The initial guess for velocity is calculated in the same way as for Cartesian velocity components. For details, see Automatic Values.

If you suspect that the flow is predominantly in a particular direction through the domain (for example, in the direction of an Inlet specification), you can initialize the velocity components to this direction. You should not set a velocity higher than the inlet velocity.

If you believe that the automatic prediction will be unsuitable for your simulation, but do not want to specify Cartesian components, setting the velocity scale to zero imposes a zero pressure and velocity field throughout the domain. For details, see Velocity Scale.

An explanation of how coordinate frames interact with velocity components is available in Coordinate Frame.

Information on modeling multiphase flow is available in Initial Conditions for a Multiphase Simulation.