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Apr 1991

Volume 89, Issue 4B, pp. 1851-2015

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back to top Session 9PA: Physical Acoustics: General Topics
Contributed Papers
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Large amplitude wall pressure events beneath turbulent boundary layers (A)

Carolyn Karangelen

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2006-2006 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Examination of the fluctuating wall pressures can yield much information on the turbulent structures within a flow. The wall pressure signal may be considered a footprint of these structures. The pressure fluctuations measured at the wall are produced from velocity fluctuations within the flow that are propagated through the flow medium. The sporadic large amplitude wall pressure events are thought to be footprints of turbulent bursting events in the near wall layer. Large amplitude events were captured using conditional sampling techniques and analyzed for their statistical, temporal, and spectral characteristics. The results of this analysis will be presented. [Funded by ONR and IBM.]
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Interior pressure field in an accelerated spherical container (A)

Christopher L. Morfey

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2006-2006 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The pressure field inside a rigid hollow container, filled with uniform fluid and subjected to an impulsive translational acceleration a(t), is examined analytically by time‐domain methods. In the incompressible limit, a uniform pressure gradient −ρa(t) exists throughout the interior. However, compressibility introduces acoustic wave phenomena, which are investigated in detail in this paper for a spherical container. The exterior version of the problem is discussed by Pierce, and a solution given, in his book [A. D. Pierce, Acoustics (1982)]. The present interior problem has received some attention in the biomechanics literature, as a step toward modeling the dynamics of closed‐head injury: In this context, it describes events on a short time scale, of order 20 μs rather than 20 ms (which is the typical impact duration in blunt head trauma).
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Ultrasonic focus degradation produced by abdominal wall (A)

Robert C. Waag

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2006-2006 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Focus degradation produced by human abdominal wall has been studied experimentally using measurements of ultrasonic pulses from a special curved transducer that emits a hemispheric wave and simulates a point source. The pulse waveforms were measured in a two‐dimensional aperture after propagation through a water path and after propagation through five different specimens of human abdominal wall. The time history of the virtual point source was reconstructed from the measurements by removing the time delay produced by geometric path differences, finding the complex amplitudes of the temporal harmonics across the aperture, calculating the Fraunhofer diffraction pattern of each harmonic, and summing the patterns. Comparison of the time history in the plane of the virtual source reconstructed from signals that propagated through the abdominal wall specimens and the time history obtained from signals without the abdominal wall specimens present shows that the main features of the reconstructed source waveform are relatively unaffected by the presence of the human abdominal wall but that significant differences in signals greater than 40 dB down from the peak result from propagation through the specimens employed in this study.
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Excitation of resonators by blood flow in arteries (A)

T. Douglas Mast

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2007-2007 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Resonators are known to exist in human arteries. These include aneurysms and weakened sections of the arterial wall. The present paper gives a quantitative theory of how such resonators are excited under physiological conditions. Considered mechanisms of excitation include turbulence and flutter. The pulsatile nature of blood flow is taken into account, as are the flexible walls of arteries. The time‐varying frequency, bandwidth, and amplitude of such resonances are related to parameters that characterize the cardiovascular system. [Work supported by the William E. Leonhard endowment to Penn State Univ. The author acknowledges the advice of A. D. Pierce.]
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Spherical cavity resonator: Singular boundary‐shape perturbation? (A)

James B. Mehl

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2007-2007 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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A nearly spherical cavity resonator has been investigated using boundary‐shape perturbation theory. The cavity parts consist of two perfect hemispheres of radii a and b = a(1 − ϵ), aligned along their common axes and connected by a plane surface at θ = π/2. The lowest‐frequency nonradial modes with acoustic pressure proportional to Φz  =  jt(kr)cos θ and Φx  =  jt(kr)sin θ cos ϕ have been investigated. These modes have fairly uniform velocity fields oscillating along the z and x axes, respectively. Boundary‐shape perturbation theory has been applied to develop an expression for the eigenfrequency perturbation in powers of the small parameter ϵ. The first‐order perturbations of the eigenfrequencies vanish for both modes. The second‐order perturbation series for the Φx mode converges; the results agree with numerical calculations based on a boundary‐integral‐equation (BIE) technique. The second‐order perturbation series for the Φz case diverges. The BIE numerical results suggest the presence of a nonsingular term proportional to ϵ2 log ϵ2.
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A simple circuit model for the thermodynamics of thermoacoustic devices (A)

