An experimental investigation into the breakdown of riblet drag reduction at post-optimal conditions

Abstract

A long-standing question in riblet research is why drag reduction only occurs within a small, non-dimensionally scaled envelope, outside of which drag is significantly increased. For riblets with viscous-scaled spacings that are much larger than those required for drag reduction, one hypothesis is that the riblets exhibit k-type, ‘fully rough’ behaviour. However, this seems counter-intuitive since fully rough behaviour is typically associated with a dominance of pressure drag over viscous drag, and yet riblets can sustain no pressure drag.

This study aims to investigate this issue by conducting single normal hot-wire traverses above a trapezoidal riblet surface, over a range of drag-increasing viscous-scaled riblet spacings. Novelty was added by also measuring within the riblet valleys, providing a unique look at the turbulent behaviour within them.

Previously proposed mechanisms for the breakdown in drag reduction have included lodgement of turbulence within the riblet valleys, and the development of a Kelvin–Helmholtz instability, but neither mechanism appears active in our results. They instead show a reduction in turbulent energy as riblet spacing increases, despite a significant increase in drag, which does seem to be approaching a k-type roughness asymptote as hypothesised. This may be caused by the generation of time-invariant secondary flows above the riblet tips and corners of the riblet valleys, although this will require further investigation.