TURBULENT BOUNDARY-LAYER IN AN ADVERSE PRESSURE-GRADIENT - EFFECTIVENESS OF RIBLETS

Measurements of skin friction by means of a drag balance and of veloci ty profiles by means of a hot wire have been carried out in the bounda ry layer over a flat plate in a wind tunnel. A convergent/divergent te st section produces a moderate adverse pressure gradient in the diverg ent part. The purpose of the present study is to investigate the chara cteristics of this boundary layer and to examine the effectiveness of riblets for skin-friction reduction. We find that the mean velocity fo llows a logarithmic profile that is shifted downward with respect to t he logarithmic law for a zero-pressure-gradient boundary layer The pro file of the streamwise velocity fluctuations exhibits two maxima. Appl ication of riblets in this case leads to a skin-friction reduction of 13%, which implies a considerable increase over the 6% obtained in the zero-pressure-gradient boundary layer. Measurements with a hot wire s how that only close to the wall is the mean velocity affected by the p resence of riblets and that streamwise velocity fluctuations are reduc ed here.