Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Shock Waves in Cloud Cavitation
Shock Waves in Cloud Cavitation
Pages 756-771
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From page 756... ...
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. The experiments investigate the ejects of reduced frequency, cavitation number and tunnel velocity on the magnitude of these pressure pulses.
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From page 757... ...
They speculated that the collapse of a cloud of bubbles involves the formation and inward propagation of a shock wave and that the geometric focusing of this shock at the center of cloud creates the enhancement of the noise and damage potential associated with cloud collapse. Recently Wang and Brennen (1, 2)
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From page 758... ...
The high speed movies also showed that the local impulses were caused both by crescent-shaped regions of low void fraction and by small bubbly structures. These regions appear to be bounded by bubbly shock waves which are associated with the large pressure pulses.
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From page 759... ...
The output signals are characterized by very large amplitude pressure pulses of very short duration. As exemplified by figure 1, the magnitudes of these pulses are of the order of tens of atmospheres with typical durations of the order of tenths of milliseconds.
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From page 760... ...
The distinction between these two types of pressure pulse can be further elucidated by examining the high speed movies. These movies indicate that local pulses are caused by local disturbances in the cavitation structure while global pulses are generated by a larger scale cloud cavitation collapse.
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From page 761... ...
Consecutive high speed movie frames taken during time interval B of figure 1. by all of the foil transducers and the transducer located in the tunnel floor.
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From page 762... ...
In each figure the calculated impulses are presented for both the global and local pressure pulses detected by the four foil mounted transducers and the transducer located in the tunnel test section floor. Each average acoustic impulse presented in the figures is based on data collected during 40 individual foil oscillation cycles.
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From page 763... ...
Conversely, global pulses always produce subtantial radiated noise which is registered by the floor transducer. The effects of the magnitude of the tunnel velocity are illustrated in figures 10 and 11 for the static and oscillating foils, respectively (in the latter case, since the foil oscillation frequency was held constant at 12.7Hz, there is also a small variation in the reduced frequency as the tunnel velocity changes)
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From page 764... ...
As shown in figure 2 and the photographs, the void fraction increases sharply downstream of the third foil transducer since the sheet cavtiation remains have been carried away from the leading edge of the foil prior to the global cloud collapse. The approximate origins of the global pulses were calculated by noting which transducer registered the earliest global pulse.
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From page 765... ...
Note the formation and inward propagation of a shock wave whose typical structure is illustrated in figure 15. This structure which involves a series of rebounds and secondary collapses, is very similar to that of the gas/liquid shocks investigated by Noordij and van Wijngaarden (20)
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From page 766... ...
of the initial cloud size to the dimension of the low pressure region, Ao/D, were exercised in order to explore the range of possible phenomena. Wang and Brennen (1, 2)
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From page 767... ...
Moreover, there are further but weaker shocks which arrive at the center and thus produce a train of acoustic impulses which, eventually, lead to a regular oscillation ofthe cloud at the first cloud natural frequency. These results suggest that shock focusing may be one of the major mechanisms for the enhanced noise and damage potential associated with cloud cavitation.
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From page 768... ...
The discrete boundaries of the crescent-shaped regions and the motion of these boundaries relative to the bubbly flow suggest that these boundaries are bubbly shock waves. The collapse of vapor bubbles within these waves could explain the large pressure impulses measured on the foil surface.
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From page 769... ...
The reduced frequency is an important parameter in the determination of the total impulse level and the local and global pulse distribution. The cavitation number has a significant effect on the global impulse strength, but large impulses are still present on the foil surface at values of a where acoustic radiation is minimal and global pulses are rare or non-existent.
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From page 770... ...
and Brennen, C.E., "On the acoustical dynamics of bubble clouds," ASME Cavitation and Multiphase Flow Forum, 1983, pp.
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From page 771... ...
ASME Cavitation and Polyphase Flow Forum, 1981, pp.
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