Rotating boundary in fluid flow problem

Hi,

I am quite new to Comsol and am trying to model the stationary fluid flow around a rotating circle within a rectangle. I'm a bit unsure of how to speicfiy the rotating boundary condition upon the internal circle. I've tried defining the angular velcoity as two parameters and speicifying this as the boundary velocity; U_x = -(sqrt(x^2+y^2)RPMsin(atand(y/x))) U_y = (sqrt(x^2+y^2)RPMcos(atand(y/x))) Any help is much apperciated.

Thanks, Dónal