# Region of Influence Parameters

When Region of influence parameters is turned On the following options are available: Influence type, Influence size factor and Influence distance.

Region of influence is a size control that allows you to control the size and growth rate of the surface and volume mesh surrounding a surface face based on the distance from the surface. Two different options are available for region of influence meshing: Simple and Directional. Each method is detailed in a later section. Toggling this option on enables region of influence meshing for the faces in the surface group. The volume mesh size is controlled as a function of the surface mesh size. This mesh control comes with some important implications.
• The meshing process may be significantly slower due to the need to compute distances from the surface and evaluate the sizes repeatedly.
• The volume mesh size will still be constrained to the octree sizes (as described in the introduction). Therefore, you should not expect to see smooth size transitions or exact size enforcement in the resulting volume mesh in unstructured regions.
• The base size used for the region of influence size calculation is the value specified for the Absolute or Relative mesh size on the surface group. This feature does not take into account curvature refinement, sizes inherited from parent entities, zone refinements, and so on.
Influence type has two options: Simple and Directional.
• Simple: A simple region of influence type specifies that the mesh size will be a linear function of the distance from the surface. When specifying this type of meshing control, you only need to enter the Influence size factor and Influence distance. The mesher will then grade the mesh size as a function of the distance from the surface. For example, if you want the volume mesh to be .01 meters at the surface, then transition to a size of .04 meters at a distance of 2 meters from the surface, the following parameters would be specified:

Absolute mesh size: .01 meters

Region of influence parameters = on

Influence type = Simple

Influence size factor: 4

Influence distance: 2.0 meters

• Directional: A direction region of influence type specifies that the mesh size will be a function of the distance from the surface, but will have different behaviors in the upstream and downstream distance. When specifying this type of meshing control, you need to enter the Influence size factor, Influence direction, Upstream influence distance, Downstream influence distance and Influence start angle. The influence direction tells the mesher which direction the flow is directed. The mesh is then refined in the upstream direction to a distance of upstream influence distance, and in the downstream direction to a distance of downstream distance. The influence start angle is specified to tell the mesher the angle at which to expand the refinement zone in the cross stream direction as a function of downstream distance. As with simple region of influence type, the mesh base mesh size is equal to the surface mesh size. The size can be scaled as a function of distance by specifying Influence size factor. The intent for the directional region of influence is to create automatic wake refinement regions for bluff body flows.

For example, if you want to simulate flow past a bluff body (in the x-direction) where the goal is to have the volume mesh be .01 meters at the surface, then transition to a size of .04 meters at a distance of 2 meters from the surface (with upstream refinement of 1 meter, downstream refinement of 5 meters, and a wake expansion angle of 20 degrees), the following parameters would be specified:

Absolute mesh size: .01 meters

Region of influence parameters = on

Influence type = Directional

Influence direction = 1,0,0

Influence size factor = 4

Upstream influence distance = 1.0 meters

Downstream influence distance = 5.0 meters

Influence start angle = 20 degrees

When Directional is selected the following options are available:
• Influence direction: Direction vector used to determine the upstream and downstream distances.
• Upstream influence distance: Distance in the upstream distance for which the meshing control should be specified.
• Downstream influence distance: Distance in the downstream distance for which the meshing control should be applied.
• Influence start angle: The expansion angle that should be used in the downstream distance to increase the cross-stream extent of the wake refinement region.