Many applications in aerodynamic, hydrodynamic and in pipelines can be greatly benefited from any significant amount of drag reduction. Longitudinal microgrooves aligned in the flow direction known as riblets stand as the most convenient and practical techniques applied in real applications for drag reduction. Considerable studies have been devoted to the development of an optimum shape of riblets in order to improve the net drag reduction. Different shapes including the L, V, and U cross-sections have been investigated experimentally and numerically. Initial experimental studies on riblets were conducted by Walsh [1, 2] at NASA Langley Research Center. They were devoted to optimize the riblet size and shape for a maximum drag reduction. Suzuki and Kasagi [3] used PTV for measuring the velocity field in near triangular riblet region while Park and Wallace [4] used hot-wires in their measurements. Both Choi [5] and Tang and Clark [6] examined the influence of L-shaped riblets on burst frequency using flow visualization and hot wire anemometry. Bechert et al. [7, 8] have investigated more thoroughly different configurations of riblets including rectangular, scalloped and shark-skin-shape riblets.
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