The Noise Generated by the Collapse of a Cloud of Cavitation Bubbles
CaltechAUTHORS
View Archive InfoField  Value  
Title 
The Noise Generated by the Collapse of a Cloud of Cavitation Bubbles


Creator 
Wang, Y.C.
Brennen, C. E. 

Subject 
Caltech Library Services


Description 
The focus of this paper is the numerical simulation of the dynamics and acoustics of a cloud of cavitating bubbles. The prototypical problem solved considers a finite cloud of nuclei that is exposed to a decrease in the ambient pressure which causes the cloud to cavitate. A subsequent pressure recovery then causes the cloud to collapse. This is typical of the perturbation experienced by a bubble cloud as it passes a headform or the blade of a ship propeller. The simulations employ the fully nonlinear, nonbarotropic, homogeneous flow equations coupled with the RayleighPlesset dynamics for individual bubbles. This set of equations is solved numerically by an integral method. The computational results confirm the early speculation of Morch and his coworkers (Morch 1980 & 1981, Hanson et al. 1981) that an inwardly propagating shock wave may be formed in the collapse of a cavitating cloud. The structure of the shock is found to be similar to that of the steady planar shocks analyzed by Noordij and van Wijngaarden (1974). The shock wave grows rapidly not only because of the geometric effect of an inwardly propagating spherical shock but also because of the coupling of the single bubble dynamics with the global dynamics of the flow through the pressure and velocity fields (see also Wang and Brennen 1994). The specific circumstances which lead to the formation of such a shock are explored. Moreover, the calculations demonstrate that the acoustic impulse produced by the cloud is significantly enhanced by this shockfocusing process. Major parameters which affect the dynamics and acoustics of the cloud are found to be the cavitation number, [sigma], the initial void fraction, [alphazero], the minimum pressure coefficient of the flow, [C Pmin], the natural frequencies of the cloud, and the ratio of the length scale of low pressure perturbation to the initial radius of the cloud, [D/Azero], where D can be, for example, the radius of the headform or chord length of the propeller blade. We examine how some of these parameters affect the far field acoustic noise produced by the volumetric acceleration of the cloud. The nondimensional farfield acoustic impulse produced by the cloud collapse is shown to depend, primarily, on the maximum total volume of the bubbles in the cloud normalized by the length scale of the low pressure perturbation. Also, this maximum total volume decreases quasilinearly with the increase of the cavitation number. However, the slope of the dependence, in turn, changes with the initial void fraction and other parameters. Nondimensional power density spectra for the farfield noise are presented and exhibit the [equation] behavior, where n is between 0.5 and 2. After several collapse cycles, the cloud begins to oscillate at its natural frequency and contributes harmonic peaks in its spectrum. 

Publisher 
American Society of Mechanical Engineers


Date 
199508


Type 
Book Section
PeerReviewed 

Format 
application/pdf


Identifier 
http://authors.library.caltech.edu/172/1/WNG153.pdf
Wang, Y.C. and Brennen, C. E. (1995) The Noise Generated by the Collapse of a Cloud of Cavitation Bubbles. In: Cavitation and GasLiquid Flow in Fluid Machinery and Devices, 1995: Presented at the 1995 ASME/JSME Fluids Engineering and Laser Anemometry Conference and Exhibition, August 1318, 1995, Hilton Head, South Carolina. Fluids Engineering Division. Vol.FED226. No.226. American Society of Mechanical Engineers , New York, NY, pp. 1729. ISBN 0791814815 http://resolver.caltech.edu/CaltechAUTHORS:WNGcglffmd95 <http://resolver.caltech.edu/CaltechAUTHORS:WNGcglffmd95> 

Relation 
http://resolver.caltech.edu/CaltechAUTHORS:WNGcglffmd95
http://authors.library.caltech.edu/172/ 
