TURBULENT BOUNDARY-LAYER AND THE EFFECT OF CRITICAL ROUGHNESS REYNOLDS-NUMBERS ON THE RECOVERY LENGTH BEHIND AN ISOLATED SPHERICAL ROUGHNESS ELEMENT UNDER VARIABLE PRESSURE-GRADIENT

The transition characteristics and the boundary layer development behind an isolated spherical roughness element were investigated in an open-circuit, suction-type wind tunnel. The experiments were performed upon a smooth aluminiuim flat plate placed in the test section of the tunnel. The desired pressure gradient was obtained by means a false roof placed in the test section. In this article, emphasis is given to the recovery length of the turbulent boundary layer behind an isolated spherical roughness element under zero, favorable, and adverse pressure gradienst. The recovery length of the turbulent boundary layer is correlated with the critical roughness Reynolds number REkce and Rekcs