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Twenty-Second Symposium on Naval Hydrodynamics
both in the dynamics and the acoustics. In many of these cases the coherent collapse of the cloud of bubbles can cause a substantial increase in the radiated noise and potential for damage. Much recent interest has focused on the dynamics and acoustics of finite clouds of cavitation bubbles because of these very destructive effects (see, for example, Knapp (5), Bark and van Berlekom (6), Soyama et al. (7)). The purpose of the computational investigations outlined in this paper is to explore the various phenomena which can occur in the dynamics anfd acoustics of these bubbly cavitating flow and, where possible, to compare these with experimental observations.
1.Some Recent Experiments on Cloud Cavitation
Numerous investigators (for example, Knapp (5), Wade and Acosta (8), Bark and van Berlekom (6), Shen and Peterson (9,10), Bark (11), Franc and Michel (12), Hart et al. (13), Kubota et al. (14,15), Le et al. (16), de Lange et al. (17), Kawanami et al. (18)) have studied the complicated flow patterns involved in the production and collapse of cloud cavitation on a hydrofoil. The basic features of the cyclic process of cloud formation and collapse (whether on a stationary or oscillating foil) are as follows. The growth phase usually involves the expansion of a single attached cavity, at the end of which a re-entrant jet penetrates the cavity from the closure region. This penetration breaks the cavity into a bubbly cloud which collapses as it is convected downstream.
The radiated noise all occurs during this bubbly part of the cycle. It consists of pressure pulses of very short duration and large magnitude. The pulses have been observed and measured by a number of investigators including Bark (11), Bark and van Berlekom (6), Le et al. (16), Shen and Peterson (9,10), McKenney and Brennen (19) and Reisman et al. (20). More recently, Reisman et al. (1) have made measurements of the impulsive pressures on the suction surface of a hydrofoil (within the cloud cavitation), have simultaneously measured the radiated pulses and have correlated these pressure traces with images from high-speed movies. Very large pressure pulses were recorded by the surface transducers, with typical magnitudes as large as 10bar and durations of the order of 10−4s. These help to explain the enhanced noise and cavitation damage associated with cloud cavitation. For example, the large impulsive surface loadings due to these pulses could be responsible for the foil damage reported by Morgan (21), who observed propeller blade trailing edges bent away from the suction surface and toward the pressure surface.
Reisman et al. (1) also correlated high-speed movies of the clouds with the pressure measurements and found that the pressure pulses recorded (both on the foil surface and in the far field) were associated with specific structures (more precisely, the dynamics of specific structures) which are visible in the movies. Indeed, it appears that several types of propagating structures (shock waves) are formed in a collapsing cloud and dictate the dynamics and acoustics of collapse. One type of shock wave structure is associated with the coherent collapse of a well-defined bubble cloud which separates from the rear of the cavitation zone and is convected downstream. This type of structure causes the largest impulsive pressures and radiated noise. The pulses it produces are termed global pulses since they are recorded almost simultaneously by all transducers. The high-speed motion pictures showed that the cavitation cloud undergoes a rapid and coherent collapse and that these collapses generate the global pulses. It was noted that such cloud collapses do not involve large and rapid changes in the volume of the cloud. Rather, they involve rapid and propagating changes in the void fraction distribution within the cloud.
Unexpectedly, two other types of structures were observed. Typically, the pulses produced by these structures were registered by only one transducer and these events were therefore termed local pulses. They are recorded when an ephemeral, localized and transient low void fraction structure forms in the bubbly cavitating cloud and happens to pass over the face of the transducer. While these local events are smaller and therefore produce less radiated noise, the pressure pulse magnitudes are almost as large as those produced by the global events. How and why these low void fraction structures (shock waves) form in the flow is not at all clear.
Finally, we note that injection of air into the cavitation on the suction surface can substantially reduce the magnitude of the pressure pulses produced (Ukon (22), Arndt et al. (23), Reisman et al. (24)). However Reisman et al. (25) have shown that the bubbly shock wave structures still occur; but with the additional air content in the bubbles, the pressure pulse magnitudes are greatly reduced.