Abstract
Experimental study of a new three-dimensional (3-D) blade riblet is carried out. The lateral spacing of our 3-D riblet surface is periodically changed in the streamwise direction. Obtained drag reduction rate is about 6% at Re_τ= 150. The flow structure over the 3-D riblet was also analyzed in instantaneous velocity fields by using 2-D particle image velocimetry on a horizontal near-wall plane and compared with the corresponding flow over the flat surface in an attempt to identify the physical mechanisms that cause drag reduction. Moreover, the spanwise two-point correlation R_<u'u>(Δz^+) length increases on the riblet surface, indicating that the local friction velocity u_τ is decreased. It is also found that the streamwise persistence of the streaky structures becomes shorter on the drag reducing surface.
