Observations of the Dynamics and Acoustics of Travelling Bubble Cavitation

TLDR: In this article, the growth and collapse of cavitation bubbles generated by axisymmetric headforms were investigated using a surface electrode probe and the results were compared with relevant theoretical and emperical predictions.

Abstract: Individual travelling cavitation bubbles generated on two axisymmetric headforms were detected using a surface electrode probe. The growth and collapse of the bubbles, almost all of which were quasi-spherical caps moving close to the headform surface, were studied photographically. Although the growth patterns for the two headforms were similar, the collapse mechanisms were quite different. These differences were related to the pressure fields and viscous flow patterns associated with each headform. Measurements of the acoustic impulse generated by the bubble collapse were analyzed and found to correlate with the maximum volume of the bubble for each headform. Numerical solutions of the Rayleigh-Plesset equation were generated for the same flows and compared with the experimental data. The experiments revealed that for smaller bubbles the impulse-volume relationship is determinate, but for larger bubbles the impulse becomes more uncertain. The theoretical impulse was at least a factor of two greater than the measured impulse, and the impulse-volume relationship was related to the details of the collapse mechanism. Acoustic emission of individual cavitation events was spectrally analyzed and the results were compared with relevant theoretical and emperical predictions. Finally, the cavitation nuclei flux was measured and compared to the cavitation event rate and the bubble maximum size distribution through the use of a simple model. The nuclei number distribution was found to vary substantially with tunnel operating conditions, and changes in the nuclei number distribution significantly influenced the cavitation event rate and bubble maximum size distribution. The model estimated the cavitation event rate but failed to predict the bubble maximum size distribution. With the above theoretical and experimental results, the cavitation rate and resulting noise production may be estimated from a knowledge of the non-cavitating flow and the free stream nuclei number distribution.