Hi all, I am using moving wall boundary condition (velocity at wall is given so that it behaves like a momentum supply through wall but no motion of wall)...I am facing problem of density going negative near the wall if the wall velocity is very high. Can anybody suggest why it happens and what is the treatment for the problem. Thank you.

If it is happening when the wall is moving away from the fluid then you may simply have too large a time step. If it is happening when the wall moves into the fluid, you may have a sign error. How do you have this condition implemented? What are you solving for? How are the pressure and velocity behaving in the wall region?