Assigning velocity as a boundary condition

Hi
I am modeling an electrophoretic phenomena where the governing equations are
- Navier Stokes equati
- Poisson equation
- Nernst Plank equation

This is an axisymmetric model containing a half circle (sphere) at the symmetry boundary. I need to assign a z- velocity over the circle. The available BC are sliding wall and Moving/Leaking wall. The sliding wall seems to give the tangential component of velocity which should work good for the plane boundary not for a circular boundary. And the Moving/Leaking wall it gives phi velocity edit field which made me really confused. What is this Phi velocity? Is it z-velocity here? If so why such named?

With regards
Tariq