885095963

1:45

2:45

9PA4. Excitation of resonators by blood flow in arteries. T. Douglas Mas! (Graduate Próg. in Acoust., Pcnn State Univ., AppL Sci. Bldg., Univcrsity Park, PA 16802)

Resonators are known to exist in human arteries. These include aneurysms and weakened sections of the arterial wali. The prescnt paper gives a quantitative theory of how such resonators are excited under physiological conditions. Considercd mechanisms of excitation include turbulence and flutter. The pulsatile naturę of blood flow is taken into account, as are the flexible walls of arteries. The time-varying frequency, bandwidlh, and amplitudę of such resonances are related to parameters that charactcrize the cardiovascular system. [Work supported by the William E. Leonhard endowment to Pcnn State Univ. The author ac-knowledges the advicc of A. D. Pierce.J

2:00

9PA5. Spherical cavity resonator:    Singular boundary-shape

pcrturbation? James B. Mehl (Physics Dept., Univ. of Delaware, Newark, DE 19716)

A nearly spherical cavity resonator has been investigated using boundary-shape perturbation theory. The cavity parts consist of two perfect hemispheres of radu a and b = a( 1 — e), aligned along their common axes and connected by a piane surface at 0 = rr/2. The lowest-frequency nonradial modes with acoustic pressure proportional to <X>, = j_x(kr)cos 6 and <t>, = y,(A:r)sin 0cos have been investigated. These modes have fairly uniform velocity fields oscillating along the z and x axes, respectively. Boundary-shape perturbation theory has been appiied to develop an exprcssion for the eigenfrequency perturbation in powers of the smali paramcler e. The first-order perturbations of the eigenfrequencies vanish for both modes. The second-order perturbation series for the <!>, modę converges; the results agree with numerical calculations based on a boundary-integral-equation (BIE) technique. The second-order perturbation series for the <t>, case diverges. The BIE numerical results suggest the presence of a nonsingular term proportional to e² log e².

2:15

9PA6. A simple Circuit model for the thermodynamics of thermoacoustic devices. Peter H. Ceperley (Dcpts. of Electrical and Computer Eng. and Physics, George Mason Univ., Fairfax, VA 22030)

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. Morę importantly, it allows a simpler, morę 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 piąte (or slit) geometry for a rangę of slit widths and phasings. [Work supported by ONR.j

2:30

9PA7. Thermoacoustic properties of porous media. Alon Koren and Peter H. Ceperley (Depts. of Electrical and Computer Eng. and Physics, George Mason Univ., Fairfax, VA 22030)

The thermoacoustic time constant and flow resistance werc mea-sured for Steel wool. sand. and a parallel piąte geometry over a rangę of frcquencics in one atmosphere of air. The results are compared with that of the theoretical parallel piąte geometry. Suitability of various packings for thermoacoustic applications will be discussed. [Work supported by ONR.J 9PA8. Complcx cigenfrequency analysis of thermoacoustic heat engines. W. Pat Arnott, Richard Raspet, and Henry E. Bass (Natl. Ctr. for Physical Acoust. and the Dept. of Physics and Astron., P.O. Box 847, University, MS 38677)

Gas in a straight tubc, open at the cool end and closcd at the hot end, can be madę unstable with respect to acoustic ości 1 lation by pbcing a suflfciently large temperaturę gradient along the tubę. The possibility of spontaneous oscillation exists when the cxtcmally appiied temperaturę gradient is larger than the temperaturę gradient assodated with a standing acoustic wave in the tubę. Energy for oscillation in this non-equilibrium system is supplied by the heat input nccessary to maintain the temperaturo gradient. Rott and his studenta [Rott, Adv. Appl. Mech. 20, 135-175 (1980)] have investigated this instability for a va-riety of conditions. The instability analysis is given in this paper for a thermoacoustic prime mover known as a Hoffler tubę. The complex eigenfrequcncy (CEF) of a recently constructcd Holflcr tubc is com-puted as a function of temperaturę. 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 l/Q-0. Application of the CEF to thermoacoustic refrigerators is discussed. [Work supported by ONR.J

3:00

9PA9. Observation of a second soundlike modę in superfluid-filled aerogel. M. J. McKenna, Tania Slawecki, and J. D. Maynard (Dept. of Physics, Perm State Univ., University Park, PA 16802)

Superfluid ⁴He is interesting acoustically because it can support morę than one modc 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 be-havior of superfluid ⁴He in the presence of a random potential field provided by highly porous media, specifically, silica aerogcls whose po-rosities excccd 90%. Unlike other porous media, whcrc the normal fluid is clamped to the rigid matrix, these aerogcls are highly compliant, and whilc the normal fluid remains loeked to the aerogel matrix, both the matrix and the normal fluid can move in response to mechanical and thermal gradients. Thcrefore, one would not observe ordinary fourth sound but rather a sound modę 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 modę. The first experimental observation of this second soundlike modę in superfluid-filled aerogel is reported, and it is shown that it remains a high-quality modc near the critical temperaturę. Also presented are measurements of the sound modę 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 Rcsearch.J

3:15

9PA10. Optimization of spatial resołution and penetration depth using a waveguide in pulsed Doppler ultrasound Systems for measurement of blood flow ve!ocity. James J. Finneran and Mardi C. Hastings (Dept. of Mech. Eng., Ohio State Univ., 206 W. 18th Ave., Columbus, OH 43210)

Doppler ultrasound is a noninvasive method used to examine anat-omy and various physiological functions. A limitation in its use to mea-sure blood-flow velocity is the size and location of the volume within the blood vesscl from which the signal is received. The volume is primarily a function of the size of the ultrasonic transduccr and signal frequency. In this study, the feasibility of using a wavcguide to mechanically adjust the natural focus of the acoustic field is considered. The adjustable focus allows control of spatial resołution and penetration depth of the ultrasonic signal. By controlling these parameters rather than totally relying

2007 J. Acoust. Soc. Am.. Vol. 89. No. 4, Pt. 2, April 1991

121 st Meeting: Acoustical Society of America 2007