尖锐后缘尾流中的反向发卡涡
The unsteady organization and the evolution of thecoherent structures within the immediate wake of the sharpsymmetric trailing-edge of a NACA0012 airfoil is investigatedusing two-component (2C) and tomographic particle imagevelocimetry (Tomo-PIV). The inspection of the Tomo-PIVresults showed new vortical structures, introduced here ascounter-hairpin vortices, evolve in the wake region to directthe turbulent mechanism towards recovery of the wake deficit.These vortical structures are topologically similar to thehairpins of a turbulent boundary layer as they appear to be Ushapedbut with inverted orientation. The spanwise portion ofa counter-hairpin vortex is in the vicinity of the wakecenterline and the legs are inclined at an approximately 60degree to the wake axis in the downstream direction. Theypartially wrap around the high speed streaks and frequentlysweep the high-speed flow towards the wake centerline. Apattern recognition algorithm is applied to establishcharacterization based on an ensemble averaged counterhairpinvortex. The formation mechanism of the counterhairpinvortices is attributed to the shear layer produced alongthe wake centerline by the neighboring of a low and a highspeed streak which promotes formation of spanwise vorticesthat form the vortex by connection to quasistreamwisevortices.