Overset Mesh Interfaces

An overset mesh interface couples one overset region with the background region or another overset region.

There are two types of overset interface:

• Overset Mesh Interface

  This is the normal type of overset interface. The overset boundaries in the overset region define the border of the hole-cutting process. When using an regular overset mesh interface, you are advised to ensure that 4 to 5 cell layers are always present in the overlapping zone between the background and overset regions, or between two overset regions.

  You can create an overset interface between a pair of regions, or between multiple regions at the same time. You can also repeat the process several times with different pairs of regions.

• Overset Mesh ZeroGap Interface

  This type of interface is provided for cases where the boundary of one overset region comes close to, or makes contact with, the boundary of the background region or another overset region. In the case of contacting surfaces the gap between the two regions must be closed and not permit any flow between the surfaces.

  In cases where an overset zero gap interface is applied, Simcenter STAR-CCM+ can automatically cut off part of the fluid domain when wall boundaries overlap or form very small gaps. Temporary walls are generated between the two relevant regions.

  During this close-off mechanism, isolated island cells can be formed. To maintain solver stability, these island cells are deactivated from the solvers when any of the following conditions apply:

  • They are surrounded by ZeroGap boundaries.
  • They are surrounded by a ZeroGap boundary and a wall boundary.
  • They are surrounded by a ZeroGap boundary and a contact interface.
  • They only have an one-sided acceptor and donor relationship.

  The following diagrams of a ball-type check-valve show an example of such a configuration:

  This image shows the mesh in its initial state, that is, before the simulation starts. The overset region (gray) contains the mesh of the ball. The overset region extends beyond the boundaries of the background region (blue). You can see a very small gap between the background region and the overset region. The distance between the background region and the overset region is less than two cell layers and the zero gap algorithm applies.

  This image shows the mesh after the simulation starts. The cells (overset and background) in the gap and the overset cells outside of the background region are inactive.

  To check the distance between an overset region and a boundary, you can use the Overset Zero Gap Distance field function. In cases where the minimum gap size can vary with flow rate and time, you can plot the function as the solution progresses (see Overset Mesh Field Function Reference).

  When you create the overset mesh zero gap interface between two regions, a zero gap wall boundary is automatically created in each of the regions. During the simulation, if the overset hole-cutting process detects that the distance between the two zero gap wall boundaries of both regions is less than the default value of three cell layers, the cells in the gap become inactive. If you wish, you can change the number of cell layers below which Simcenter STAR-CCM+ deactivates the cells. Cell faces between active and inactive cells are placed within the zero gap wall boundaries. Isolated cell islands surrounded only be ZeroGap boundaries are detected automatically by the solver and set to inactive.

  For detailed settings of overset mesh interfaces, refer to Overset Interface Reference. For the workflow of creating overset interfaces, refer to Creating Overset Mesh ZeroGap Interface.