The geometry of this case is a rectangular vessel with two-padlle impellor inside. There are therefore 2 velocity components.
The vessel is filled with the liquid initial state of which is characterized by difference in concentration of inert contaminant: the upper part of the vessel is pure liquid while lower part contaminant concentartion is equal to unity.
The paddle is supposed to be suddenly set in rotation.
The computational task is to predict the mixing, represented by the subsequent distributions of velocity, pressure and time-average concentration.