Ultrasound propagation in bubbly fluid was numerically analyzed to simulate HIFU (high intensity focused ultrasound) in human body with microbubbles injected into tissues. HIFU is one of promising non-invasive treatments and micorbubbles are expected to enhance heating of body tissues. As ultrasound shows nonlinearity near focal area, high-order perturbation equation of acoustic wave with void fraction (gas volume fraction) was derived and discretized with a finite difference method. Bubble motions and temperature rise due to the bubble oscillations were obtained and visualized with various initial bubble radii and void fractions.

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One of the cavitation models for cavitating flow simulations is the bubble dynamics based method (bubble model). In a typical bubble dynamics based method, the Rayleigh-Plesset equation is solved for determining the volumetric motion of a bubble. It is derived for a single bubble in uniform fluid, and thus, is not adequate for a bubble in high void fraction fluid. Therefore, in the existing bubble dynamics based model, high void fraction fluid has not been treated as far as utilizing the Rayleigh-Plesset equation is concerned. In this paper, a bubble dynamics model treating high void fraction region is proposed. The present model has a threshold between low and high void fraction. Below the threshold, Rayleigh-Plesset equation is solved. Above the threshold, the second derivative of temporal difference of a bubble radius is set to be zero when the bubble is expanding, and Rayleigh-Plesset equation is again solved when the bubble is shrinking. For computational example, flow around Clark-Y11.7% and NACA0015 is calculated for validation of this approach and compared with experiment and the old bubble dynamics based method.

Numerical simulation method of High Intensity Focused Ultrasound (HIFU) propagation in bubbly fluid (microbubbles in liquid) is proposed for observing the ultrasound wave propagation, the bubble motion at focal area and the ultrasound power at the focal point in this paper. The governing equations are the acoustic wave equation derived from the equation of fluid and the Keller equation (bubble volume motion equation). These equations are discretized by the finite difference method. Additionally, the linear dispersion relation is derived from governing equations. First, the present method is validated on grid convergence and compared with experiment and theory. Second, HIFU in bubbly fluid is simulated for various initial void fraction and bubble radius to observe the difference of pressure field and difference of bubble motion.

For cavitating flow simulation using a bubble model ， oavitation inceptiQn effect was included to see the Cleray ofcavitation growth， which is expected to affect on cavitation bubble distribution and pressure distribution around a hydrofoil ． Simulated flows around NACAOO15 hydrofoil showed higher lift force than the previous simUlation because the delay ofcavitation growth ma 血 tains negative pressure peak near the leading edge ． 1． は じ め に