The sweep down flow and contra-hairpin vortex in a turbulent boundary layer

The sweep down flow and related flow structures in a turbulent boundary layer were visualized with hydrogen bubble method. The streamwise and transverse vortices induced by sweep down flow near the wall of a vessel were observed. A new kind of vortical structure, whose features are contrary to those of the hairpin vortex, was observed in the present experiments. It is termed contra-hairpin vortex here. Its head points upstream, while the head of a hairpin vortex points downstream; lift-up flow is induced between the legs of a hairpin vortex, while between the legs of this new vortical structure, sweep down flow is induced.