Effect of wavy wall tilting on turbulent separation over an airfoil surface

This study employs large eddy simulations to examine turbulent boundary-layer (TBL) separation control using tilted wall waviness embedded in a NACA 4412-type surface under adverse pressure gradient conditions at a Reynolds number based on friction velocity and TBL thickness of Reτ=2500. Building on prior work [Kamiński et al., Phys. Fluids 37, 075157 (2025)], where streamwise-aligned sinusoidal waviness (WS) increased wall shear stress (τw) and delayed separation, we examine whether tilting of the waviness improves the effect observed for WS. Two tilt directions are analyzed: counter-streamwise (WLT—waviness left tilt) and streamwise (WRT—waviness right tilt). We compare their impact on τw and the separation point with that observed for WS and an uncorrugated surface (S), which serves as the baseline. We show that the WLT shape substantially delays separation, doubling the WS effect, whereas WRT advances it, performing worse than S. These trends are linked with the near-wall flow alterations: WLT suppresses the occurrence of a localized reverse flow regions (backflows) and aligns TBL with the freestream, while WRT enhances backflows. Moreover, spectral analysis shows that wall shape markedly alters the turbulent scales and the interactions between small- and large-scales. Specifically, small-scale energy is amplified considerably for WLT, leading to a marked increase in τw. In contrast, WRT suppresses small-scale activity, reducing τw and, thus, leading to an earlier and more pronounced separation.