Mixing vessels generally use two domains. The impeller domain is a small, rotating domain that encloses the impeller. The rest of the tank is represented by a stationary domain. Different types of domain interfaces are available for the connection between the stationary and the rotating domains. The recommended types are:
Frozen Rotor: faster but cruder
Transient: slower (transient analysis) but much more accurate
The choice of a steady-state or transient simulation is dependent on the type of interface that exists between the two domains. Where a Frozen Rotor interface is used, a steady-state simulation is usually carried out. Performing a transient simulation allows you to use the transient rotor/stator frame change model to account for transient effects.
The initial guess for velocity can be set to zero in the relative frame for each domain.