  An investigation of turbulent boundary layers (TBLs) is presented for a
NACA0012 airfoil at angles of attack 9 and 12 deg. Wall-resolved large eddy
simulations (LES) are conducted for a freestream Mach number M = 0.2 and
chord-based Reynolds number Re = 4x10^5, where the boundary layers are tripped
near the airfoil leading edge on the suction side. For the angles of attack
analyzed, mild, moderate and strong adverse pressure gradients (APGs) develop
over the airfoil. Despite the strong APGs, the mean flow remains attached along
the entire airfoil suction side. Similarly to other APG-TBLs investigated in
the literature, a secondary peak appears in the Reynolds stress and turbulence
production profiles. This secondary peak arises in the outer layer and, for
strong APGs, it may overcome the first peak typically observed in the inner
layer. The analysis of the turbulence production shows that other components of
the production tensor become important in the outer layer besides the shear
term. For moderate and strong APGs, the mean velocity profiles depict three
inflexion points, the third being unstable under inviscid stability criteria.
In this context, an embedded shear layer develops along the outer region of the
TBL leading to the formation of two-dimensional rollers typical of a
Kelvin-Helmholtz instability which are captured by a spectral proper orthogonal
decomposition (SPOD) analysis. The most energetic SPOD spatial modes of the
tangential velocity show that streaks form along the airfoil suction side and,
as the APG becomes stronger, they grow along the spanwise and wall-normal
directions, having a spatial support along the entire boundary layer.
