Volume Meshers:
Different types of core mesher in STAR-CCM+
1) Tetrahedral Mesher
The tetrahedral meshing model utilizes the tetrahedral cell shape in order to build the core mesh. In Simcenter STAR-CCM+, the Delaunay method is used to construct the mesh, which iteratively inserts points into the domain, forming high-quality tetrahedra in the process. The resulting mesh strictly conforms to the triangulation of the surface at the domain boundary, so the quality of the original surface mesh must be good to ensure a good quality volume mesh.
2) Polyhedral Mesher
They are relatively easy and efficient to build, requiring no more surface preparation than the equivalent tetrahedral mesh. They also contain approximately five times fewer cells than a tetrahedral mesh for a given starting surface.
The polyhedral meshing model utilizes an arbitrary polyhedral cell shape in order to build the core mesh. In Simcenter STAR-CCM+, a special dualization scheme is used to create the polyhedral mesh from an underlying tetrahedral mesh, which is automatically created as part of the process. Simcenter STAR-CCM+ calculates the initial cell count from this underlying tetrahedral mesh. The polyhedral cells that are created typically have an average of 14 cell faces.
A tetrahedral mesh is generated initially as shown:
A dualization scheme is used to mark the center of the tetrahedral cells (red dots in example), as well as the midpoints (blue dots in example) on the boundary edges.
Polyhedral cells are generated, starting from the boundary edges.
The process carries on until all polyhedral cells are generated.
An example polyhedral core mesh is shown below:
3) Trimmed Mesher
By default, the trimmer meshing model utilizes a template mesh that is constructed from hexahedral cells at the target size from which it cuts or trims the core mesh using the starting input surface. The template mesh contains refinement that is based on the local surface mesh size and local refinement controls. Growth parameters can be used to control the transitioning of the mesh cell sizes from small to large both at the surface and far field.
The following sequence of images shows how a trimmed mesh is generated for an input surface in Simcenter STAR-CCM+.
The template is generated from hexahedral cells using the target size.
The core mesh is trimmed using the input surface.
The resulting mesh is composed predominantly of hexahedral cells with trimmed cells next to the surface. Trimmed cells are polyhedral cells but can usually be recognized as hexahedral cells with one or more corners and/or edges that are cut off.
An example trimmed cell core mesh is shown below:
4) Thin Mesher
The Thin mesher is used to generate a prismatic type volume mesh for thin volumes within parts or regions. Thick or bulk portions of the same geometry are meshed with the core volume mesher.Using this mesh improves the overall cell quality and reduces the cell count when compared with an equivalent tetrahedral or polyhedral type core mesh. The different portions of mesh are joined continuously without abrupt changes in the mesh between the bulk and thin portions of the geometry.
The Thin Mesher is typically used for thin plate geometries. In these geometries, good quality cells are required to capture the solid material thickness adequately. Heat transfer analysis can then be performed between any geometries that are in contact with each other.
For example, the Thin Mesher can be used during the mesh generation stage of an analysis involving the heat transfer between the thin plates of a car chassis.
An example thin mesh with two triangular based prismatic layers, generated using the default thin mesher (RBM), is shown below:
5) Advancing Layer Mesher
The advancing layer mesher generates layers of prismatic cells around the surfaces of regions or parts and fills the remaining void with polyhedral cells. This mesher either uses the input triangular surface mesh or converts the triangular mesh to a polygonal or quad surface mesh – depending on the Surface Element Type that is selected. This surface mesh is then advanced into the region volume to form cell layers. Some advantages of this approach are a conformal match on either side of an interface, and the ability to generate thicker and more uniform cell layers.
The prismatic cell layers help capture the boundary layer, turbulence effects, and heat transfer near wall boundaries.
An example of an advancing layer mesh is shown below. For illustration, the prismatic layers are colored blue and the core mesh is colored pink.
Why are prism Layers necessary?
The prism layer mesh model is used with a core volume mesh to generate orthogonal prismatic cells next to wall surfaces or boundaries. This layer of cells is necessary to improve the accuracy of the flow solution.
Prism layers allow the solver to resolve near wall flow accurately, which is critical in determining not only the forces and heat transfer on walls, but also flow features such as separation.
Accurate prediction of these flow features depends on resolving the velocity and temperature gradients normal to the wall. These gradients are much steeper in the viscous sublayer of a turbulent boundary layer than would be implied by taking gradients from a coarse mesh.
To perform volume mesh, the CAD geometry has to be free from surface errors, after which the boundary conditions are given followed by volume meshing.
Volume meshed sphere inside the wind tunnel:
The above mesh shows a plane section created to show the internal mesh generation