Experimental study on drag-reduction effect due to sinusoidal riblets in turbulent channel flow

The drag-reduction effect of a three-dimensional sinusoidal riblet surface is experimentally evaluated in a fully developed turbulent channel flow. The lateral spacing of the adjacent walls of the riblet is varied sinusoidally in the streamwise direction. The obtained maximum total drag-reduction rate is approximately 12 % at a bulk Reynolds number of 3,400. The flow structure over the sinusoidal riblet surface is also analyzed in the velocity field by using two-dimensional particle image velocimetry. The velocity field is compared with the corresponding flow over a flat surface. It is found through pathlines and Reynolds shear stress analyses that the drag-reduction mechanism is similar to those of two-dimensional riblets. A different point is that the present riblet respectively induces a downward and upward flows in the expanded and contracted regions, which prevent vortices from hitting the bottom wall with wider lateral spacing of the riblet. In consequence, the wetted area of the present sinusoidal riblet is smaller than those of two-dimensional riblets, resulting in the high drag-reduction effect.