Explanation needed: buoyancy->coupled->implicit...
 

Hi, I'd be greateful if somebody could explain me why using coupled solver with implicit formulation is the best choice for buoyancy driven flows? Why coupled and why implicit. BTW: segregated unsteady gave me as good results as coupled implicit steady solving Ra=10^9 square enclosure with temperature differential on the vertical walls. Does coupled solver use some pseudo time-steps? Marching or so? Cause there should be the reason for CFL number appearence in solution control. Thanks a lot.