Hi. If I want to simulate the shape of free surface in a stirred tank. In addition to normal gravity -9.8 m/sec^-2, do I need to assign the centrifugal force by omega^2*r in body force term? Or CFX can assign this centrifugal force automatically. Do I need to use cylindrical coordinate system in this question ?

You dont need to assign a centrifugal force.

If the tank is static and you stir the tank explicitely with a paddle then the free surface shape wil emerge as part of the solution assuming you have some sort of gaseous phase at the top of the tank to accomodate the distortion of the free surface.

You dont need to use a CCS.