sinusoidal boundary
 

I am trying to impose a sin(omega*t) velocity type boundary on the a cylinder sidewall in Fluent 5. In cylindrical coordinates, the velocity vector can be decomposed into (r,theta,z) components. The velocity component that I want to impose my boundary condition is the theta sometimes called the tangential component. My axis of rotation in cartesian coordinates is the y-axis. This makes the x-z plane as the plane of motion. In Fluent 5, I executed the following commands:Define->Boundary Conditions->Moving Wall->Components. This gives me the option to hook my UDF to one of the cartesian (x,y, z) components. This problem can be solved easily in Fluent 4 using the Harmonic boundary condition option. I would rather not do that because the required structured mesh is a little bit difficult ot generate. Question: How can I easily (possibly without writing additional code) specify the theta component of velocity instead of the cartesian (x, y, z) options that are available?