The characteristics of bubbly shock waves in a cavitating venturi via time-resolved X-ray densitometry*

Partial cavitation is a common form of cavitation, where unstable attached vapour cavities are formed and shed periodically, leading to cloud cavitation. Classically, it was believed that a periodically generated re-entrant jet at cavity closure, travelling upstream below the vapour cavity solely is responsible for the cloud cavitation. However, studies show that bubbly shock waves or condensation shock waves emanating from the periodic collapse of a relatively large cavitation cloud is an alternate mechanism that can cause cavity detachment. To this end, the flow conditions that favour the formation of such condensation shock waves remain less explored in the literature. Hence, quantifying the time-resolved vapour fraction of the cavitating bubbly mixtures becomes imperative. We employ the high-speed X-ray densitometry facility at TNO-Ypenburg (The Netherlands) to measure the vapour fraction in the cavitating axisymmetric venturi with a temporal resolution of 1/3600 seconds. The sharp change in density (vapour fraction) across the retracting vapour cavity measured by x-ray densitometry combined with high-frequency dynamic pressure measurements revealed that the condensation shock wave is the only cavity destabilizing mechanism at low σ. The condensation shock wave characteristics such as the propagation velocity, the vapour fraction and the pressure rise across the shock-front as a function of σ are studied. Further, we examine the variation of shock-wave characteristics as it propagates upstream. The vapour fraction fields along with static pressure measurement inside the attached vapour cavity allow us to gain insight into the possible role of compressibility of the bubbly cavitating mixture in cloud cavitation. This further help to explain the dominance of `bubbly shock wave' over 're-entrant jet' as a mechanism driving the periodic cloud shedding in an axisymmetric venturi.