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

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