An experimental study of a new three-dimensional (3-D) riblet has been carried out. The lateral spacing of our 3-D riblet surface is sinusoidally varied in the streamwise direction (see Fig. 3). In the comparison of the optimal two-dimensional (2-D) blade riblet which shows 9.9% drag reduction rate [1], the riblet height, thickness and averaged lateral spacing are respectively 0.83, 5 and 2.5 times larger than those of the optimal 2-D riblet in wall units. The net drag reduction rate of 11.7% has been confirmed in a low-speed wind channel at the bulk Reynolds number of 3400. The flow structure over the 3-D riblet mounted a wall was also analyzed in the velocity field by using 2-D Particle Image Velocimetry and was compared with the corresponding flow over the flat surface in an attempt to identify the physical mechanisms for the drag reduction. The normal turbulent intensities on the present riblet are almost same as those of the flat surface, whereas the Reynolds shear stress is much decreased, and especially becomes negative near the riblet height. These are different phenomena from those of all the previous riblets [1–7].

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