Could you tell me something abou re-circulation? Thank you.

Recirculation is where a fluid element goes round in circles. It usually occurs in separated flows, or before/after obstructions, depending on your geometry. It's just like a standing vortex. In terms of CFD, areas of circulation tend to be transient (i.e. they fluctuate) and as such are an obstacle to convergence for steady state solutions. With specific reference to CFX, the CFX inlet/outlet BC's really don't like having recirculation going on near them, and it will start putting walls on these boundaries to stop inflow. If you get this problem, try and move your boundaries away from the area of recirculation.

Thank you very much.