Overset Hole Cutting

The overset hole-cutting procedure is essentially a cell marking procedure. The hole in one region, mostly in the background region, is virtual because no cells are actually removed. They are deactivated and have no part in the solution process. This procedure takes place when the overset interfaces are initialized, and also in each time-step when mesh changes due to motion (for unsteady simulations). See also: Donor Search.

The hole-cutting procedure must be re-executed whenever the position of the overset region changes.

During an overset simulation, the two following mesh representations are available for you to examine:
  • The Overset Mesh Representation displays all the cells from all the regions. This representation is often chosen to plot overset cell types.
  • The Volume Mesh Representation displays all of the active cells, and some acceptor cells in order to fill-in visualization gaps. This representation is often chosen to plot motion and scalar quantities of the overset simulation.
After the hole-cutting procedure completes, cells are grouped into the following cell types (see Overset Cell Status):
  • Active Cells

    Within active cells, discretized governing equations are solved. The Volume Mesh representation shows the active cells and possibly some acceptor cells.

  • Inactive Cells

    Within inactive cells, no equations are solved. However, these cells can become active if the overset region moves. The Overset Mesh representation shows both active and inactive cells. Since the physical values from inactive cells are meaningless, select the Volume Mesh representation when displaying physical quantities.

  • Acceptor Cells

    For a region (background or overset), the acceptor cells separating active and inactive cells define the perimeter of the overset interface for that region.

Hole-Cutting Approach

Currently, Simcenter STAR-CCM+ provides two methods for hole-cutting approaches:

  • Layered Approach (Default)

    The main steps of the layered approach are as follows:

    • Mark the cell types in the overset region: In the overset region, a single layer of cells adjacent to the overset boundaries is defined as acceptor cells, the remaining cells are set as active cells. From the active cells, next to the acceptor cells a layer of cells are marked as potential donor cells (one type of active cells) of the overset region.
    • Mark the cell types in the background region: In the background region, a single layer of cells in immediate vicinity to the potential donor cells of the overset region become the acceptor cells. This single layer is located around 4 cell layers away from the interface with the overset region. The 4 intermediate cell layers between the acceptor cells and the overset boundaries are marked as donor cells in the background region. Background cells within the hole marked by the overset interface are set to inactive and have no part in the solution process.

    These layers of cells must form a watertight container around the overset region. The cells of the background mesh that are completely covered by the overset region cells inside this closed container become inactive.

  • Alternate Hole Cutting Approach

    This approach is more robust for cases with close gaps. For each cell of the background region, the algorithm verifies whether the cell centroid of that cell is outside or inside the overset region. If the cell centroid of a background region cell is inside the overset region, it becomes inactive.

When you create an overset interface, first use the default layered approach. This approach is computationally more efficient than the global approach and suitable for most cases. If the overset interface creation gives overset intersector errors, activate the Alternate Hole Cutting instead. The global approach is more robust when you run the simulation on many cores. The global approach is more sensitive to non-standard overset topologies where the overset boundary does not form a closed surface (see Overset Topology).