27.12 Mesh Adaption Controls

ANSYS FLUENT allows you to:

• Place restrictions on the cell zones.

• Limit adaption by cell volume or volume weight.

• Limit the total number of cells that can be produced from the adaption process.

• Modify the intensity of the volume weighting in the gradient function.

• Restrict the adaption process to refinement and/or coarsening, and control which nodes are eligible for possible elimination from the mesh during coarsening.

The parameters controlling the aspects of adaption are set in the Mesh Adaption Controls dialog box (Figure  27.12.1). You can also open this dialog box by clicking on the Controls... button in any of the adaption dialog boxes.

Adapt $\rightarrow$ Controls...

Figure 27.12.1: The Mesh Adaption Controls Dialog Box

Note:   Write a case and data file before starting the adaption process. Then, if you generate an undesirable mesh, you can restart the process with the saved files.

Limiting Adaption by Zone

You can limit the adaption process to specified cell zones. The cells composing the fluid and solid regions of the analysis generally have very different resolution requirements and error indicators. Limiting the adaption to a specific cell zone and use different adaption functions to create the optimal mesh.

To limit the adaption to a particular cell zone (or to particular cell zones), select the cell zones in which you want to perform adaption in the Zones list. By default, adaption will be performed in all cell zones.

Limiting Adaption by Cell Volume or Volume Weight

The minimum cell volume limit restricts the refinement process to cells with volumes greater than the limit. Use this to initiate the refinement process on larger cells, gradually reducing the limit to create a uniform cell size distribution. Set this limit in the Min Cell Volume field. The input that you will give in this field for a 2D axisymmetric problem will be interpreted as the minimum cell area.

In addition, the gradient volume weight can be modified. A value of zero eliminates volume weighting, a value of unity uses the entire volume, and values between 0 and 1 scale the volume weighting. Set this value in the Volume Weight field. For more information, see this section in the separate Theory Guide.

Limiting the Total Number of Cells

The maximum number of cells is a restriction that prevents ANSYS FLUENT from creating more cells than required for the present analysis. In addition, it saves the time you spent waiting for the mesh adaption process to complete the creation of these cells. However, this premature termination of the refinement process can produce undesirable mesh quality depending on the order in which the cells were visited, which is based on the cell arrangement in memory (random).

During the dynamic gradient adaption, the resulting number of cells after adaption is estimated. If this number exceeds the maximum number of cells, both the Coarsen Threshold and the Refine Threshold are updated. This is done to ensure the best possible mesh resolution with the specified number of cells. You can also specify the minimum number of cells. This is helpful if strong structures of the flow that were resolved with the adaption vanished (e.g. left the domain) and you want to resolve the remaining weaker ones. This would otherwise require modifying the Coarsen Threshold and the Refine Threshold.

You can set the total number of cells allowed in the mesh in the Max # of Cells field. The minimum number of cells in the mesh can be set in the Min # of Cells field. The default values of zero places no limits on the number of cells.

Controlling the Levels of Refinement During Hanging Node Adaption

You can control the number of levels of refinement used to split cells during nonconformal adaption by setting the Max Level of Refine. The default value of 2 is a good start for most problems. If this is not sufficient, you can increase this value.

Note:   Even in a 2D problem, the default value of 2 can increase the number of cells by a factor of 16, in the adapted regions.

A value of zero leaves this parameter unbounded, and you should use a suitable limit for Min Cell Volume. For more information on hanging node adaption, see this section in the separate Theory Guide. For guidelines for limiting cell sizes and number of cells during dynamic gradient adaption, see Section  27.4.1.