Numerical Study on the Dynamics and Thermal Effects of Bubble Stable Cavitation in Focused Ultrasound Fields

In order to investigate the bubble dynamics and thermal effects of stable cavitation under different acoustic fields, this study computes and analyzes a series of DNS (Direct Numerical Simulation) approaches with the VOF (Volume of Fluid) method. The analysis focuses on bubble clusters with a radius of 1.5 μm and a void ratio of 10−6, commonly encountered in ultrasound therapy. Firstly, the results show that the thermal effects of bubble cavitation are non-linearly positively correlated with the ultrasound amplitude and the volume changes of the bubbles. Meanwhile, acoustic scattering caused by ultrasound passing through the bubbles leads to acoustic pressure focusing, intensifying cavitation. Secondly, the thermal effect is most evident at an acoustic frequency of 250 kHz. When the ultrasound input frequency is higher than 250 kHz, acoustic attenuation occurs, while at frequencies lower than 250 kHz, the efficiency of bubbles’ energy absorption reduces. Finally, when the acoustic pressure amplitude on the bubble surface is above 210 kPa, the thermal effect of cavitation is significantly enhanced. However, the temperature rise in the flow domain gradually slows with time and eventually reaches a fixed rate. To sum up, to optimize and control the thermal effects of ultrasound therapy, the ultrasound frequency and amplitude must be carefully selected based on the targeted bubble cluster.