Abstract

The time-dependent flow over riblets of various shapes is calculated using a control volume finite element simulation technique and assuming a model of the near-wall viscous flow region of a turbulent boundary layer. The cross-sectional shapes considered include thin element blade riblets and saw-toothed riblets (but with flat valleys), which are similar to the more common continuous V-shaped riblets. Turbulence statistics derived from the calculations include skewness and flatness factors. It is concluded that to achieve maximum drag reduction, riblets not only need to ensure a thicker than usual viscous laminar-like flow in their grooves, but also the transient momentum exchange must be minimized between this inner region and the larger-scale outer motions associated with quasistreamwise vortices.

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