Diverging Rotating Analyses

Issue

Some rotating-region analyses diverge after appearing to converge if allowed to continue running.

Solution

There are two typical scenarios that lead to divergence in rotating region simulations:

Scenario 1:

This can simply be due to the need for solution under-relaxation

Try using the following solution control parameters (accessed with on the Solution Control button on the Solve dialog).

pressure = 0.25
velocities = 0.25
turbulence = 0.1
eddy_visc = 0.1

These settings improve most “difficult” cases.

Another option is to enable Intelligent Solution Control (from the Solution control button on the Solve dialog). This limits the rotation to 3 degrees/time step, but is often effective for eliminating severe local velocity variations.  

Scenario 2:

This problem can be caused by a poor quality mesh.

Enable the Stream Function result (Solve dialog>Results Quantities>Stream Function).

This writes out the Nodal Aspect Ratio as a results quantity. Plot the Nodal Aspect Ratio to determine the location of poor mesh quality. Areas that show values of 10,000 and higher should be adjusted. Either adjust the mesh size or modify the CAD model to eliminate the geometry that is causing the severe mesh gradients.