrahedron (Patch Conforming) No inflation Programmed Controlled Inflation 18

MultiZone Sweep Meshing Based on ICEM CFD Hexa Blocking Automatic geometry decomposition With the swept method, this part would have to be sliced into 3 bodies to get a pure hex mesh With MultiZone, it can be meshed directly!

MultiZone for Pipe Intersection 2 1 3 4

MultiZone for Pipe Intersection Free block in center (here meshed with tets)

Adding Inflation to MultiZone As for the tetrahedral meshers, inflation is scoped to bodies and defined for faces

MultiZone Mesh with Inflation

CFX-Mesh Method Tet/prism mesher or extruded meshes for geometries with a periodic translation or rotation CFX-Mesh uses a ‘loose’ integration Selecting Right Mouse ‘Edit…’ on the Method launches CFX-Mesh as a separate window that is different than the Workbench Meshing environment. No Meshing Application sizings are respected or transferred to CFX-Mesh 24

Workshop 3.1 Static Mixer with Patch Conforming Tetrahedrons and Sweep Methods

Goals This workshop will illustrate combining the Patch Conforming Tetrahedrons and Sweep Methods for a multibody part to yield a conformal mesh with hybrid tet/prism and hex elements The use of Inflation is also demonstrated for both the Sweep and Patch Conforming methods 26

Specifying Geometry Copy the sm.agdb file from the tutorial files folder to your working directory Start Workbench and double-click the Mesh entry in the Component Systems panel at the right Right-click on Geometry in the Mesh entry in the Project Schematic and select Import Geometry/Browse Browse to the sm.agdb file you copied and click Open Note that the Geometry entry in the Project Schematic now has a green check mark indicating that geometry has been specified The first 9 steps repeat the process followed for Tutorial 2.1. 27

Initial Mesh Double-click the Mesh entry in the schematic or right-click and select Edit Expand the Geometry entry in the Outline and note that there is a single part with 4 bodies Left click on the Mesh entry and set the Physics preference to CFD and select the FLUENT solver Right click on Mesh and Insert a Mesh Method. Select the three cylindrical bodies from the Model View and choose the Sweep Method Set the Src/Target Selection to Manual Source and select the three end faces of the cylindrical bodies 1 2 3

Patch Conforming Tetrahedrons Right click on Mesh and Insert a Mesh Method. Select the central conical bodies from the Model View and choose the Tetrahedrons Method with the Patch Conforming Algorithm

Initial Mesh (no Inflation) Expand the Inflation entry in the Mesh settings and set the Use Automatic tet Inflation option to None as you will manually inflate the two different methods Make sure the mesh settings are as shown at right Right-click on Mesh and generate the mesh. Notice that the mesh is conformal

Inflating the Sweep Method Right-click on the Sweep Method and choose Inflate this Method. The inflation will be scoped to the three source faces For the boundary, you will need to select the three outer circular edges of the faces (you may need to enable the Select Edges toggle to simplify this). Set the maximum thickness to 0.2 m, leaving the other settings at defaults. Right-click

Inflating the Tetrahedrons Method Right-click on the Tetrahedrons Method and choose Inflate this Method. The inflation will be scoped to the central body. Select the 2 outer radial faces of the conical body Set the Inflation Option to Total Thickness and set the Maximum Thickness to 0.2 m, leaving the other settings at defaults. Right-click

Generating the Mesh Generate the mesh. Note that the swept regions still produce hexes while the central body produces prisms and tetrahedrons. Verify that all meshes are conformal and save your project.

Interior View of Inflated Mesh