77 巻 (2011) 782 号 p. 1892-1903
An experimental study of a three-dimensional (3-D) blade-type riblet has been performed. The lateral spacing of the 3-D riblet surface is periodically changed in the streamwise direction. The net drag reduction rate of about 9% has been confirmed in a low-speed wind channel at the bulk Reynolds number of 4000. The flow structure over the 3-D riblet was also analyzed in the velocity field by using 2-D Particle Image Velocimetry in x-z and x-y planes and was compared with the corresponding flow over the flat surface in an attempt to identify the physical mechanisms for drag reduction. Under the drag-reducing condition, the streamwise length of the streaky structures becomes shorter, which is the different phenomenon from those for well-known 2-D riblets. Moreover, quadrant analysis has been conducted to investigate the events contributing to the Reynolds shear stress. The second (ejection) and forth (sweep) quadrants are decreased, whereas it was interesting to note that the first (outward) and third (inward) quadrants are increased near the 3-D riblet.