Anthony A. Atchley

The crevice model of bubble nucleation

The crevice model for heterogeneous nucleation of bubbles in water in response to a decreasing liquid pressure is studied. The model neglects gas‐diffusion effects and is therefore more suited for acoustic than for flow cavitation. It is argued that previous work has overlooked the essential requirement of unstable growth of the interface in the crevice. As a consequence, the available results are incorrect in some cases. Another feature of the model which is considered is the process by which the interface moves out of the crevice. It is concluded that, depending on circumstances, the conditions for this step may be more stringent than those for the initial expansion of the nucleus inside the crevice. Some numerical examples are given to illustrate the complex behavior of nuclei, depending of geometrical parameters, gas saturation, contact angles, and other quantities.

Thresholds for cavitation produced in water by pulsed ultrasound

The threshold for transient cavitation produced in water by pulsed ultrasound was measured as a function of pulse duration and pulse repetition frequency at both 0.98 and 2.30 MHz. The cavitation events were detected with a passive acoustic technique which relies upon the scattering of the irradiation field by the bubble clouds associated with the events. The results indicate that the threshold is independent of pulse duration and acoustic frequency for pulses longer than approximately 10 acoustic cycles. The threshold increases for shorter pulses. The cavitation events are likely to be associated with bubble clouds rather than single bubbles.

The role of nonlinear effects in the propagation of noise from high-power jet aircraft

To address the question of the role of nonlinear effects in the propagation of noise radiated by high-power jet aircraft, extensive measurements were made of the F-22A Raptor during static engine run-ups. Data were acquired at low-, intermediate-, and high-thrust engine settings with microphones located 23-305 m from the aircraft along several angles. Comparisons between the results of a generalized-Burgers-equation-based nonlinear propagation model and the measurements yield favorable agreement, whereas application of a linear propagation model results in spectral predictions that are much too low at high frequencies. The results and analysis show that significant nonlinear propagation effects occur for even intermediate-thrust engine conditions and at angles well away from the peak radiation angle. This suggests that these effects are likely to be common in the propagation of noise radiated by high-power aircraft.

Simultaneous measurement of acoustic and streaming velocities in a standing wave using laser Doppler anemometry

Laser Doppler anemometry (LDA) with burst spectrum analysis (BSA) is used to study the acoustic streaming generated in a cylindrical standing-wave resonator filled with air. The air column is driven sinusoidally at a frequency of approximately 310 Hz and the resultant acoustic-velocity amplitudes are less than 1.3 m/s at the velocity antinodes. The axial component of fluid velocity is measured along the resonator axis, across the diameter, and as a function of acoustic amplitude. The velocity signals are postprocessed using the Fourier averaging method [Sonnenberger et al., Exp. Fluids 28, 217-224 (2000)]. Equations are derived for determining the uncertainties in the resultant Fourier coefficients. The time-averaged velocity-signal components are seen to be contaminated by significant errors due to the LDA/BSA system. In order to avoid these errors, the Lagrangian streaming velocities are determined using the time-harmonic signal components and the arrival times of the velocity samples. The observed Lagrangian streaming velocities are consistent with Rotts theory [N. Rott, Z. Angew. Math. Phys. 25, 417-421 (1974)], indicating that the dependence of viscosity on temperature is important. The onset of streaming is observed to occur within approximately 5 s after switching on the acoustic field.

On the Perception of Crackle in High-Amplitude Jet Noise

Crackle is a phenomenon sometimes found in supersonic jet noise and can comprise an annoying and dominant part of the overall perceived noise. In the past, crackle has been commonly quantified by the skewness of the tune waveform. In this investigation, a simulated waveform with a virtually identical probability density function and power spectrum as an actual F/A-18E afterburner recording has been created by nonlinearly transforming a statistically Gaussian waveform. Although the afterburner waveform crackles noticeably, playback of the non-Gaussian simulated waveform yields no perception of crackle at all, despite its relatively high skewness

A precise technique for the measurement of acoustic cavitation thresholds and some preliminary results

A description is given of a precise technique for measuring the threshold for acoustic cavitation inception. The system, which is automated so as to remove operator involvement, utilizes a slow ramping of the acoustic pressure amplitude until cavitation occurs. The detection criterion is the generation of a sufficiently intense sonoluminescent signal. Measurements made in filtered water show a well-defined, reproducible, and stable cavitation threshold. Measurements of the dependence of the threshold on filter size, on time, and on the concentration of dissolved ions for various salts are also presented. Many of these results appear anomalous.

