On the relationship between drag modification and vertical velocity fluctuations in flow over riblets

We carry out DNS of flow over triangular and rectangular riblets in a wide range of size and Reynolds number, with the goal of establishing the mechanisms controlling drag reduction. It is found that drag reduction is primarily associated with the capability of inhibiting vertical velocity fluctuations at the plane of crests, as in SHS/LIS devices. This is efficiently achieved in DNS by artificial suppression of v, which yields large drag decrease, proportional to the riblets size. A parametrization of the drag reduction effect in terms of the vertical velocity variance is found to be quite successful to account for variation of the controlling parameters. A Moddy-like diagram is introduced which incorporates the effect of slip velocity and a single,geometry-dependent parameter. Lower drag-reduction efficiency of SHS/LIS-like riblets is found as compared to cases with artificially imposed slip velocity.