The mesh has a great influence on the solver convergence and solution of every CFD simulation. The user is strongly advised to check the quality of a numerical mesh. Several parameters that reflect the quality of a mesh are available. In CFD-GEOM, under the tab Mesh/Mesh Query Options, the user can check 2D as well as 3D mesh quality. In figure 1, 3D Mesh Queries for Aspect Ration has been called. See figure 2 for an example for the Centroid to Face Angle.

Figure 1. Mesh Queries tool

Aspect Ratio
If the model contains thin layers, which are mostly used as boundary layer or for the different layers of the MEA inside fuel cells (see figure1), it may require placing several cells normal to the plate (thickness). Due to this, aspect ratios of several thousands can occur (as shown in figure 1). According to our experience, aspect ratios for such models up to 1500 are acceptable, for flow, heat and electrochemistry. If the same geometry is used for a solid stress analysis, this ratio must be reduced down to 15. The reason for this great discrepancy may be in the different solver structure. It is strongly recommended to check this ratio in CFD-GEOM before writing out the DTF file.

Centroid/Face Angle
Especially in unstructured meshes, very small centroid to face angle can occur (see figure 2), mostly inside thin layers but also in arbitrary interfaces. When arbitrary interfaces are called, a common, new interface with additional cells is automatically created the solver. The quality of these cells will be checked by the solver and printed out in the CFD-ACE output file to inform the user about the status of this new interface (see figure 3). Models containing many bad cells as shown in figure 3 will not run successfully. By changing the edges mesh distribution and/or density as well as the number of cells, one can reduce the number of bad cells. As soon as the warning message no longer occurs, the solver will start running satisfactorily.

Figure 2. Centroid/Face Angle quality tool

Figure 3. Arbitrary interface data in output file

Volume
Curved surfaces or splines can create overlapping areas. Due to this, cells with negative volumes can occur. Mesh refinement and/or repairing of geometrical data can resolve this problem. The user can determine the number of such bad cells in CFD-GEOM as well as in the output file.

If you have any questions about this tip or would like us to discuss other topics in the future, please let us know.

Regards,
Hans W. Mindt
ESI CFD Support Team