Dissecting turbulence structures arising in extreme wall-shear stress events of adverse pressure gradient boundary layers

Extreme events and intermittent features within turbulent boundary layers (TBLs) can manifest as coherent motions that influence the boundary layer dynamics. In order to investigate the effects of adverse pressure gradients (APGs) on coherent flow structures, wall-resolved large-eddy simulations (LES) are performed for a NACA0012 airfoil at 9 and 12 deg. angle of attack. The simulations are conducted for a Reynolds number Re = 4 x 10⁵ and freestream Mach number M = 0.2. The boundary layers are tripped near the leading edge to ensure bypass transition. Despite the high angles of attack, the mean flow remains attached on the suction side although the APG, computed in terms of the Clauser parameter, exhibits a sharp increase toward the trailing edge. The APG effects on extreme wall-shear stress events are assessed with respect to both extreme positive (EP) and negative (backflow, BF) events. These events are selected when the wall-shear stress exceeds 2 times its root-mean-square value, and are evaluated regarding the turbulence statistics and spatial support for different airfoil chord positions. A spatio-temporal conditional analysis shows that sweeps are the coherent motions that prevail for the formation of both EP and BF events. In the case of EP, a sweep structure transports high-speed fluid from the outer region towards the wall, and interacts with a high-speed streak. This interaction generates streamwise-aligned vortices which are more elongated in positions where the APG is lower, and that become shortened at downstream positions where the APG is higher. In contrast, for BFs, a sweep generated by a large-scale outer structure interacts with a near-wall low-speed streak, leading to the formation of a BF vortex that is spanwise aligned at positions of higher APGs, and becomes more streamwise aligned for lower APG conditions.