Mesh models in STAR-CCM+

As it is very important to stablish a proper grid resolution before attempting to solve any CFD problem, I’ll try to analyse the different mesh models available in STAR-CCM+. Fortunately, most of this meshing techniques are common to various CAE tools.

So, according to the User Guide [1], a mesh is a discretized representation of the computational domain, which the physics solvers use to provide a numerical solution. There are different meshing strategies, and each one has its pros and cons, being more suited for one or other application.

Surface mesh

• Surface wrapper: wraps the initial surface to provide a closed and manifold surface mesh from a complex geometry.
  For poor quality or complex CAD, this procedure ensures that the geometry is closed and of sufficient quality for generating surface and volume meshes. The surface wrapper comes with a tool for leak detection and is usually used in conjunction with the surface remesher.
• Surface remesher: remeshes the initial surface to provide a quality discretized mesh that is suitable for CFD.
  It is used to retriangulate the surface based on a target edge length supplied and can also omit specific surfaces or boundaries preserving the original triangulation from the imported mesh.

Volume mesh

• Trimmer: generates a volume mesh by cutting a hexahedral template mesh with the geometry surface.
  It is recommended when an underlying custom mesh needs to be used or if the surface quality is not good enough for a polyhedral mesh. Besides, it is useful in modeling external aerodynamic flows due to its ability to refine cell in a wake region —unsteady and turbulent fluid caused by boundary layer separation.
• Polyhedral mesher: generates a volume mesh that is composed of polyhedral-shaped cells.
  It is numerically more stable, less diffusive, and more accurate than an equivalent tetrahedral mesh. Moreover, also contains approximately five times fewer cells that a tetrahedral mesh for a given starting surface.
• Tetrahedral mesher: generates a volume mesh that is composed of tetrahedral-shaped cells.
  According to CD-adapco [2], tetrahedral meshes are only recommended when comparisons have to be made with legacy tetrahedral models.
• Advancing layer mesher: creates a volume mesh composed of prismatic cell layers next to wall boundaries and a polyhedral mesh elsewhere. The mesher creates a surface mesh on the wall and projects it to create the prismatic cell layers.
  The prismatic cell layers help capture the boundary layer, turbulence effects, and heat transfer near wall boundaries.
• Thin mesher: generates a prismatic layered volume mesh for thin geometries, where good quality cells are required to capture the solid material thickness adequately.

Optional models

• Prism layer mesher: adds prismatic cell layers next to wall boundaries. The mesher projects the core mesh back to the wall boundaries to create prismatic cells.
  This layer of cells are created next to wall boundaries to improve the accuracy of the flow solution as prediction of various flow features —e.g., drag or pressure drop— 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 that would be implied by taking gradients from a coarse mesh.
• Extruder: generates an extruded mesh region from a boundary that one of the core volume meshers has meshed.
  It is typically used for inlet and outlet boundaries to extend the volume mesh beyond the original dimensions of the starting surface, so that a more representative computational domain is obtained.
• Generalized cylinder mesher: generates a volume mesh appropriate for elongated cylindrical regions.
  It uses extruded prismatic cells to reduce the overall cell count and improve the rate of convergence in some cases.
• Shelling mesher: generates a shell mesh region from a boundary that one of the core volume meshers has meshed. This model is specifically for modelling casting methods.
• Embedded thin mesher: similar to the default thin mesher —it is also used to generate a prismatic type mesh in predominantly thing geometries—, it assumes that the thin geometries are entirely contained within another region.

References

[1] User Guide STAR-CCM+ Version 8.06. 2013.
[2] CD-adapco. CFD Basics. Americas Agency Training Document. 2008.