Dissecting coherent motions in extreme wall shear-stress events within adverse-pressure-gradient turbulent boundary layers

Coherent motions associated with extreme wall shear-stress events are investigated for adverse-pressure-gradient turbulent boundary layers. The analyses are performed using wall-resolved large eddy simulations of a NACA0012 airfoil at angles of attack of $9$ and $12^{\circ}$ and Reynolds number $4\times 10^5$ . The suction side exhibits attached TBLs which develop under progressively stronger APGs. A quadrant decomposition of Reynolds shear stress shows that sweeps and ejections dominate the momentum exchange between the mean and fluctuating fields, with the intensity of sweeps near the wall growing more rapidly with APG strength. Probability density functions of wall shear stress reveal a higher frequency of backflow events and an increased distribution symmetry with stronger APGs. Conditional statistics indicate that backflow events originate from inner-layer sweep motions that transport high-momentum fluid toward the wall, followed by ejections that drive local deceleration. In such cases, the intensity of ejections is modulated by the APG strength. In contrast, extreme positive events are primarily characterised by sweep motions. For both types of extreme events, inward and outward interactions become increasingly significant as the APG strengthens. The dynamics of the coherent turbulent structures and their interactions are examined using conditional flow field analyses. For extreme positive events, stronger APGs lead to shorter high-speed streaks, while the associated sweep motions generate spanwise velocities that increasingly influence the near-wall dynamics. In the case of backflows, stronger APGs shorten low-speed streaks and amplify high-speed structures associated with sweep motions, promoting spanwise alignment of vortical structures. Overall, APGs modify the structure and dynamics of extreme near-wall events by reshaping the balance and spatial organisation of sweep- and ejection-dominated motions.