Riblets in a turbulent boundary layer under favorable pressure gradients

The drag-reducing effects of riblets have been extensively studied in zero-pressure-gradient (ZPG) turbulent wall-bounded flows. However, their performance in non-equilibrium turbulent boundary layers (TBLs) remains largely unexplored. We perform direct numerical simulations of TBLs subjected to a favorable pressure gradient (FPG), comparing one case over a smooth wall with another over resolved riblets with resolution up to 7200x308x4800. The FPG is induced by an accelerating freestream velocity designed to initiate the FPG at $Re_{\delta}=6,800$ and ultimately increase the flow velocity threefold. It creates a bell-shaped acceleration parameter distribution along the 80$\delta$-length of the TBL, inducing both re-laminarization and re-transition. Prior to the FPG, the riblets are in the conventional ZPG drag-reducing regime ($s^+=16$, $l^+=10$). Shortly after the FPG begins to act, while still very mild, the riblets become drag augmenting. In this region, $s^+$ and $l^+$ remain within the ZPG drag-reducing range. Drag penalty from eddy penetration, common in ZPG flows, does not explain the increase here. Instead, it is linked to a delayed turbulence response to elevated shear. Following re-laminarization, riblets also accelerate and augment re-transition. The formation of strong spanwise-oriented roller vortices near the riblet crest and their interaction with residual upstream turbulence characterize this process, leading to another local rise in wall stress.