Automated Meshing for Internal Flow

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7.5. Automated Meshing for Internal Flow#

This tutorial shows how to create a mesh for internal flow using the automated meshing process.

Geometry#

As shown in Fig. 7.5.1, the geometry is a wind tunnel enclosing a sphere that is supported by four cylindrical struts. The corresponding CSM file is provided, so the readers can easily reconstruct the geometry on their local machines.

Several details are worth emphasizing in the CSM file:

  1. For internal flow, a single solid body representing the fluid must be provided to the automated meshing process. To do this, once the wind tunnel (box) and the sphere with struts are constructed, the next step is to subtract the sphere and struts from the wind tunnel.

  2. Fig. 7.5.1 shows the faceName and groupName attributes on the faces of the wind tunnel. In general faces with the same faceName will be treated as the same surface patch for assigning surface (maxEdgeLength) and volume (firstLayerThickness) mesh attributes. Conversely, the user can set different surface and volume mesh attributes for faces with a different faceName. Faces with the same groupName will be considered as the same boundary when exporting the CGNS file.

../../_images/geometry.svg

Fig. 7.5.1 Sphere in wind tunnel, geometry with face attributes. sideNegY is hidden to show the interior of the wind tunnel.#

Meshing#

The meshing parameters is set up as follows:

 1 geometry.group_faces_by_tag("faceName")
 2 farfield = fl.UserDefinedFarfield(name="farfield")
 3 params = fl.SimulationParams(
 4     meshing=fl.MeshingParams(
 5         defaults=fl.MeshingDefaults(
 6             surface_max_edge_length=1.2 * fl.u.m,
 7             curvature_resolution_angle=15 * fl.u.deg,
 8             surface_edge_growth_rate=1.2,
 9             boundary_layer_first_layer_thickness=1e-6,
10             boundary_layer_growth_rate=1.2,
11         ),
12         refinement_factor=1.0,
13         volume_zones=[farfield],
14         refinements=[
15             fl.SurfaceRefinement(
16                 name="sphere", max_edge_length=0.1, faces=[geometry["sphere"]]
17             ),
18             fl.SurfaceRefinement(name="strut", max_edge_length=0.01, faces=[geometry["strut"]]),
19             fl.BoundaryLayer(
20                 name="floor", first_layer_thickness=1e-5, faces=[geometry["floor"]]
21             ),
22             fl.PassiveSpacing(
23                 name="adjacent2floor", type="projected", faces=[geometry["adjacent2floor"]]
24             ),
25             fl.PassiveSpacing(name="ceiling", type="unchanged", faces=[geometry["ceiling"]]),
26         ],
27     ),
28 )

  1. UserDefinedFarfield is used to generate the farfield volume zone based on the supplied geometry.

  2. MeshingDefaults defines the global settings for surface and volume meshing. For example, surface_max_edge_length governs the mesh size on all faces.

  3. Regarding the details refinements setting, please note that:

    • When setting up parameters for the specific faces, the faceName tag defined in the CSM file can be used to select a group of faces:

      • The grouping tag MUST be chosen first (geometry.group_faces_by_tag("faceName")) to allow the usage of group selection.

      • All the keys of geometry (e.g. "floor" in geometry["floor"]) need to match the resulting grouped face names under faceName tag in the CSM file.

    • To overwrite the global surface_max_edge_length setting, we define local SurfaceRefinement.max_edge_length for the sphere and strut faces.

    • For the floor faces, we set BoundaryLayer.first_layer_thickness as 1e-5 to overwrite the global value of 1e-6. The first layer thicknesses provided here are just for illustration. The users need to adjust these values according to the target \(y^+\) in their simulations.

    • For the ceiling faces, PassiveSpacing.type is set as unchanged, hence the anisotropic layers won’t be grown from the ceiling. Meanwhile, the surface mesh on the ceiling remains unaltered when generating the volume mesh.

    • For the faces adjacent to the floor adjacent2floor, PassiveSpacing.type is set as projected. The surface mesh on those faces is updated when populating the volume mesh. As shown in Fig. 7.5.2, the “spacing” of anisotropic layers grown from the floor are “projected” onto the faces adjacent to the floor.

../../_images/mesh.png

Fig. 7.5.2 Sphere in wind tunnel, populated volume mesh.#

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