Experimental characterisation of Kelvin-Helmholtz rollers over riblet surfaces

Abstract

The formation of Kelvin-Helmholtz-like rollers (referred to as K-H rollers) over riblet surfaces has been linked to the drag-increasing behaviour seen in certain riblet geometries, such as sawtooth and blade riblets, when the riblet size reaches sufficiently large viscous scales (Endrikat et al. (2021a), J. Fluid Mech. 913, A37). In this study, we focus on the sawtooth geometry of fixed physical size, and experimentally examine the response of these K-H rollers to further increases in viscous scaled riblet sizes, by adopting the conventional approach of increasing freestream speeds (and consequently, the friction Reynolds number). Rather than continual strengthening, the present study shows ..