### 1.11.3. Mesh Adaption Limitations

Mesh Adaption in ANSYS CFX 18.2 is subject to the following limitations:

- Mesh adaption is limited to allocate 2^31-1 words of 4 byte integer stack space; this limits the maximum problem size for mesh adaption to approximately 80 million elements (structured) and 200 million elements (unstructured).

- Mesh adaption cannot be used in multi-domain simulations or in cases with domain interfaces, porous domains, elapsed time control, or external solver coupling. Mesh adaption also cannot be used for transient, mesh-motion, radiative-tracking, or particle-transport cases.

- The mesh may not be coarsened such that original mesh elements are combined. Mesh elements that have been refined in earlier adaption steps may be coarsened, back to their original mesh elements, but not beyond.

- You must specify how much adaption takes place by specifying the maximum number of nodes in the final mesh. You cannot specify that mesh adaption should take place until the mesh stops changing or until a particular solution criterion is satisfied.

- The method used is not well-suited to meshes with many high-aspect ratio elements, because it can only refine elements in an isotropic manner.

- The method cannot improve the quality of the existing mesh.

#### 1.11.3.1. Mesh Adaption Tips

- More than one adaption step applied to a surface mesh with no underlying geometry can lead to convergence problems with transonic problems.

- If you set Mesh Adaption Criteria Method to `Solution Variation` without specifying Minimum Edge Length, then you may over-refine in regions where there are discontinuities. If you want to adapt meshes containing geometrically small features, you can exploit the adaption capability without limitation by not setting any minimum edge length.

- If you set Mesh Adaption Criteria Method to `Variation * Edge Length`, then you will not over-refine discontinuities. This method places an emphasis on adapting long edges with large variations of solution variable in preference to short edges with large variations of solution variable. This method will also refine long edges with a small variation of solution variable.

- Adaption criteria values for each node will be computed as the average value of all edges connected to the node, and these values will be stored in the results file for visualization purposes.

- If you find that the mesh adaption appears to have missed a discontinuity in a solution by refining the mesh in front of or behind the discontinuity, then your solution was not sufficiently converged before mesh adaption took place. As a solution containing a discontinuity develops, the location of such a discontinuity may move. If the solution is not sufficiently converged before mesh adaption takes place, then the mesh will be refined at the location of the discontinuity, which will move as convergence continues. A lower Target Residual can be set on the Mesh Adaption/Advanced Parameters tab to correct this problem.

Release 18.2 - © ANSYS, Inc. All rights reserved.