Index of content:
Volume 124, Issue 4, October 2008
- NONLINEAR ACOUSTICS 
124(2008); http://dx.doi.org/10.1121/1.2968687View Description Hide Description
Knowledge of the theoretical relation between acoustic radiation force and ultrasonic power is important for measuring the emitted power, particularly from medical ultrasonic equipment, using a radiation force balance. A far-field method is applied to calculate the radiation force exerted by rectangular field types on a large absorbing target in a nonabsorbing fluid. This is an extension of previous work on fields with circular symmetry. Pistonlike, apodized, and weakly focusing transducer behavior including diffractioneffects is dealt with. If diffraction is neglected, i.e., in the geometric or high-frequency limit, the result can be given as a closed-form expression.
124(2008); http://dx.doi.org/10.1121/1.2973235View Description Hide Description
For high gas supersaturation levels in liquids, on the order of 300% as predicted in capillaries of marine mammals following a series of dives [D. S. Houser, R. Howard, and S. Ridgway, J. Theor. Biol.213, 183–195 (Year: 2001)], standard mathematical models of both static and rectified diffusion are found to underestimate the rate of bubble growth by 10%–20%. The discrepancy is demonstrated by comparing predictions based on existing mathematical models with direct numerical solutions of the differential equations for gas diffusion in the liquid and thermal conditions in the bubble. Underestimation of bubble growth by existing mathematical models is due to the underlying assumption that the gas concentration in the liquid is given by its value for a bubble of constant equilibrium radius. This assumption is violated when high supersaturation causes the bubble to grow too fast in relation to the time scale associated with diffusion. Rapid bubble growth results in an increased gas concentration gradient at the bubble wall and therefore a growth rate in excess of predictions based on constant equilibrium bubble radius.