Indeed the literature on the linearized dynamics of clouds of bubbles has grown rapidly (see, for example, Omta ( 23), d'Agostino et al. Later investigators explored numerical methods which incorporate the individual bubbles (Chahine ( 19)) and continuum models which, for example, analyzed the behavior of shock waves in a bubbly liquid (Noordzij and van Wijngaarden ( 20), Kameda and Matsumoto ( 21)) and identified the natural frequencies of spherical cloud of bubbles (d'Agostino and Brennen ( 22))). ( 17) have studied the complicated flow patterns involved in the production and collapse of a cavitating cloud on a hydrofoil.Īnalytical studies of the dynamics of cavitation clouds can be traced to the work of van Wijngaarden ( 18) who first attempted to model the behavior of a collapsing layer of bubbly fluid next to a solid wall. Many authors such as Wade and Acosta ( 6), Bark and van Berlekom ( 4), Shen and Peterson ( 7, 8), Bark ( 9), Franc and Michel ( 10), Le et al. Much recent interest has focused on the dynamics and acoustics of finite clouds of cavitation bubbles because of the very destructive effects which are observed to occur when such clouds form and collapse in a flow (see, for example, Knapp ( 3), Bark and van Berlekom ( 4), Soyama et al.
Common examples of imposed fluctuations are (a) the interaction between rotor and stator blades in a pump or turbine or (b) the interaction between a ship's propeller and the non-uniform wake created by the hull. This temporal periodicity may occur naturally as a result of bubble-filled vortical structures or it may be the response to a periodic disturbance imposed on the flow. In many cavitating flows of practical interest one observes the periodic formation and collapse of a “cloud” of bubbles. However, by combining the computational and experimental observations, we suggest some specific mechanisms which may be active in the dynamics and acoustics of these more complex flows. How these shocks are formed and propagate in the much more complex cloud geometry associated with cavitating foils, propeller or pump blades is presently not clear. Understanding such bubbly flow and shock wave processes is important because these flow structures propagate the noise and produce the impulsive loads on nearby solid surfaces in a cavitating flow. Here we review how the radiated acoustic pulses depend on the governing parameters such as the bubble population density, the cavitation number and the ratio of the bubble size to the cloud size. This confirmed the idea put forward by Mørch and his coworkers who speculated that collapse of the cloud involved the formation of a bubbly shock wave on the surface of the cloud and that inward propagation and geometric focussing of this shock would lead to very large localized pressure pulses.
The computational component continues the earlier work of Wang and Brennen ( 1, 2), which presented numerical solutions of the growth and collapse of a spherical cloud of bubbles. The experiments investigate the effects of reduced frequency, cavitation number and tunnel velocity on the magnitude of these pressure pulses. Two distinct types of pressure pulse were identified from high-speed films: “local pulses” which are registered by individual transducers and appear to be associated with the propagation of localized bubbly shocks and “global pulses” which result from larger scale, coherent collapses of bubble clouds.
#Motion pulse shockwave free download series#
Piezo-electric transducers mounted at a series of locations on the suction surface measured very large positive pressure pulses with amplitudes of the order of tens of atmospheres and with durations of the order of tenths of milliseconds. The experimental program focuses on cloud cavitation formed on the suction surface of a hydrofoil, both static and oscillating. Recent studies have confirmed that the interactions between bubbles as they are manifest in the dynamics of bubble clouds lead to the generation of very large impulsive pressures which, in turn, cause substantial enhancement of the radiated noise and the material damage which results from this form of cavitation. This paper describes experimental and computational investigations of the dynamics of clouds of cavitation bubbles.