Physics
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........51k&link_type=abstract
Thesis (PH.D.)--CALIFORNIA INSTITUTE OF TECHNOLOGY, 1992.Source: Dissertation Abstracts International, Volume: 52-11, Section: B
Physics
Bubble Layers, Harmonic Cascading
Scientific paper
This work investigates two aspects of cavitation noise. First, some nonlinear interactive effects have been investigated by studying the frequency response of a bubble layer next to a wall oscillating normal to itself. Next, the collapse process of a bubble has been acoustically investigated. The flow in a bubble layer has been modelled by continuum conservation equations and the Rayleigh-Plesset equation has been used to include bubble dynamics. Finite thickness of a bubble layer results in characteristic natural frequencies of the layer, all of which are less than the natural frequency of a single bubble. Natural frequencies of the layer are determined by the void fraction and ratio of the layer thickness to the bubble radius. The response is maximum for excitation at the smallest layer natural frequency and consists of equally significant first and second harmonics. This response is increased by proximity of the lowest layer natural frequency to about half of the bubble natural frequency. The response is seen to form standing waves in the layer for excitation frequencies less than the bubble natural frequency but decreases rapidly with distance from the wall for larger excitation frequencies. A phenomenon termed harmonic cascading is seen to take place when the layer consists of bubbles with distribution of sizes. In this phenomenon, a strong second harmonic response is observed when the layer contains bubbles with natural frequency equal to twice the excitation frequency. The ratio of the second harmonic to the first harmonic increases when number of small bubbles is increased relative to the number of large bubbles. In acoustical study of the bubble collapse, the collapse process was observed to consist of up to two successive collapses with each collapse producing a single acoustic pulse. Each of these pulses was seen to have one or more peaks. These features characterized the acoustic emission and their occurrence and effects on some measurable characteristics such as spectra have been investigated for two different velocities and three different cavitation numbers. The investigation covered travelling bubble cavitation around two different headforms with different pressure fields and flow patterns.
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