Volume 124, Issue 4, October 2008
Index of content:
- ULTRASONICS, QUANTUM ACOUSTICS, AND PHYSICAL EFFECTS OF SOUND 
Numerical modeling of the sound propagation through a rarefied gas in a semi-infinite space on the basis of linearized kinetic equation124(2008); http://dx.doi.org/10.1121/1.2967835View Description Hide Description
The sound propagation through a rarefied gas is investigated on the basis of the linearized kinetic equation. A plate oscillating in the direction normal to its own plane is considered as a sound wave source. It is assumed a fully established oscillation so that the solution of the kinetic equation depends on time harmonically, while its dependence on the spatial coordinates is obtained numerically. The problem is solved over a wide range of the oscillation speed parameter defined as a ratio of the intermolecular collision frequency to the sound frequency. In order to evaluate the influence of the momentum and energy accommodation coefficients on the solution of the problem, the Cercignani–Lampis scattering kernel is applied as the boundary condition. An analysis of wave characteristics near the source surface shows that they are significantly different from those far from the surface even if the oscillation is slow, i.e., the solution is not harmonic in the space.
Torsional waves propagation along a waveguide of arbitrary cross section immersed in a perfect fluid124(2008); http://dx.doi.org/10.1121/1.2968677View Description Hide Description
Guided torsional waves in a bar with a noncircular cross section have been exploited by previous researchers to measure the density of fluids. However, due to the complexity of the wave behavior in the noncircular cross-sectional shape, the previous theory can only provide an approximate prediction; thus the accuracy of the measurement has been compromised. In this paper, a semianalytical finite element method is developed to model accurately the propagation velocity and leakage of guided waves along an immersed waveguide with arbitrary noncircular cross section. An accurate inverse model is then provided to measure the density of the fluid by measuring the change of the torsional wave speed. Experimental results obtained with a rectangular bar in a range of fluids show very good agreement with the theoretical predictions. Finally, the potentials to use the model for sensor optimization are discussed.
124(2008); http://dx.doi.org/10.1121/1.2973229View Description Hide Description
The shock-induced collapse of a pre-existing nucleus near a solid surface in the focal region of a lithotripter is investigated. The entire flow field of the collapse of a single gas bubble subjected to a lithotripter pulse is simulated using a high-order accurate shock- and interface-capturing scheme, and the wall pressure is considered as an indication of potential damage. Results from the computations show the same qualitative behavior as that observed in experiments: a re-entrant jet forms in the direction of propagation of the pulse and penetrates the bubble during collapse, ultimately hitting the distal side and generating a water-hammer shock. As a result of the propagation of this wave, wall pressures on the order of 1 GPa may be achieved for bubbles collapsing close to the wall. The wall pressure decreases with initial stand-off distance and pulse width and increases with pulse amplitude. For the stand-off distances considered in the present work, the wall pressure due to bubble collapse is larger than that due to the incoming shockwave; the region over which this holds may extend to ten initial radii. The present results indicate that shock-induced collapse is a mechanism with high potential for damage in shockwave lithotripsy.
Angular influence on the scattering of fundamental shear horizontal guided waves by a through-thickness crack in an isotropic plate124(2008); http://dx.doi.org/10.1121/1.2968697View Description Hide Description
The angular influence on the scattering of cylindrical-crested waves of the fundamental shear horizontal guided mode by through-thickness cracks in an isotropic plate is studied in the context of array imaging using ultrasonic guided waves.Finite element simulations are used to obtain trends which are subject to analytical study and experimental confirmation. The influence of the incidence angle on reflection behavior is first studied in terms of two complementary cases, that of normal incidence and that of specular reflection at various oblique incidence angles. The normal incidence study suggests that for a given incidence angle, the peak reflection is concentrated around the specular direction, while the oblique incidence studies show that maximum specular reflection occurs in the case of normal incidence. The variation of diffraction with both the angle of incidence and that of monitoring is then taken up and this shows that when the first diffraction from the crack edges can be separated, its angular dependence can be obtained from literature on similar bulk elastic wave scattering problems.