Natasha A. Chang

Ray-based acoustic localization of cavitation in a highly reverberant environment

Acoustic detection and localization of cavitation have inherent advantages over optical techniques because cavitation bubbles are natural sound sources, and acoustic transduction of cavitation sounds does not require optical access to the region of cavitating flow. In particular, near cavitation inception, cavitation bubbles may be visually small and occur infrequently, but may still emit audible sound pulses. In this investigation, direct-path acoustic recordings of cavitation events are made with 16 hydrophones mounted on the periphery of a water tunnel test section containing a low-cavitation-event-rate vortical flow. These recordings are used to localize the events in three dimensions via cross correlations to obtain arrival time differences. Here, bubble localization is hindered by reverberation, background noise, and the fact that both the pulse emission time and waveform are unknown. These hindrances are partially mitigated by a signal-processing scheme that incorporates straight-ray acoustic propagation and Monte-Carlo techniques for compensating ray-path, sound-speed, and hydrophone-location uncertainties. The acoustic localization results are compared to simultaneous optical localization results from dual-camera high-speed digital-video recordings. For 53 bubbles and a peak-signal to noise ratio frequency of 6.7 kHz, the root-mean-square spatial difference between optical and acoustic bubble location results was 1.94 cm. Parametric dependences in acoustic localization performance are also presented.

The acoustic emissions of cavitation bubbles in stretched vortices

Pairs of unequal strength, counter-rotating vortices were produced in order to examine the inception, dynamics, and acoustic emission of cavitation bubbles in rapidly stretching vortices. The acoustic signatures of these cavitation bubbles were characterized during their inception, growth, and collapse. Growing and collapsing bubbles often produced a sharp, broadband, pop sound. The spectrum of these bubbles, and the peak resonant frequency can generally be related to quiescent flow bubble dynamics and corresponding resonant frequencies. However, some elongated cavitation bubbles produced a short tonal burst, or chirp, with frequencies on the order of a few kilohertz. Theses frequencies are too low to be related to resonant frequencies of a bubble in a quiescent flow. Instead, the frequency content of the acoustic signal during bubble inception and growth is related to the volumetric oscillations of the bubble while it interacted with vortical flow that surrounds the bubble (i.e., the resonant frequency of the vortex-bubble system). A relationship was determined between the observed peak frequency of the oscillations, the highly stretched vortex properties, and the water nuclei content. It was found that different cavitation spectra could relate to different flow and fluid properties and therefore would not scale in the same manner.

Cavitation visualization of vorticity bridging during the merger of co-rotating line vortices

The flow of two co-rotating, merging line vortices was visualized with cavitation, and the images reveal the complex flow interactions associated with the vortex merger. While the developed cavitation has an influence on the vortical flow, the basic features of the noncavitating flow can be discerned through the judicious application of different free-stream cavitation numbers. By inducing vapor in the vortex cores, the existence of radial “bridging” vortices in the merger zone was confirmed. These visualizations are consistent with the numerically predicted vortex interactions presented by R. L. Bristol, J. M. Ortega, P. S. Marcus, and O. Savas [J. Fluid Mech. 517, 331 (2004)] and the experimentally inferred flow features are reported by A. L. Chen, J. D. Jacob, and O. Savas [J. Fluid Mech. 382, 155 (1999)] and W. J. Devenport, C. M. Vogel, and J. S. Zsoldos [J. Fluid Mech. 394, 357 (1999)].

Cavitation inception during the interaction of a pair of counter-rotating vortices

Pairs of unequal strength, counter-rotating vortices were produced to examine the inception and dynamics of vortex cavitation as the vortices undergo a long-wavelength instability. The instability causes the weaker, secondary vortex to be turned and stretched by the stronger primary vortex. Folding and stretching of the secondary vortices result in sharp reductions of the core pressure. Here, these sharp and transient reductions in the secondary vortex core pressure produced incipient cavitation at static pressures that were as much as 20 times higher than that required for inception in the core of the unstretched secondary vortex. In addition, the majority of nuclei measured was of the order of 1 μm in size, which requires tension on the order of 100 kPa for cavitation inception to occur. The flow parameters that lead to the instability and cavitation inception in the secondary vortex are examined, and the measured event rates are correlated to freestream nuclei populations and static pressure. These mea...

Bragg cell laser intensity modulation: effect on laser Doppler velocimetry measurements

In most laser Doppler velocimetry (LDV) systems, the frequency of one of the two laser beams that intersect to create the probe volume is shifted with an acousto-optic element. It is shown here that Bragg shifting can impose a problematic fluctuation in intensity on the frequency-shifted beam, producing spurious velocity measurements. This fluctuation occurs at twice the Bragg cell frequency, and its relative amplitude to the time average intensity is a function of the ratio of the laser beam diameter to the Bragg cell acoustic wavelength. A physical model and a configuration procedure to minimize adverse effects of the intensity modulations are presented.

