[Lee97]

Hello guys, I am trying to solve rotor aerodynamics using Actuator Surface Model.

When I run calculation, the calculation convergence well for 20 ~ 30 iterations.
But, after 20 ~ 30 iteration the velocity is exploded, and residuals are divergence.

Do you have any solutions or wisdoms?

[Pavlord]

Sounds to me like you have some areas where your velocity increases over 20 iterations enough to make your simulation unstable.

Have you monitored the maximum Courant number for your later iterations?

I would guess that sometimes before the 20th iteration the max Courant number passes it's critical value. If that is the case you should probably look to refine your mesh in the places where you expect the highest velocities.

If you are not familiar with this, I recommend looking up a video about this on the Fluid Mechanics 101 YouTube channel.

[kinematic_presser]

There are a lot of reasons why this could be happening. As mentioned by Pavlord the Courant number being too high can cause this. But also choosing bad boundary conditions can cause this as well. You need to provide more detail.