Peter H. Ceperley

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2007-2007 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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A simple circuit model of the thermodynamics of thermoacoustic devices will be presented. This model allows the simple calculation of thermoacoustic gain, efficiency, and thermal current. More importantly, it allows a simpler, more intuitive approach to optimizing the parameters of a thermoacoustic device. Results calculated using this model will be compared with the exact results calculated for a parallel plate (or slit) geometry for a range of slit widths and phasings. [Work supported by ONR.]
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Thermoacoustic properties of porous media (A)

Alon Koren and Peter H. Ceperley

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2007-2007 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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The thermoacoustic time constant and flow resistance were measured for steel wool, sand, and a parallel plate geometry over a range of frequencies in one atmosphere of air. The results are compared with that of the theoretical parallel plate geometry. Suitability of various packings for thermoacoustic applications will be discussed. [Work supported by ONR.]
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Complex eigenfrequency analysis of thermoacoustic heat engines (A)

W. Pat Arnott, Richard Raspet, and Henry E. Bass

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2007-2007 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Gas in a straight tube, open at the cool end and closed at the hot end, can be made unstable with respect to acoustic oscillation by placing a sufficiently large temperature gradient along the tube. The possibility of spontaneous oscillation exists when the externally applied temperature gradient is larger than the temperature gradient associated with a standing acoustic wave in the tube. Energy for oscillation in this non‐equilibrium system is supplied by the heat input necessary to maintain the temperature gradient. Rott and his students [Rott, Adv. Appl. Mech. 20, 135–175 (1980)] have investigated this instability for a variety of conditions. The instability analysis is given in this paper for a thermoacoustic prime mover known as a Hoffler tube. The complex eigenfrequency (CEF) of a recently constructed Hoffler tube is computed as a function of temperature. The frequency of oscillation and the quality factor, or Q, are determined from the real and imaginary parts of the CEF. Spontaneous oscillation can occur in the limit 1/Q→0. Application of the CEF to thermoacoustic refrigerators is discussed. [Work supported by ONR.]
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Observation of a second soundlike mode in superfluid‐filled aerogel (A)

M. J. McKenna, Tania Slawecki, and J. D. Maynard

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2007-2007 (1991); (1 page)

Online Publication Date: 14 Aug 2005

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Superfluid 4He is interesting acoustically because it can support more than one mode of sound propagation, and these modes may be used in combination to determine the critical properties of the superfluid. Recently, there has been considerable interest in the critical behavior of superfluid 4He in the presence of a random potential field provided by highly porous media, specifically, silica aerogels whose porosities exceed 90%. Unlike other porous media, where the normal fluid is clamped to the rigid matrix, these aerogels are highly compliant, and while the normal fluid remains locked to the aerogel matrix, both the matrix and the normal fluid can move in response to mechanical and thermal gradients. Therefore, one would not observe ordinary fourth sound but rather a sound mode intermediate between first and fourth sound. In addition, the superfluid can move in a direction opposite to the normal fluid/aerogel matrix, resulting in a second soundlike mode. The first experimental observation of this second soundlike mode in superfluid‐filled aerogel is reported, and it is shown that it remains a high‐quality mode near the critical temperature. Also presented are measurements of the sound mode intermediate between first and fourth sound, and a theoretical model that gives good agreement with the observed new sound modes. [Work supported by NSF Grant DMR 9000549 and the Office of Naval Research.]
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Optimization of spatial resolution and penetration depth using a waveguide in pulsed Doppler ultrasound systems for measurement of blood flow velocity (A)

James J. Finneran and Mardi C. Hastings

J. Acoust. Soc. Am. Volume 89, Issue 4B, pp. 2007-2008 (1991); (2 pages)

Online Publication Date: 14 Aug 2005

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Doppler ultrasound is a noninvasive method used to examine anatomy and various physiological functions. A limitation in its use to measure blood‐flow velocity is the size and location of the volume within the blood vessel from which the signal is received. The volume is primarily a function of the size of the ultrasonic transducer and signal frequency. In this study, the feasibility of using a waveguide to mechanically adjust the natural focus of the acoustic field is considered. The adjustable focus allows control of spatial resolution and penetration depth of the ultrasonic signal. By controlling these parameters rather than totally relying on digital signal processing of received signals, the Doppler system may be used to examine blood vessels of variable size and shape. Measurements of beam profiles in a wave‐guided system are compared with theoretical predictions. Preliminary design of a waveguide for a probe to measure the blood flow velocity at different points through the aorta is presented. [Work supported by NSF.]
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