Estimating the mechanical properties of acrylic plates via acoustic transmission experiments and modeling.

Clear acrylic [polymethylmethacrylate (PMMA)] is commonly used to separate instruments from the test‐section flow in water tunnel experiments. Thus, for some hydroacoustic studies, knowledge of the sound transmission properties of acrylic plates may be essential. Unfortunately, the actual mechanical properties of PMMA plates cover a relatively large range (e.g., the elastic modulus can vary from 2 to 5.5 GPa) and these mechanical properties may be frequency dependent. In this presentation, numerical and experimental results for acoustic pulse transmission through submerged flat PMMA plates are compared to identify plate properties and to calibrate an approximate plate transmission model. The experiments were conducted in a fish tank at sound incidence angles of 0–35 deg with a spherical wave source and two receivers. The transmitted sounds were short pulses with a nominal frequency range of 40–200 kHz. Several plate thicknesses, d, were tested for kd values (k is the acoustic wave number in the water) fro...

Ray-Based Acoustic Localization of Discreet Sound Sources in a Highly Reverberant Environment

An acoustic localization method is applied in a reverberant environment to locate the sources of discrete sounds having unknown timing and waveform. In particular, the localization method is applied to study low event rate cavitation in a vortical flow in a water-tunnel test-section with characteristic cross section dimension of 0.3 m. The primary frequency and bandwidth of the acoustic pulses from the small isolated cavitation bubbles are 10 kHz and 200 kHz respectively, and the measured pulse duration is ∼15–20 micro-seconds. The localization method involves using an array of receiving hydrophones to record the cavitation sound pulses. These hydrophone recordings, which include direct-path signal, reflected path signal, and noise, are time windowed and cross-correlated to obtain direct-path arrival-time differences. These arrival time differences are used in conjunction with a simple ray-based acoustic model to estimate the source location in three dimensions via a robust Monte-Carlo routine. The ratio of the primary-frequency wavelength to the water-tunnel cross-section dimension is ∼1/2. Consequently the time-windowing is tight; only 1 to 1.5 center-frequency cycles at the beginning of a signal pulse are readily useful for localization purposes. The remainder of the signal is contaminated by reflections and is not used in the present effort. To check and validate the results of the acoustic method, two-camera high-speed video data was taken synchronously with the acoustic data for 53 cavitation events. The acoustic localization scheme provided an unambiguous location estimate for all 53 cavitation bubbles. The average distance between the optical and acoustic measurement of the bubble location was 18.4 mm, or ∼1/8 of the wavelength of the primary signal frequency.Copyright

Correlation of vortex‐induced cavitation bubble dynamics and acoustic emissions

The bubble dynamics and corresponding acoustic emission from low‐event rate cavitation inception due to the interactions of a pair of parallel counter‐rotating vortex was studied experimentally in a water tunnel. The measured bandwidth of the acoustic pulse from the growth/collapse of a small isolated cavitation bubble is more than 200 kHz, and the measured pulse duration is ∼15–20 μs. The underlying vortical flow, static pressure, and nuclei distribution were characterized and are reported. These fluid and flow parameters influenced the 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 kHz and 6 kHz. Here, the frequency content of the acoustic signal during inception and growth was related to the volumetric ...

Acrylic plate acoustic transmission experiment and theory

An approximate model of the transmission of sound waves in water through a plate of PMMA was developed for a point source. The model is based on spherical wave propagation and plane‐wave transmission through a solid layer, and it is calibrated by identifying the properties of the plastic. By minimizing the error between the modeled and the experimentally measured acoustic wave, the mechanical properties of PMMA can be estimated for PMMA plates of varying thickness d for the d/fluid range of 0.04 to 2.5, and sound‐incident angles of 0 deg to 35 deg. This calibration requires only the compression wave ray‐path information and was shown to achieve 90% correlation between experimental and predicted waveforms for synthetic cavitation pulses with a nominal bandwidth from 40 kHz to 200 kHz. At larger angles of incidence as measured based on the compression wave, it was necessary to track the various waves that occur in the solid, i.e., compression, shear, and evanescent. The acoustic pressure waves that they gen...

Ray‐based acoustic localization applied in a highly reverberant environment

Acoustic localization techniques for finding and identifying broadband sound sources is of interest for studying low event rate cavitation, and other hydroacoustic noise sources, in water tunnels and other reverberant environments. The measured center frequency and bandwidth of the acoustic pulse from the collapse of a small isolated cavitation bubble are both approximately 100 kHz, and the measured pulse duration is ∼15−20 μs. Here, a sound projector is used to mimic such a pulse, and 16 receiving hydrophones are used to estimate the source location in the test section of a laboratory water tunnel when neither the timing of the event nor the exact signal waveform is presumed known. Fortunately, the direct‐path signal arrival is generally distinct enough at each receiver for acoustic localization. The signal coda is not used in the present effort. The received direct‐path signals are cross correlated between hydrophones to obtain arrival time differences. These arrival time differences are used in conjunc...