I run a case about swirl flow. Z-axis is centre axis. Is it related to the coodinates to show tangetial velocity results?

The way to do this is:

Define the rotation axis of the fluid in >Define >Boundary Conditions ( 0, 0, 1 (x,y,z) in your case). NB you can either do this pre- or post- solution.

Now you can simply report cylindrical velocities (axial, tangential and radial) anywhere in the domain.

Hope this helps,

Angus