19.18. Convergence Acceleration for Stretched Meshes (CASM) with the Local Pseudo Time Step Method

When using the Local Time Step pseudo time method with the pressure-based coupled solver, you can accelerate the convergence of your solution on highly-stretched and anisotropic meshes (like those used when modeling external aerodynamic problems) by enabling the Convergence Acceleration For Stretched Meshes option in the Solution Methods task page. This option increases the local pseudo time step size for each high-aspect-ratio cell by multiplying it by the aspect ratio or by a user-defined cut-off multiplier (whichever is smaller). This is available for steady and transient simulations. You can also specify how the high-aspect-ratio cells are identified by enabling either a standard or an enhanced formulation of CASM.

To use the CASM option with the local pseudo time step method, perform the following steps:

Enable beta feature access (as described in Introduction).
Select the pressure-based coupled solver, and select Local Time Step from the Pseudo Time Method drop-down list in the Solution Methods task page. For details on this setup (along with the models that limit the availability of the Local Time Step method or render it unsupported), see Using the Local Pseudo Time Step Method with the Pressure-Based Coupled Solver.
Enable the Convergence Acceleration For Stretched Meshes option in the Solution Methods task page.
Figure 19.7: The Solution Methods Task Page

Note that by default, the cut-off multiplier is set to 100.
You have the following options:
- To use the standard formulation for CASM with a user-defined cut-off multiplier (for example, 20), perform the following in the text user interface:
```
>solve/set/pseudo-time-method/convergence-acceleration-for-stretched-meshes
Use convergence acceleration for stretched meshes (CASM)? [yes] yes
Enter CASM cut-off multiplier : [] 20
```
- To use the enhanced formulation for CASM with a user-defined cut-off multiplier (for example, 20), perform the following in the text user interface:
```
>solve/set/pseudo-time-method/convergence-acceleration-for-stretched-meshes
Use convergence acceleration for stretched meshes (CASM)? [yes] no
Use enhanced CASM formulation? [no] yes
Enter CASM cut-off multiplier : [] 20
```
  The enhanced formulation for CASM may be good for meshes where the cells have very high aspect ratios and the flow is aligned with these highly-stretched cells.
Note:
- A good initial solution field is sometimes needed for the CASM option to work efficiently.
- The maximum benefit of the CASM option is realized when the local flow is aligned with the mesh stretching.