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Some theoretical and experimental studies of cavitation noise

Citation

Kumar, Sanjay (1992) Some theoretical and experimental studies of cavitation noise. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-01052006-142036

Abstract

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This work investigates two aspects of cavitation noise. The first part models some nonlinear interactive effects in bubbly mixtures generated in cavitating flows, and the second part focuses on an acoustical study of the collapse process of a single bubble in travelling bubble cavitation.

The nonlinear interactive effects in a bubbly cloud have been studied by investigating the frequency response of a bubble layer bounded by a wall oscillating normal to itself. First, a Fourier analysis of the Rayleigh-Plesset equation is used to obtain an approximate solution for the nonlinear response of a single bubble in an infinite fluid. This is used in an approximate solution of the oscillating wall problem for bubble layers of finite and infinite thickness in which all the bubbles have the same equilibrium size and a semi-infinite layer containing bubbles with a distribution of size. Particular attention is paid to the generation of harmonics that is due to nonlinear effects.

The finite thickness of the layer results in characteristic natural frequencies of the bubble mixture, all of which are less than bubble natural frequency. These characteristic natural frequencies are functions of the void fraction and the ratio of layer thickness to the bubble radius. In general, the lowest characteristic natural frequency is found to dominate the response. The amplitude of the response increases as the excitation frequency, [...], is reduced from [...] to around 0.5[...] and decreases with further decrease in excitation frequency. The characteristic frequencies disappear in the limit of a semi-infinite layer. The bubble size oscillation in a semi-infinite layer is maximum at the excitation frequency of [...]. The pressure oscillation is minimum at the excitation frequency of [...] with equally significant first and second harmonic components.

For sub-resonant and trans-resonant excitation [...], the response consists of standing wave patterns with an amplitude that decays slowly with distance from the oscillating wall. This decay is different from that found in spherical bubble clouds (d'Agostino and Brennen 1988a) because of the geometric effects of propagating disturbance. However, for super-resonant excitation the amplitude of oscillation rapidly decays with distance from the source of excitation.

A phenomenon termed harmonic cascading is seen to take place when the bubble layer consists of bubbles with a distribution of bubble sizes. In this phenomenon a large response is observed at twice the excitation frequency when the layer contains bubbles with a natural frequency equal to twice the excitation frequency. The effect is manifest as an increase in the ratio of the second harmonic to the first harmonic as the number of bubbles with small radii gets larger relative to the number of bubbles with large radii. Also, a similar change in the bubble size distribution, while holding the equilibrium void fraction constant, results in a weaker response. This reduction in amplitude of pressure oscillation may be due to the increased number of bubbles. Larger void fraction and smaller amplitudes of wall oscillation are observed to produce a weaker response. Reduced effects of viscocity and surface tension that are due to changes in ambient conditions result in a larger response.

In the second part the collapse processes of single bubbles in the travelling bubble cavitation around two axisymmetric headforms have been studied acoustically to understand the collapse process of a cavitation bubble and to characterize the sound emission in travelling bubble cavitation. The bubbles were observed to collapse and then sometimes to rebound and collapse again, resulting in one or two pulses in the acoustic signal from a cavitation event. It was observed that each of the pulses could contain more than one peak. This phenomenon is called multipeaking and is clearly distinct from rebounding. The occurrence of rebounding and multipeaking and their effects on some characteristic measures of the acoustic signal such as power spectra are examined in this chapter. Two particular head-forms (I.T.T.C. headform and Schiebe headform) with distinct flow characteristics were investigated.

Both rebounding and multipeaking increased with reduction in cavitation number in case of the I.T.T.C. headform. However, multipeaking decreased and rebounding increased with the reduction in cavitation number for the Schiebe headform. Smaller flow velocity, smaller cavitation number and multipeaking delay the rebound. The peak amplitude of the sound emitted from the first collapse was seen to be twice as large as the peak amplitude of sound from the second collapse suggesting a repeatable process of bubble fission during the collapse process. The multipeaking and rebounding increased the characteristic measures of the acoustic signal. These characteristic measures have larger magnitudes for smaller flow velocity. Also, the values of these characteristics are larger for the I.T.T.C. headform than for the Schiebe headform.

Theoretical calculations based on the Rayleigh-Plesset equation were seen to predict correctly the order of magnitude for most of these characteristic measures. However, the distribution of spectral energy is not properly predicted by the model based on the Rayleigh-Plesset equation; bubble fission during the collapse is thought to account for this discrepancy. Reduction in the cavitation number and multipeaking are observed to decrease the fraction of spectral energy contained in the high frequency range (30 kHz-80kHz).

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Mechanical Engineering
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Brennen, Christopher E.
Thesis Committee:
  • Unknown, Unknown
Defense Date:2 August 1991
Record Number:CaltechETD:etd-01052006-142036
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-01052006-142036
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:33
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:05 Jan 2006
Last Modified:26 Dec 2012 02:26

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