Attached Decelerating Turbulent Boundary Layers over Riblets

TL;DR

This study uses direct numerical simulation to explore how riblets influence drag in turbulent boundary layers under adverse pressure gradients, revealing enhanced drag reduction and vortex dynamics compared to zero-pressure-gradient conditions.

Contribution

It provides new insights into the effects of adverse pressure gradients on riblet-induced drag reduction and vortex formation, extending understanding beyond zero-pressure-gradient flows.

Findings

Riblets reduce drag more effectively under APGs than ZPG flows.

Extreme cases show riblets can generate negative drag (upstream force).

Kelvin-Helmholtz vortices are intensified during APGs, affecting drag.

Abstract

Turbulent boundary layers over riblets subjected to adverse pressure gradients (APGs) are investigated by direct numerical simulation. Multiple APG strengths and riblet sizes are examined, permitting evaluation of drag modification by riblets, and associated physical mechanisms, in various regimes established for zero-pressure-gradient (ZPG) riblet flows. The APG strengths are selected such that the flow remains attached. It is found that during APGs, riblets reduce drag beyond what has been achieved in ZPG flows. In extreme cases, an upstream force (i.e., negative drag) is attained. The significant drag reduction is found to be a product of Kelvin-Helmholtz roller vortices forming near the riblet crest, which are augmented in size, strength, and frequency during the APG. The preliminary results reported here indicate the need to modify existing metrics to predict drag reduction and the onset of KH rollers by riblets when the pressure gradient is non-negligible. Further analysis will be documented in the final paper.