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Fig. 13. Vorticity contour plots of pipe jet events with increasing levels of background flow (U/up) for the cases of L/D=4.3 (A,C,E) and L/D=16 (B,D,F). (A,B) Background flow velocity less than piston velocity, U/up=0.5 (U=5 cm s-1; up=10 cm s-1; D=2.39 cm). (C,D) Background flow velocity equal to piston velocity, U/up=1.0 (U=5 cm s-1; up=5 cm s-1; D=2.39 cm). (E,F) Background flow velocity greater than piston velocity, U/up=2.0 (U=5 cm s-1; up=2.5 cm s-1; D=2.39 cm). The magnitudes of vorticity contours are 0.4, 0.5, 1.0, 1.5 and 2.0 rad s-1. See explanation of contour identification in Fig. 2. (A) t=3 s, t/{Delta}tj=3, (B) t=3 s, t/{Delta}tj=0.8, (C) t=4 s, t/{Delta}tj=2, (D) t=4.7 s, t/{Delta}tj=0.6, (E) t=8 s, t/{Delta}tj=2, and (F) t=6 s, t/{Delta}tj=0.4.