Attached Decelerating Turbulent Boundary Layers over Riblets

Turbulent boundary layers (TBLs) over riblets subjected to freestream adverse pressure gradients (APGs) are investigated by direct numerical simulation. The reference zero-pressure-gradient (ZPG) boundary layer is initialized at a Reynolds number of $Re_{\delta} =6800$ before entering the APG region. Downstream, the TBLs then develop under the APG for 30 boundary layer thickness ($\delta$). We examine two APG strengths (peak $\beta \approx$ 5 and 10), three riblet sizes ($s^+$=16, 26, and 60), and corresponding smooth-wall cases. This case matrix allows us to evaluate riblet-induced drag modification and the associated physical mechanisms across drag-modulation regimes previously identified for ZPG riblet flows. The flow remains attached in all the cases. An immersed boundary method, together with fine grid resolution, is employed to resolve the flow within the riblet grooves and the TBL. The simulations show that, under APGs, riblets can reduce drag far beyond what has been achieved in ZPG flows. For large riblet sizes under strong APG, the riblets even generate an upstream force, corresponding to negative drag. The substantial drag reduction is attributed to Kelvin–Helmholtz (KH) roller vortices that form near the riblet crest. The vortices’ size, strength, and wavelength are founded to be governed primarily by the local APG at their generation site. The results presented here suggest that existing drag-characterization metrics, as well as criteria for the onset of KH rollers, may require modifications when the pressure gradient is non-negligible.