Overset Gap Treatment

When the wall boundaries of moving bodies approach each other, small gaps can occur between the wall boundaries. In many applications they require special treatment.

In conjunction with overset technique, the gap treatment methods are listed as follows:

• Closed Gap Treatment when using Overset Mesh ZeroGap Interface

  This option is preferred when the gap closes due to the body motion, for example, a valve. Small gaps are closed automatically if the overset assembly process detects that the distance between the zero gap wall boundaries of both regions is less than the default value of three cell layers. If you wish, you can change the number of cell layers below which Simcenter STAR-CCM+ deactivates the cells.

• Small Gap Treatment when using Overset Mesh Interface

  When you use a regular overset interface, the flow in the gap is always resolved. The overset assembly process requires a minimum of 4 layers of complete cells between body wall and the overset boundary, in both the overset and the background mesh. Providing this amount of cells for an entire motion trajectory in which small gaps occur would result in a large cell count.

  The two following methods can be applied to reduce the required number of cells:

  • Uniform Gap Refinement

    In the adaptive mesh refinement (AMR) method, you can apply uniform gap refinement to achieve the specified refinement only for the cells within a certain gap size. For more details, refer to Uniform Gap Refinement.

  • Prism Layer Shrinkage

    By default, the overset intersector deactivates cells in a small gap between bodies. The gap thickness at which the cells are set inactive is determined by the cell size and the overset intersector. If the inactive cells are large, a void between the wall boundaries can remain.

    To avoid such problems, you activate the prism layer shrinkage model, which morphs the cells of the prism layers between the walls of the approaching bodies. Overset mesh modeling requires a minimum of 2-3 active cell layers to resolve a gap between two wall boundaries. Prism layer shrinkage approach requires that you create a minimum of 5 prism layers on all boundaries for which you expect close contact with a neighbour body. See also: Setting up Prism Layer Shrinkage.

    The following diagram illustrates the concept (for the background mesh only):

    
    
    

  When applying the Prism Layer Shrinkage in conjunction with Uniform Gap Refinement, refer to the specific workflow in the section Prism Layer Shrinkage with Adaptive Mesh Refinement.