Statistical scale of hairpin packets in the later stage of bypass transition induced by cylinder wake

The hairpin packet's structure and its statistical scale in the later stage of bypass transition induced by a cylinder wake are investigated by time-resolved particle image velocimetry from the side and top view, respectively. Linear stochastic estimation is used to achieve the conditionally averaged velocity fields. For the side view case, the conditionally averaged structure consists of a series of swirling motions located along a line inclining at a large angle (18A degrees) from the wall and a low-speed region occupied by the cylinder wake appearing right above them. In the (x, z)-plane at the wall-normal height y/delta A = 0.2, the dominant structures are shown to be the large-scale regions of low momentum elongated almost over 3 delta along the streamwise. The low-speed regions are consistently bordered by small-scale counter-rotating vortice pairs organized in the streamwise with a statistical spanwise width of 0.55 delta. The results suggest that in the later part of the transitional zone, the upward induction of the cylinder wake enhances both the wall-normal and spanwise extent of the hairpin packets.