Volume 106, Issue 6, December 1999
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
106(1999); http://dx.doi.org/10.1121/1.428181View Description Hide Description
The problem concerning the propagation of plane harmonic waves of small amplitudes in monatomic ideal gases is studied using an extended kinetic model that replaces (in the same manner as in the Bhatnagar–Gross–Krook model) the Boltzmann collision operator with a single relaxation-time term. Numerical results for the speed and attenuation of sound are derived and compared with the experimental data of Meyer and Sessler, Greenspan, and Schotter for monatomic gases. At low frequencies, the correct continuum limits for the absorption and dispersion are recovered, while at high frequencies a complete agreement between theory and experiments is observed.
106(1999); http://dx.doi.org/10.1121/1.428182View Description Hide Description
An investigation of the internal flow field for a drop at the antinode of a standing wave has been carried out. The main difference from the solid sphere case is the inclusion of the shear stress and velocity continuity conditions at the liquid–gas interface. To the leading order of calculation, the internal flow field was found to be quite weak. Also, this order being fully time dependent has a zero mean flow. At the next higher order, steady internal flows are predicted and, as in the case of a solid sphere, there is a recirculating layer consisting of closed streamlines near the surface. In the case of a liquid drop, however, the behavior of this recirculating Stokes layer is quite interesting. It is predicted that the layer ceases to have recirculation when where is the liquid viscosity, is the exterior gas-phase viscosity, and M is the dimensionless frequency parameter for the gas phase, defined by with a being the drop radius. Thorough experimental confirmation of this interesting new development needs to be conducted. Although it seems to agree with many experiments with levitated drops where no recirculating layer has been clearly observed, a new set of experiments for specifically testing this interesting development need to be carried out.
106(1999); http://dx.doi.org/10.1121/1.428155View Description Hide Description
An analysis was made of the SH-interfacial wave propagation on an interface between semi-infinite piezoelectric media. In particular, a sufficient condition for the existence of the SH-wave on the metallized rigid contact was derived. It has been shown that on the metallized rigid contact between two completely identical piezocrystals the SH-localized wave always exists, with the exception of at most two orientations of the interface. An investigation was performed of specific features of the sound reflection—transmission at incident angles corresponding to the resonant excitation of the leaky wave originating from the “supersonic” interfacial wave due to misorientation of crystals that constitute the bicrystal. Also, it has been established that in 6 mm crystals an interfacial “supersonic” wave may remain purely localized even when the direction of propagation is not perpendicular to the axis 6 or when the interface is not parallel to this axis.
106(1999); http://dx.doi.org/10.1121/1.428183View Description Hide Description
A refined expression for the secondary Bjerknes force between two spherical gas bubbles in a viscous incompressible fluid is derived, allowing for the dipole oscillations of the bubbles and acoustic streaming around them. The investigation is motivated by the fact that the existing theory [see, for example, E. A. Zabolotskaya, Sov. Phys. Acoust.30, 365–368 (1984)] neglects both of these processes, taking into account only linear viscouseffects due to the radial oscillations of the bubbles. More correct calculations reveal that, provided the viscous penetration depth in the surrounding fluid is comparable to the bubble size, the forces on the bubbles differ noticeably from values given by the former theory, and, in addition, are no longer equal and opposite to each other. Such conditions are shown by numerical examples to be quite realistic ones, even for low-viscosity fluids like water.
An acoustical helicoidal wave transducer with applications for the alignment of ultrasonic and underwater systems106(1999); http://dx.doi.org/10.1121/1.428184View Description Hide Description
A simple four-panel transducer capable of producing a beam with a screw dislocation along its axis was constructed and evaluated. A screw dislocation in a wavefront is characterized by a phase dependence about the dislocation axis that varies as where m is an integer and φ is the angle about the axis. At the axis, the phase is indeterminate and as a result there is a corresponding null in the pressure magnitude. The screw dislocation in the transducer beam is along the beam axis and is found to exist in both the far- and near-fields of the transducer. This null then clearly indicates the axis of the beam at all distances and has the potential to be used as an aid in the alignment of objects in sonar experiments or other similar applications. The helicoidal wave is also shown to possess axial angular momentum. A related transducer was summarized previously [J. Acoust. Soc. Am. 103, 2971 (1998)] and is also discussed here for the purposes of comparison.