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Cavitation Inception Scale Effects: I. Nuclei Distributions in Natural Waters II. Cavitation Inception in a Turbulent Shear Flow

O'Hern, Timothy John (1987) Cavitation Inception Scale Effects: I. Nuclei Distributions in Natural Waters II. Cavitation Inception in a Turbulent Shear Flow. California Institute of Technology , Pasadena, CA. (Unpublished)

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Cavitation scale effects can be grouped into two major categories: susceptibility of the water to cavitation, i.e., the amount, size, and type of microbubbles or microparticulates in the water acting as inception nuclei, and flow field effects due to such factors as velocity and pressure distributions, body size and shape, viscous effects, and turbulent phenomena. Experimental investigations into these two aspects of scale effects were performed in the present study. Field investigations of marine nuclei populations were made using underwater holography to observe microbubbles and particulates, including microplankton in oceanic waters of Los Angeles Harbor, San Pedro Channel and near Santa Catalina Island. Holographic detection was shown to be a reliable method of measuring the nuclei number concentration density distributions. Overall, very high concentrations of the various types of potential cavitation nuclei were observed at all of the test sites and depths examined, although the statistical significance of these results is strong only in the smaller size ranges (less than 50 μm), where a significant number of counts were made. Relatively high bubble concentrations during calm sea conditions, and their population inversion below the thermocline where organism activity was high, indicate a possible biological source of bubble production rather than the usual surface mechanisms of breaking waves and whitecaps. The measured population of particulates is somewhat higher than comparable data in the ocean or in cavitation test facilities, and the number density distribution of particulates decreases approximately as the fourth power of the particle size, as often reported in the literature. An increase in particle concentration near the bottom of the thermocline in clear coastal waters is observed. The total concentration of particles and bubbles in a liquid provides an upper bound on the number of potentially active cavitation nuclei. The measured bubble sizes can be used to indicate that the average tensile strength of the ocean waters examined in this study should be on the order of via few thousand Pascals, with a minimum expected value of about one hundred Pascals. The present results support the recommendation of Billet (1985), that a concentration of at least 3 bubbles per cm^3 in the 5 to 20 μm radius range is needed in test facility water in order to model marine conditions. Experimental studies were also made on the inception processes in a large turbulent free shear layer generated by a sharp edged plate in a water tunnel at Reynolds numbers up to 2 X 10^6. Two distinct types of vortex motion were evident in the shear layer, the primary span wise and the secondary longitudinal vortices. Cavitation inception occurs consistently in the secondary shear layer vortices and more fully developed cavitation is visible in both structures, with the streamwise cavities primarily confined to the braid regions between adjacent spanwise vortices. A Rankine vortex model indicates that the secondary vortex strength is always less than 10% of that of the primary structure. Measurements of fluctuating pressures in the turbulent shear layer are made by holographically monitoring the size of air bubbles injected into the non-cavitating flow, showing that pressure fluctuations were much stronger than previously reported, with positive and negative pressure peaks as high as 3 times the freestream dynamic pressure, sufficient to explain the occurrence of cavitation inception at high values of the inception index. Cavitation inception indices display a strong dependence on the dissolved air content and thus on the availability of freestream bubble cavitation nuclei. The present inception data do not display a clear dependence on freestream velocity (or Reynolds number) but do fall into the overall range of data of previous bluff body investigations. The occurrence of inception in the secondary vortices of the shear layer, and previous reports of velocity dependence of these cores (Bernal 1981) may provide the key to explaining the commonly observed Reynolds number scaling of the inception index in shear flows.

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Additional Information:©1987 Timothy John O'Hern. Report No. Eng. 183.15. This work was supported by the Office of Naval Research under Contract No. N00014-83-K-0506, and by the Naval Sea Systems Command General Hydromechanics Research Program administered by the David Taylor Naval Ship Research and Development Center under Contract No. N000167-85-K-0165. Special thanks to my thesis advisor, Professor A.J. Acosta, whose insight and direction were irreplaceable in the course of this work. I especially appreciate the opportunities I have had to attend a number of technical meetings to present various aspects of this work. Thanks also to Professors R.F. Sabersky, C.E. Brennen and A. Roshko for their continuing interest in my research and welfare. Professor J. Katz of Purdue University, my immediate predecessor on these projects, was a valuable colleague and motivating force during the early stages of this research. Special thanks also to Luca d' Agostino, a current colleague, who has provided much valuable insight in the course of our work together. The friendship and assistance of Elton Daly, Joe Fontana, Rich Eastvedt and Leonard Montenegro of the W .M. Keck Laboratory of Hydraulics and Water Resources were of particular importance in construction and trouble-shooting of the experimental apparatus. I feel indebted to Cecilia Lin for her long hours of drawing and figure preparation needed for this manuscript. Several students have been involved in the data collection and reduction aspects of these experiments, and I would especially like to acknowledge Theresa Rueen-Fang Wang, Sheldon Green, Richard Arrieta and Cindy Morss for their assistance in the holographic testing and analysis. Judy Goldish and Matt Compton were also of great assistance for their support during some of the water tunnel experiments. I very much appreciate the financial support provided by the Earle C. Anthony Fellowship for my first year of graduate study. I would also like to give special acknowledgement of the financial support of myself and this research provided by the Office of Naval Research and by the Naval Sea Systems Command General Hydromechanics Research Program administered by the David W. Taylor Naval Ship Research and Development Center. This support is deeply appreciated. Special thanks to Dr. T.T. Huang of DTNSRDC for his personal involvement and interest in this work. Finally, deepest thanks go to my family for their understanding and support over the years of my formal education. I dedicate this thesis with love to my wife, Charlotte Jesse Welty, whose encouragement and tolerance were often needed and always appreciated.
Funding AgencyGrant Number
Office of Naval ResearchN00014-83-K-0506
Naval Sea Systems Command General Hydromechanics Research ProgramN000167-85-K-0165
Subject Keywords:Cavitation, Cavitation Inception, Cavitation Nuclei, Holography, Ocean Nuclei Distributions, Microbubbles, Shear Flows, Turbulent Structure, Pressure Fluctuations
Record Number:CaltechAUTHORS:20140509-142715209
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:45651
Deposited By: Kristin Buxton
Deposited On:12 May 2014 20:26
Last Modified:12 May 2014 20:26

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