Reisman, Garrett Erin (1997) Dynamics, acoustics and control of cloud cavitation on hydrofoils. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-03302004-140539
Cloud-cavitation, often formed by the breakdown and collapse of a sheet or vortex cavity, is responsible for severe cavitation noise and erosion damage. This thesis describes an experimental investigation of the dynamics and acoustics of cloud cavitation on a three dimensional hydrofoil and examines the injection of air as a means of noise suppression.
Part one of this work examines the large amplitude impulsive pressures which were measured on the suction surface of an oscillating hydrofoil experiencing cloud cavitation and these pressure pulses are correlated with the observation of shock waves propagating through the bubbly mixture. Recess mounted transducers were used to measure unsteady pressures at four locations along the chord of the suction surface of a hydrofoil. By examining the transducer output, two distinct types of pressure pulses were identified. Local pulses occurred at a single transducer location and were randomly distributed in position and time. Conversely, global pulses were registered by all the transducers almost simultaneously. The location of the global pulses relative to the foil oscillation was quite repeatable and these events produced substantial far-field noise. Correlation of the transducer output with high speed movies of the cavitation revealed that the global pulses were produced by a large scale collapse of the bubble cloud. Conversely, local pulses were generated by local disturbances in the bubbly mixture characterized by large changes in void fraction.
The large pressure pulse associated with the local and global cavitation structures, the geometric coherence of their boundaries and the nearly discrete change in void fraction across the boundaries of these structures indicate that these structures consist of bubbly shock waves. Qualitative and quantitative comparisons between the current experiments and the numerical, analytic and experimental bubbly shock wave analysis of other investigators support this conclusion.
Part two of this work examines the dramatic reduction in cloud cavitation noise due to both continuous and pulsed injection of air into the cavitating region of the foil. At sufficient air flow rates, the radiated noise could be reduced by a factor greater than 200 relative to the noise produced without air injection. Unsteady surface pressure measurements also showed a reduction in the acoustic impulse with air injection by a factor of up to two orders of magnitude. An explanation for this noise reduction can be found by examining the high speed motion pictures. The presence of the non-condensible gas in the cavitation cloud is shown to prevent any rapid or coherent collapse process. Although the formation of local structures is still observed, the presence of air in the bubbles diminishes both the magnitude and the frequency of occurrence of local pressure pulses. Finally, pulsed air injection results in a lower acoustic impulse than the impulse obtained by injecting the same mass of air continuously over the entire oscillation cycle.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Division:||Engineering and Applied Science|
|Major Option:||Mechanical Engineering|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||24 September 1996|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||30 Mar 2004|
|Last Modified:||26 Dec 2012 02:36|
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