Bubbly shock propagation as a mechanism for sheet-to-cloud transition of partial cavities

Abstract

Partial cavitation in the separated region forming from the apex of a wedge is examined to reveal the flow mechanism responsible for the transition from stable sheet cavity to periodically shedding cloud cavitation. High-speed visualization and time-resolved X-ray densitometry measurements are used to examine the cavity dynamics, including the time-resolved void-fraction fields within the cavity. The experimentally observed time-averaged void-fraction profiles are compared to an analytical model employing free-streamline theory. From the instantaneous void-fraction flow fields, two distinct shedding mechanisms are identified. The classically described re-entrant flow in the cavity closure is confirmed as a mechanism for vapour entrainment and detachment that leads to intermittent shedding of smaller-scale cavities. But, with a sufficient reduction in cavitation number, large-scale periodic cloud shedding is associated with the formation and propagation of a bubbly shock within the high void-fraction bubbly mixture in the separated cavity flow. When the shock front impinges on flow at the wedge apex, a large cloud is pinched off. For periodic shedding, the speed of the front in the laboratory frame is of the order of half the free-stream speed. The features of the observed condensation shocks are related to the average and dynamic pressure and void fraction using classical one-dimensional jump conditions. The sound speed of the bubbly mixture is estimated to determine the Mach number of the cavity flow. The transition from intermittent to transitional to strongly periodic shedding occurs when the average Mach number of the cavity flow exceeds that required for the generation of strong shocks.

Investigation of Unsteady Sheet/Cloud Cavitation in the Divergent Section of a Nozzle With Emphasis on the Mechanism of Shock Wave Propagation

“…It should be noted that during the propagation of shock wave, there exists sharp decrease in void fraction at the shock front, while during the development of re-entrant flow, no evident void fraction change is found. The sharp decrease in void fraction within the cavity will significantly change the cavity dynamics through changing the local speed of sound, and thus the local Mach number [21]. The propagation speed of the shock wave within the attached cavity based on both the high-speed video and the averaged gray level contour is estimated to be 7.12 m/s.…”

“…To confirm the characteristics of the shock wave, the one-dimension bubbly shock wave relationship is applied. Ignoring the bubble dynamics of the shocking process, the speed upstream of the discontinuity in the reference frame of the shock is obtained in eqn (9) [21]. ,…”

“…This indicates the possibility of the existence of shock wave propagation mechanism of cavitation instabilities. Reisman et al [18], Arndt et al [19], Leroux et al [20], and Ganesh et al [21], have discussed the role of bubbly shock wave in the breakup and shedding of cloud cavity. Although the re-entrant flow mechanism and shock wave propagation mechanism of sheet/cloud cavitation are identified in the experiments, more detailed experiments are still needed to investigate the difference of the interaction between the synchronized cavity developments, and the corresponding pressure fluctuations under the two breakup and shedding mechanisms of sheet/cloud cavitation.…”

Fluid dynamics of acoustic and hydrodynamic cavitation in hydraulic power systems

“…The motion of the re-entrant liquid jet is central to the periodic shedding of the cavitation cloud, but the mechanism that drives the phenomenon is still not fully known [54]. It has been observed [60] that the re-entrant jet is dominant during the earlier stages of the instability, whereas a propagating shock wave appears during the later stages for the intensive cloud-shedding phase [61].…”

The speed of sound in a gas–vapour bubbly liquid

“…For example, the experiments of Reisman et al [1] showed that condensation shocks cause large pressure spikes which influence the behaviour of sheet cavities (e.g. [2,3]), and recent work has demonstrated that condensation shocks are responsible for the transition from stable to cloud-shedding cavities [4].…”