NEAR-WALL CHARACTERISTICS OF A TURBULENT BOUNDARY-LAYER OVER RIBLETS

This paper deals with the turbulent boundary layer developing over triangular riblets in a water tunnel. The riblet height (h) and spacing (s) are quite large, which enables a point wise determination of both mean and fluctuating velocities over and within the grooves. All measurements are performed with laser Doppler anemometry. Emphasis is given here on experimental results obtained very close to the wall, where most of the modifications induced by the riblet surface are confined. Combinations of normal to the crest plane and spanwise distributions are presented for the mean longitudinal velocity, rms values of the fluctuations of the three velocity components and for the skewness and flatness factors of the streamwise velociy over riblets. Measurements made within the grooves provide new information about the turbulent field over drag reducing surfaces.