Volume 116, Issue 5, November 2004
Index of content:
- NONLINEAR ACOUSTICS 
116(2004); http://dx.doi.org/10.1121/1.1802533View Description Hide Description
An evolution equation for nonlinear shear waves in soft isotropic solids is derived using an expansion of the strain energy density that permits separation of compressibility and shear deformation. The advantage of this approach is that the coefficient of nonlinearity for shear waves depends on only three elastic constants, one each at second, third, and fourth order, and these coefficients have comparable numerical values. In contrast, previous formulations yield coefficients of nonlinearity that depend on elastic constants whose values may differ by many orders of magnitude because they account for effects of compressibility as well as shear. It is proposed that the present formulation is a more natural description of nonlinear shear waves in soft solids, and therefore it is especially applicable to biomaterials like soft tissues. Calculations are presented for harmonic generation and shock formation in both linearly and elliptically polarized shear waves.
116(2004); http://dx.doi.org/10.1121/1.1810139View Description Hide Description
Several studies have proved that the geometry of an oscillating acoustic resonator strongly influences its resonance frequencies and the nonlinear standing pressure waveform generated within the cavity. The research presented herein uses a quasi-one-dimensional numerical model to solve the acoustic field and is validated by comparing with experimental results. A quasi-Newton type numerical scheme is used to optimize the axisymmetric cavity contour by maximizing the pressure compression ratio, defined as the ratio of maximum to minimum gas pressure at one end of the oscillating resonator. Cone, horn-cone, and cosine resonator contours are each optimized for a fixed amplitude of the periodic external force oscillating the cavity. Different optimized shapes are found when starting with different initial guesses, indicating multiple local extrema. The maximum pressure compression ratio value of 48 is found in an optimized horn-cone shape. This represents a 241% increase in the compression ratio over any previously published results.
Numerical simulation of acoustic streaming generated by finite-amplitude resonant oscillations in an enclosure116(2004); http://dx.doi.org/10.1121/1.1795332View Description Hide Description
Acoustic streaming motion in a compressible gas filled two-dimensional rectangular enclosure is simulated and the effects of the sound field intensity on the formation process of streaming structures are investigated numerically. The oscillatory flow field in the enclosure is created by the vibration of the left wall of the enclosure. The frequency of the wall vibration is chosen such that the lowest acoustic mode propagates along the enclosure. The fully compressible form of the Navier–Stokes equations is considered and an explicit time-marching algorithm is used to track the acoustic waves. The formation of the wave field in the enclosure is computed and fully described and the acoustic boundary layer development is predicted. The interaction of the wave field with viscouseffects and the formation of streaming structures are revealed by time-averaging the solutions over a given period. The strength of the pressurewaves associated with the acoustic effect and the resulting streaming flow pattern is found to be strongly correlated to the maximum displacement of the wall during a vibration cycle. The wave form determines the shape of the steady streaming structures in the oscillatory flow field.
Using light scattering to measure the response of individual ultrasound contrast microbubbles subjected to pulsed ultrasound in vitro116(2004); http://dx.doi.org/10.1121/1.1795334View Description Hide Description
Light scattering was used to measure the radial pulsations of individual ultrasound contrast microbubbles subjected to pulsed ultrasound. Highly diluted Optison® or Sonazoid®microbubbles were injected into either a water bath or an aqueous solution containing small quantities of xanthan gum. Individual microbubbles were insonified by ultrasound pulses from either a commercial diagnosticultrasound machine or a single element transducer. The instantaneous response curves of the microbubbles were measured. Linear and nonlinear microbubble oscillations were observed. Good agreement was obtained by fitting a bubble dynamics model to the data. The pulse-to-pulse evolution of individual microbubbles was investigated, the results of which suggest that the shell can be semipermeable, and possibly weaken with subsequent pulses. There is a high potential that light scattering can be used to optimize diagnosticultrasound techniques, understand microbubble evolution, and obtain specific information about shell parameters.