Premature transition to supercritical flow with bubbly flow around a circular cylinder

Vortex induced vibration (VIV) experienced during flow past a cylinder can reduce equipment performance and in some cases lead to failure. Previous studies have shown that the shift in shedding frequency and vibration amplitude under the influence of gas injection at the upper subcritical range can produce a premature shift to supercritical flow (and the drag crisis). To date, the influence of the gas distribution along the cylinder span has not yet been investigated. Time-resolved particle image velocimetry (TR-PIV), proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) of the wake structures, as well as bubble image velocimetry (BIV) are used to assess the flow topology changes under the influence of spanwise uniform and spanwise discontinuous gas injection. We demonstrate that for gas injected along the span of the cylinder, a premature shift to supercritical flow occurs even at volumetric qualities below 0.025%, which is lower than has been previously shown in literature. For gas injected along the central 1.3 D of the channel (30% of the channel width), a local transition to supercritical flow occurs at the channel centerline; however, the wake recovers to that of subcritical flow by 3.6 D downstream, as mixing occurs with the predominantly single-phase flow to either side of the bubble injection. This downstream transition in the shedding frequency resembles that of single-phase dual step cylinders, which to the author’s knowledge has not yet been shown to occur under two-phase conditions. At two-phase supercritical flow, for R e D = 360,000, we demonstrate a significant shift in near-wake gas motion and vortex shedding frequency, with gas motion driven by vortex interaction in the separated shear layer. • Time-resolved flow and vibration measurements were conducted for two-phase flow over a cylinder. • A premature transition to supercritical flow was observed at volumetric qualities as low as 0.02%. • For gas injection only along the central cylinder span, a subcritical vortex street was found to reform downstream. • Potential mechanisms are proposed for the transition to supercritical flow, as well as the reformation of the subcritical vortex street. • Gas was found to preferentially collect in the separated shear layer for supercritical flow at Re D = 360,000.