Turbulent boundary layer in an adverse pressure gradient - Effectiveness of riblets

Measurements of skin friction by means of a drag balance and of velocity profiles by means of a hot wire have been carried out in the boundary layer over a flat plate in a wind tunnel. A convergent/divergent test section produces a moderate adverse pressure gradient in the divergent part. The purpose of the present study is to investigate the characteristics of this boundary layer and to examine the effectiveness of riblets for skin-friction reduction. We find that the mean velocity follows a logarithmic profile that is shifted downward with respect to the logarithmic law for a zero-pressure-gradient boundary layer. The profile of the streamwise velocity fiuctuations exhibits two maxima. Application 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 show that only close to the wall is the mean velocity affected by the presence of riblets and that streamwise velocity fluctuations are reduced here.