Study of a thermoacoustic prime mover below onset of self‐oscillation

The frequency response of a thermoacoustic prime mover has been measured as a function of the mean gas pressure and temperature gradient across the prime mover stack. The quality factor Q and resonance frequency can be determined from the response. As the temperature gradient is increased, the Q increases, indicating a decrease in attenuation across the stack. At sufficiently large temperature differences (∼300 K), the resonator goes into self‐oscillation, indicating negative attenuation. Measurements are reported for helium and argon at pressures ranging from 170–500 kPa and temperature gradients ranging from zero to that required for onset of self‐oscillation. The results are explained in terms of a counterpropagating, plane‐wave analysis, based on techniques commonly used in porous media investigations. In general, the predictions of the analysis are in good agreement with experiment. The predictions of Q and the change in resonance frequency with mean gas pressure are within approximately 5% and 0.4% ...

Influences of a temperature gradient and fluid inertia on acoustic streaming in a standing wave

Following the experimental method of Thompson and Atchley [J. Acoust. Soc. Am. 117, 1828-1838 (2005)] laser Doppler anemometry (LDA) is used to investigate the influences of a thermoacoustically induced axial temperature gradient and of fluid inertia on the acoustic streaming generated in a cylindrical standing-wave resonator filled with air driven sinusoidally at a frequency of 308 Hz. The axial component of Lagrangian streaming velocity is measured along the resonator axis and across the diameter at acoustic-velocity amplitudes of 2.7, 4.3, 6.1, and 8.6 m/s at the velocity antinodes. The magnitude of the axial temperature gradient along the resonator wall is varied between approximately 0 and 8 K/m by repeating measurements with the resonator either surrounded by a water jacket, suspended within an air-filled tank, or wrapped in foam insulation. A significant correlation is observed between the temperature gradient and the behavior of the streaming: as the magnitude of the temperature gradient increases, the magnitude of the streaming decreases and the shape of the streaming cell becomes increasingly distorted. The observed steady-state streaming velocities are not in agreement with any available theory.

Stability curves for a thermoacoustic prime mover

The purpose of this paper is to investigate the stability curves of the fundamental and second modes in a helium filled prime mover. The predicted and measured stability limits are in reasonable agreement for both modes at most mean pressures. There is, however, evidence that the stability of one mode is affected by the presence of the other. It is also observed that one mode can suppress the other. Measurements are also reported on a prime mover modified to selectively inhibit the fundamental mode. Results indicate that the reduced fundamental amplitude allows the stability curve for the second mode to extend into the regions where the fundamental mode previously dominated. This produces a region where both modes are simultaneously excited. Analysis of the waveforms show that the resulting oscillations are quasiperiodic.

Finite amplitude standing waves in harmonic and anharmonic tubes

Recent developments in thermoacoustic devices have generated a renewed interest in finite amplitude standing waves in resonant cavities. Such devices can generate standing waves with acoustic pressure amplitudes on the order of 10% of ambient pressure. The similarity between previously observed finite amplitude waveforms in closed tubes and those in prime movers could be modeled as anharmonic resonating cavities. Measurements of the energy transfer into the harmonics and the resonant modes of both harmonic and anharmonic closed tubes driven at resonance (∼200 Hz) will be presented. These measurements were compared with calculations using Coppens and Sanders’ formulation [A. B. Coppens and J. V. Sanders, ‘‘Finite‐amplitude standing waves within real cavities,’’ J. Acoust. Soc. Am. 58, 1133–1140 (1975)], which requires the measured quality factors and resonance frequencies. [Work supported by ONR and the NPS Research Program.]