Extruding Surfaces from a Parts-Based Volume Mesh

Use the extruder to extend the existing volume mesh beyond the original dimensions of the starting surface, normal to the existing volume mesh, or in a specified direction.

The extruder is typically used to extend the simulation domain with orthogonal extruded cells for inlet and outlet boundaries, and for special applications such as in catalysts.

Extending the inlet and outlet boundaries is useful for certain types of geometries, such as curved or short pipes near the boundary surface, where the flow would not otherwise have enough distance to fully develop in the original shape of the geometry. Using orthogonal extruded cells to extend the volume mesh at these boundaries allows the flow to develop fully while maintaining a low cell count. This practice saves cell count where the solution does not change much in the flow direction by creating anisotropic (stretched) regular prismatic cells (even when the grid in the meshing volume is polyhedral). Extrusion also avoids possible disturbances due to abrupt changes in cell size. In the following example, a small simulation domain is extended using the surface extruder in parts.

The extruder mesher is made up of two operations:
  • Surface Extruder — The surface extruder extends the edges of the input surface to form the bounding surfaces of the extruded region. You specify the extrusion distance in the surface extruder operation.
  • Volume Extruder — The volume extruder fills in the extruded region with extruded volume cells from the input surface. You control the number of divisions in the extruded region of the volume extruder operation.

To use the extruder mesher using parts-based meshing:

  1. Set up and generate a parts-based volume mesh for the simulation domain. See The Meshing Workflow.
    The surfaces of the volume mesh act as a starting point for the extruder mesh.
  2. Create and define the surface extruder operation: Right-click the Operations node and select New > Surface Preparation > Surface Extruder.
  3. In the Create Surface Extruder Operation dialog:
    1. Choose one of the following grid types for the extrusion:
      • O Grid
      • H Grid
      The image below illustrates the differences between O-Grid and H-Grid extrusion for an example case. The surfaces marked for extrusion (S1 and S2) share a common edge E1. For the O-Grid option, both surfaces are extruded simultaneously, with the edge extruded at an average angle between the surfaces. For the H-Grid option,, one of the surfaces (for example, S1) is extruded first, with the edge E1 sweeping out an additional surface, which is then extruded in conjunction with the other surface S2.

      For more information, see Surface Extruder Properties.

    2. Set Input Parts to the part that you want to extrude.
    3. Set Input Part Surfaces to the surface that you want to extrude.
      You can only choose surfaces that are associated with the selected input parts.
    4. In Create New Part, choose how new parts are treated in the extrusion. See Surface Extruder: Create New Part.
    5. In Output Part Surfaces, choose how the input part surface is to be extruded. See Surface Extruder: Output Part Surfaces.
      If only one part surface is chosen, these options will produce the same result. However, the assignment of the boundaries may differ, depending on the geometry.
    6. Set the Extrusion Distance.
    7. Set the options for creating the surface and volume extruder operations.
      To automatically create the Volume Extruder operation, activate the Create Volume Extruder Operation from Output Parts option.
      To execute the Surface Extruder operation upon closing the Create Surface Extruder Operation dialog, activate the Execute Surface Extruder Upon Creation option.
    8. Click OK.
    A Surface Extruder node is added to the Operations node and automatically executed. Here, you can change the direction and distance properties. See Surface Extruder Properties. A Volume Extruder node is also added to the Operations node. See Volume Extruder Properties.
  4. If required, create additional surface extruder operations to extend the simulation domain in multiple directions, and by different amounts.
    This is not necessary if you want to extrude the simulation domain by the same amount in all directions.
  5. Set-up and execute the Volume Extruder operation:
    1. Expand the Operations > Volume Extruder > Controls node.
    2. Select the Set the Number of Layers node and set Number of Layers to the desired amount.
    3. Select the Stretching Function node and choose a Stretching Method to specify the cell-distribution along the extruded distance.
      For more details, see Volume Extruder Controls.
Assign the extruded parts to regions:
  1. Right-click the Surface Extruder node and select Assign Parts to Regions. For guidance, see Regions Layout Workflow.
    If you have multiple surface extruder operations, assign all extruded parts to a separate region.
  2. To create multiple extruded regions with different parameters, such as increasing the number of layers, create additional Volume Extruder operations as described in the previous steps.
    NoteIf a number of extruded regions have the same properties, additional volume extruder operations are not required. In this case, simply multi-select the surface extruder parts before setting the properties.
  3. Right-click the Operations > Volume Extruder node and select Execute.
The selected regions are extended.