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26.12 Grid Adaption Controls

FLUENT allows you to:

The parameters controlling the aspects of adaption are set in the Grid Adaption Controls panel (Figure  26.12.1). You can also open this panel by clicking on the Controls... button in any of the adaption panels.

Adapt $\rightarrow$ Controls...

Figure 26.12.1: The Grid Adaption Controls Panel
figure

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



Controlling the Type of Adaption


You can choose to use hanging node or conformal adaption. You can also restrict the adaption process to the addition of grid resolution through refinement and/or the removal of grid density through coarsening.



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 grid.

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 Section  26.4.1.



Limiting the Total Number of Cells


The maximum number of cells is a restriction that prevents FLUENT from creating more cells than required for the present analysis. In addition, it saves the time you spent waiting for the grid adaption process to complete the creation of these cells. However, this premature termination of the refinement process can produce undesirable grid 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 grid 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 grid in the Max # of Cells field. The minimum number of cells in the grid 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 Section  26.2.1. For guidelines for limiting cell sizes and number of cells during dynamic gradient adaption, see Section  26.5.1.



Controlling Node Removal During Conformal Coarsening


You can control the removal of nodes during coarsening by modifying the node removal flags. The node removal flags control which nodes are eligible for possible elimination from the grid.

Note:   The node removal flags apply to conformal adaption only.

For hanging node adaption, only refinement nodes can be removed during coarsening, and they are always removed.

Nodes introduced by refinement are called refinement nodes, and nodes that existed in the mesh before refinement are called original nodes. FLUENT maintains a section in the case file with the node flags. If this section doesn't exist (i.e., when you first read a grid), it identifies all nodes as original nodes. It also distinguishes between nodes on boundary, internal, and periodic zones for both original nodes and nodes created by adaptive refinement.

To guarantee that the original shape of the domain boundaries is maintained, only the nodes introduced by refinement are removed. For example, consider the grid of a rectangular domain. If one of the nodes on the edges of the rectangle is removed the shape is not modified, but if one of the corner nodes of the rectangle is eliminated, the shape is changed from a quadrilateral to a pentagon.

You can modify this default behavior by changing the node removal flags. The most common modification is to allow the removal of original internal nodes. Removing internal nodes does not destroy the shape of the boundary. In fact, it can be very helpful if the initial grid has substantial resolution in a region with minor or no changes in physical features.

Note:   Removing the original boundary or periodic nodes can alter the shape, and in some instances may even destroy the topology, producing a worthless grid. Therefore, be careful when removing original boundary and periodic nodes. Write a case and data file before starting the adaption process. Then, if you generate an undesirable grid, you can restart the process with the saved files.

By default, only the removal of refinement nodes is allowed, as indicated by the enabled status of Boundary - Refined, Internal - Refined, and Periodic - Refined and the disabled status of Boundary - Original, Internal - Original, and Periodic - Original in the Grid Adaption Controls panel. If you want to disable the removal of these types of nodes, do so by turning off the associated check button. Similarly, if you want to enable their removal, turn on the associated check button.


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