Experimental investigation on hairpin vortex packets in the later stage of bypass transition induced by cylinder wake

The hairpin vortex packets structure and characteristicscale in the later stage of bypass transition induced by cylinder wake areinvestigated by TR PIV technique for both the side-view and top-view cases.The two-dimensional velocity components are correlated with \lambda_ci criterion, and linear stochastic estimation is used to estimate theconditional average of the velocity field. For the side-view case, theconditionally averaged structure consists of a series of swirling motionslocated along a line inclined at a larger angle (17^\circ) from the wall, anda low-speed region occupied by cylinder wake appearing right above it in thefree stream. In the (x, z)-plane at the top of the log layer (y/δ=0.2), the dominant motions are shown to be large-scale regions of lowmomentum elongated along the streamwise and within a spanwise width of0.55δ, whichare consistently bordered by small-scale counter-rotating vortex pairsorganized in the streamwise. The results suggest that in the later part ofthe transition zone, due to the upward induction of wake vortex, the hairpinvortex packets statistically perform a skeleton frame of larger scale androbust physique.