Adverse-pressure-gradient effects on extreme events and intermittency in the turbulent boundary layer of a NACA0012 at 12° angle of attack

We investigate the effects of adverse pressure gradients (APGs) on extreme and intermittent events in the turbulent boundary layer (TBL) of a NACA0012 airfoil at 12 deg. angle of attack. A wall-resolved large-eddy simulation (LES) is performed for a Reynolds number Re = 4 x 10(5) and freestream Mach number M = 0.2. Boundary-layer tripping is enforced near the leading edge to produce bypass transition. Despite the high angle of attack, the mean flow remains attached throughout the airfoil suction side, although the adverse pressure gradient exhibits a steep rise towards the trailing edge. Results of a quadrant analysis for the Reynolds shear-stress distribution show that sweeps are predominant near the wall, while ejections dominate in the outer region. The former are the main contributor to the inner peak of turbulence production. Due to the strong APG, a secondary peak of production arises in the outer layer, and the combination of both sweeps and ejections contribute to such peak. A backflow characterization is performed, demonstrating that as the APG increases, the magnitude of the friction coefficient decreases, leading to a higher probability of such events near the trailing edge.