Pulse-burst PIV of an impulsively started cylinder in a shock tube for Re > 10⁵

The impulsive start of a circular cylinder in a shock tube was characterized with time-resolved particle image velocimetry measurements (TR-PIV) at 50 kHz using a pulse-burst laser. Three Reynolds numbers Re of 1.07, 1.63 and 2.46 × 10⁵ were studied adding insight into the transient process near the drag crisis. In all cases, vorticity was maximum in the first pair of vortices formed. In a fashion analogous to previous studies at Re ≤ 10⁴, a single symmetric vortex pair was first shed from the cylinder at Re = 1.07 × 10⁵ prior to the eventual transition to a von Kármán vortex street. In contrast, at Re ≥ 1.63 × 10⁵, two or more symmetric vortex pairs were first shed. The non-dimensional time for the wake to begin to exhibit asymmetry was also found to be lower at the two higher Re. The time required to reach a fully antisymmetric wake (peak von Kármán shedding) was roughly five times the asymmetric onset time. Altogether, the study indicates a transformation in the impulsive wake structure and associated time scales to occur at Re near 1.6 × 10⁵.