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Sina Ghaemi

Fulvio Scarano
Department of Aerodynamics Delft University of Technology Kluyverweg 2, 2629HT Delft, the Netherlands


The unsteady organization and the evolution of the coherent structures within the immediate wake of the sharp symmetric trailing-edge of a NACA0012 airfoil is investigated using two-component (2C) and tomographic particle image velocimetry (Tomo-PIV). The inspection of the Tomo-PIV results showed new vortical structures, introduced here as counter-hairpin vortices, evolve in the wake region to direct the turbulent mechanism towards recovery of the wake deficit. These vortical structures are topologically similar to the hairpins of a turbulent boundary layer as they appear to be U-shaped but with inverted orientation. The spanwise portion of a counter-hairpin vortex is in the vicinity of the wake centerline and the legs are inclined at an approximately 60 degree to the wake axis in the downstream direction. They partially wrap around the high speed streaks and frequently sweep the high-speed flow towards the wake centerline. A pattern recognition algorithm is applied to establish characterization based on an ensemble averaged counter-hairpin vortex. The formation mechanism of the counter-hairpin vortices is attributed to the shear layer produced along the wake centerline by the neighboring of a low and a high speed streak which promotes formation of spanwise vortices that form the vortex by connection to quasistreamwise vortices.