Cavitation inception acoustic characterization and localization in a reverberant environment
Dr. Natasha Chang
3:30 PM on 2008-04-09
Akerman Hall 227
Cavitation remains the primary phenomenon that limits the performance of hydrodynamic machinery. In Naval applications, identifying the parametric region of cavitation inception – the boundary that separates non-cavitating from cavitating flow – is an enduring priority. Traditionally, cavitation inception has been studied optically, but for cases where the cavitation bubbles are sub-visual, randomly distributed in time throughout a non-trivial region of space, or not optically accessible, their cavitation acoustic signature could be used. In this presentation, an acoustic localization method is applied in a reverberant environment (a water tunnel test section) to locate and characterize cavitation inception from pairs of parallel unequal strength, counter-rotating vortices. Cavitation occurs in this flow as the vortex pair undergoes the Crow instability, and the weaker, secondary vortex is deformed and stretched by the stronger, primary vortex. The stretching results in greatly reduced secondary-vortex core pressures. Cavitation nuclei present in the flow can then incept in the secondary vortex at static pressures higher than that required for cavitation inception in either unmodified vortex. Such vortex-interaction inception occurs randomly in time and in a relatively large spatial domain. The acoustic localization method uses an array of receiving hydrophones to record the cavitation sound pulses, and subsequent signal processing produces an estimated source location. This acoustic localization method was validated by direct comparisons to optical localization of 53 cavitation bubbles. The acoustic scheme provided an unambiguous location estimate for all 53 bubbles. The average distance between the optical and acoustic bubble locations was 18.4 mm, or ~1/8 of the wavelength of the primary acoustic-signal frequency (10kHz). Also, the frequency content of the acoustic signal during bubble inception and growth was related to the volumetric oscillations of the bubble. The maximum correlation was of 84%. Furthermore, it was found that only about half of the bubbles produced a sharp, broadband, popping sound. The other half produced a short tone burst with frequencies on the order of 1 to 6 kHz. (Sponsored by ONR Codes 333 and 321)