Acoustic characterization of cavitation in reverberant environments.

Abstract

Acoustic localization techniques are applied in a water tunnel to study low event rate cavitation in a vortical flow. The timing and waveform of these cavitation pulses are not known. An array of 16 receiving hydrophones is used to measure acoustic pulses, and array-signal-processing techniques are utilized to estimate the source location in the water tunnel test section. The measured bandwidth of the acoustic pulse from the growth/collapse of a small isolated cavitation bubble is more than 200kHz, and the measured pulse duration is ∼15-20 micro-seconds. The direct path signal between the cavitation source and the receiving hydrophones used for this effort may be obscured by background hydrodynamic noise in the water tunnel. Fortunately some of the direct-path signal arrivals are distinct enough for acoustic detection and localization. The direct-path signals are windowed and cross-correlated to obtain arrival time differences between hydrophones. These arrival time differences are used in conjunction with a simple ray-based acoustic model to estimate the source location in three dimensions. The source location estimate can be used in conjunction with a back-propagation routine, which was developed but not validated fully, to recover part of the original cavitation pulse waveform and improve the localization estimate. Once developed, the acoustic localization method was used in an experimental examination of the locations of cavitation inception and bubble growth, and of the dynamics of cavitation bubbles in a pair of parallel counter-rotating vortices. The underlying vortical flow, static pressure, and nuclei distribution were characterized in separate studies. These fluid and flow parameters influenced the location, acoustic signal, and dynamics of the cavitation bubbles. Details of the acoustic signature of the cavitation bubble were investigated during its inception, growth, splitting, and collapse. In the chosen flow field, it was found that during bubble growth the acoustic signal is the strongest with the bulk of the signal energy in frequencies between 1 and 6kHz. Here the frequency content of the acoustic signal during inception and growth was related to the volumetric change of the bubble measured from images taken with a high-speed video camera with a correlation of 84%.