Abstract
Experimental study of a 3-D riblet has been carried out. The lateral spacing of the 3-D riblet surface is sinusoidally varied in the streamwise direction. The obtained maximum drag-reduction rate is about 12% at the bulk Reynolds number of 3400. The flow structure over the 3-D riblet is analyzed in the velocity field obtained by using 2-D Particle Image Velocimetry and is compared with the corresponding flow over the flat surface. Although the lateral spacing of the riblet is larger than the diameter of stramwise vortices, vortices are inhibited approaching to the wall due to characteristic flows in the region near the sinusoidal riblet surface.
