Observations of Cavitating Flow
This paper will present a review of some of the recent advances in our understanding of the dynamics and acoustics of cavitating flows. We focus first on the individual events which evolve from a single travelling nucleus and describe observations of the intricate micro-fluid-mechanics which affect both the bubble shape and the subsequent emission of noise. These phenomena have important consequences in terms of their implications for the scaling of cavitation damage and noise. We also present calculations of the interaction between the individual traveling bubbles and the irrotational flow outside of the boundary layer of the headform. Comparisons of predicted and experimentally observed bubble shapes show qualitative agreement but further work is necessary to understand the details of the interactions between the viscous boundary layer and the bubble. To model the processes of cavitation inception, noise and damage it is necessary to generate a model of the cavitation event rate which can then be coupled with the consequences of the individual events. In the second part of this paper we describe recent efforts to connect the observed event rates to the measured distributions of cavitation nuclei in the oncoming stream. Such studies necessarily raise questions regarding the nuclei distributions in water tunnels and in the ocean and it would seem that we still know little of the nuclei population dynamics in either context. This is illustrated by a few observations of the population dynamics in a particular facility. The third subject addressed in this paper is the question of the noise produced by an individual travelling cavitation event. It is shown that the distortions in the shape of cavitation bubbles leads to acoustic impulses which are about an order of magnitude smaller than those predicted by the spherical bubble dynamics of the Rayleigh-Plesset equation. However, at the higher cavitation numbers, the upper bound on the experimental impulses scales with speed and size much as one would expect from the spherical bubble analysis. Initially, as the cavitation number is decreased, the impulse increases as expected. But, beyond a certain critical cavitation number, the noise again decreases in contrast to the expected increase. This phenomenon is probably caused by two effects, namely the interaction between events at the higher event densities and the reduction in the impulse due to a change in the dominant type of cavitation event. From the single event we then move to the larger scale structures and the interactions which occur when the density of the events becomes large and individual bubbles begin to interact. One of the more important interaction phenomena which occur results from the behaviour of a cloud of cavitation bubbles. Most previous theoretical studies of the dynamics of cavitating clouds have been linear or weakly non-linear analyses which have identified the natural frequencies and modes of cloud oscillation but have not, as yet, shown how a cloud would behave during the massively non-linear response in a cavitating flow. We present non-linear calculations which show the development of an inwardly propagating shock wave during the collapse phase of the motion. These observations confirm the earlier speculation of Mørch and his co-workers.
In writing this review, I came to recognize, again, the debt I owe to a series of outstanding graduate students who contributed much of the work described herein. My profound thanks to Luca d'Agostino, Steven Ceccio, Douglas Hart, Sanjay Kumar, Yan Kuhn de Chizelle, Beth McKenney, Zhenhuan Liu, Yi-Chun Wang and Garrett Reisman. As always, my friend and colleague Allan Acosta provided valuable insights and inspiration. I am also deeply grateful for the support of the Office of Naval Research who sponsored much of the research and supported the preparation of this paper under Contract N00014-91-J-1295.