Volume Meshing in STARCCM+

This project was done as a part of the course- External Aerodynamics using STARCCM+

A mesh is a representation of a larger geometric domain by smaller discrete cells. In CFD, meshes are used to compute solutions of partial differential equations. A mesh partitions space into elements over which the equations can be solved, which then approximates the solution over the larger domain. Element boundaries may be constrained to lie on internal or external boundaries within a model. Higher-quality (better-shaped) elements have better numerical properties, where what constitutes a "better" element depends on the general governing equations and the particular solution to the model instance.

1. Core Meshers:

1. Tetrahedral Mesh:

Tetrahedral Mesh constitutes of tetrahedra having 4 faces per cell. These meshes are simple and efficient for a complex geometry. It is also the fastest and uses the least amount of memory for a given number of cells. These meshes are created using the Delaunay method, where mesh construction is done by creating points iteratively inside the geometry domain. One shortfall of these meshes is that it requires a large number of cells (5 or 6 times that of polyhedral and trimmer mesh) to create a mesh of similar quality. These meshes are used in fluid-structure interaction simulations and simulations of electromagnetic phenomena.

                 2D & 3D representation of Tetrahedral Mesh

2. Polyhedral Mesh:

Polyhedral mesh consists of polyhedra with an average of 14-16 faces per cell. These meshes are also easy and efficient and do not require any more surface preparation than tetrahedral meshes. They require 5 times fewer cells than that of an equivalent tetrahedral cell. Polyhedral cells are created from the existing tetrahedral cells using the dualization technique. In this, the centroids of each tetrahedron are marked along with the centre of cell edges at the boundaries. The polyhedral cells start to grow from the boundary and then merge towards the centre of the domain. These meshes are ideally used for Heat Transfer, Swirling Flow and Complex Fluid Flow problems.

                                                Polyhedral Mesh

3. Trimmer Mesh:

The trimmed cell mesher provides a robust and efficient method of producing a high quality grid for both simple and complex mesh generation problems. It combines a number of highly desirable meshing attributes in a single meshing scheme, viz predominantly hexahedral mesh with minimal cell skewness, automatic curvature and proximity refinement, surface quality independence, alignment with a user specified coordinate system. In this, hexahedral cells are created all over the domain and then cut along the boundaries of the geometry. Growth parameters can be used to transition the cell size. These are predominantly used for external flow and electronic cooling simulations.

                      Trimmer Mesh

2. Need for Prism Layer:

A prism layer mesh is composed of orthogonal prismatic cells that usually reside next to wall boundaries in the volume mesh. They are required to accurately simulate the turbulence and heat transfer. The thickness, number of layers and distribution of the prism layer mesh is determined primarily by the turbulence model used; typically, for wall function based models, one to three layers are used, while for low Reynolds number and two-layer schemes, anywhere from 15 to 25 layers is normal. Boundary layer is a complex phenomenon which is very thin and the solution gradients are very high. To capture this phenomenon correctly, we could use a fine mesh throughout but would be a costly affair. Instead adding up prism layers near the wall would be time and cost effective giving good convergence and fairly accurate results. Prior to the core mesh being created, a subsurface is generated at the specified prism layer thickness values, in effect "shrinking" (for internal flows) or "expanding" (for external flows) the starting surface. The core mesh is built using this subsurface. The prism layer mesh is then generated by extruding the cell faces from the core mesh back to the original starting surface.

Tetrahedral mesh with prism layer

3. CAD to Volume Mesh:

The CAD model of a sphere with radius 30mm was the given geometry. To create a new geometry, go to geometry --> 3D CAD Models --> (Right Click) --> New.

Here, select the XY plane, right click and click 'Create Sketch'. Once inside the create sketch menu, draw a circle with radius 30mm, draw a construction line along the diameter and trim one half of the semicircle.

Then close the sketch, right click on the sketch and select the 'Revolve' menu. Make sure the revolution angle is 360 degrees before clicking on the Create button. Thus, the spherical geometry is created. 

Once the geometry is created, right click on the geometry and click 'New geometry part'. The geometry will then get added to parts.

                                                   Geometry Sketch

                                            Desired Geometry (Sphere)

The next step is to create a wind tunnel. To do so, right click on parts --> New Shape Part --> Block. Create the block around the sphere leaving suitable space in all three directions for fluid flow. Keep the length behind the sphere (Output side) a little longer to capture the wake region properly.

Once the block is complete, right click on the surfaces tab inside the block and select 'Split by Patch'. In this window, select and rename the Input and Output patches. After exiting the window, rename the other surfaces as 'Walls' for better identification. Then select the Block (Wind Tunnel) and the Sphere, right click, and select Combine. This will combine the two parts into a single part. 

Then right click on this combined part and select 'Assign Parts to Regions'.

The next step is to check for any surface errors. For this, right click on the Geometry and select 'Repair Surface'. Then on the right side of the screen, go to Manage, select all the 6 error types and then check for any surface errors. In this case, no surface errors were found. Hence, surface repair or surface wrapping was not required.

                                               Surface Repair Window

Then, under the parts section, right click on 'Operations' --> New --> Mesh --> Automated Mesh. Once inside the Automated Mesh window, select the Geometry and select the follwing Meshers:

Surface Remesher

Automatic Surface Repair

Trimmed Cell Mesher

Prism Layer Mesher

Under 'Default Controls', select the Base Size as 1mm. Here, since the geometry is simple, no other properties were needed to be changed. All other properties were set as default.

Then right click on the 'Automated Mesh' Button and select 'Execute'.

Once the meshing is complete, open the Mesh Scene to view the mesh.

                                                       Generated Mesh

To check the detailed mesh around the sphere, a plane needs to be created which passes through the sphere. To create the plane, right click on 'Derived Parts' --> New Part --> Section --> Plane. Adjust the plane so that it is parallel to the walls. Then hide the geometry from the Mesh Scene window and view the plane.

                                              Plane capturing mesh details

Thus, a fine volume mesh was generated around the sphere. Trimmed Cell Mesh was used because this is a case of external aerodynamic flow. Also, a trimmed cell mesher captures the boundaries of the geometry accurately and with high detail.