The main aim of this work is understanding the phenomenon of B enard cells in laminarand turbulent regimes as an application of studying the air flow in the air gap and honey comb cells that installed in the flat plate solar collector. Furthermore analyzing the turbulent Rayleigh-B enard convection components by performing direct numerical simulation of turbulent air ow in too long inclined cavity like the air gap. The fi rst two chapters treat with known problems of laminar and turbulent flow as a necessary presentation of numericalmethods used in solving the governing equation of flow motion and heat transfer, afterward the third chapter deals with the main object of investigating the turbulent and laminar Rayleigh-B enard convection flow.