Abstract

This talk will mainly review the PhD thesis of Garazi Gómez-de-Segura on anisotropic permeable substrates. Similar to riblets, these substrates are conceived to produce a change in drag through two competing mechanisms. The first, which is drag-reducing, consists in imposing different apparent origins to the mean flow and the turbulent fluctuations. The second, which is drag increasing, consists in the excitation of a Kelvin-Helmholtz-like instability just above the substrate. We will present results from direct simulations, which show that these surfaces can substantially outperform riblets, while exhibiting the same drag-altering mechanisms, by exploiting them more optimally. We will also present low-order models, obtained from linear analysis, which predict accurately the performance of these substrates.