This review provides a comprehensive overview of ultrasonic wave propagation, with a primary focus on high-power ultrasound systems where cavitation bubbles are likely to occur. The review is structured to guide readers through the historical development of cavitation models, from early works such as the Rayleigh-Plesset equation to more advanced numerical approaches. It explores the dynamics of cavitation bubbles, their physical effects, and the key factors influencing bubble formation, growth, and collapse. In addition to bubble-induced cavitation, the review addresses nonlinear wave propagation in the absence of bubbles, highlighting phenomena such as harmonic generation and shock wave formation. A detailed discussion on the numerical modelling of ultrasonic systems follows, covering linear and nonlinear approaches, boundary conditions, and the challenges of accurately simulating cavitating systems. The review concludes with an analysis of recent developments, emerging trends, and future directions in computational modelling for ultrasonic applications. By presenting a structured overview of both the theoretical and practical aspects of ultrasonic wave propagation, this work aims to provide a foundation for future research and design improvements in sonochemical and acoustic systems.
In the field of ultrasonic emulsification, the formation and cavitation collapse is one major factor contributing to the formation of micro- and nano-sized emulsion droplets. In this work, a series of experiments were conducted to examine the effects of varying the ultrasonic horn's position to the sizes of emulsion droplets formed, in an attempt to compare the influence of the simulated acoustic pressure fields to the experimental results. Results showed that the intensity of the acoustic pressure played a vital role in the formation of smaller emulsion droplets. Larger areas with acoustic pressure above the cavitation threshold in the water phase have resulted in the formation of smaller emulsion droplets ca. 250 nm and with polydispersity index of 0.2-0.3. Placing the ultrasonic horn at the oil-water interface has hindered the formation of small emulsion droplets, due to the transfer of energy to overcome the interfacial surface tension of oil and water, resulting in a slight reduction in the maximum acoustic pressure, as well as the total area with acoustic pressures above the cavitation threshold. This work has demonstrated the influence of the position of the ultrasonic horn in the oil and water system on the final emulsion droplets formed and can conclude the importance of generating acoustic pressure above the cavitation threshold to achieve small and stable oil-in-water emulsion.
Recently, multi-frequency systems were reported to improve performance in power ultrasound applications. In line with this, digital prototyping of multi-frequency sonoreactors also started gaining interest. However, the conventional method of simulating multi-frequency acoustic pressure fields in the time-domain led to many challenges and limitations. In this study, a multi-frequency sonoreactor was characterised using frequency domain simulations in 2-D. The studied system consists of a hexagonal sonoreactor capable of operating at 28, 40 and 70 kHz. Four frequency combinations were studied: 28-40, 28-70, 40-70 and 28-40-70 kHz. A semi-empirical, modified Commander and Prosperetti model was used to describe the bubbly-liquid effects in the sonoreactor. The root-mean-squared acoustic pressure was compared against experimental validation results using sonochemiluminescence (SCL) images and was noted to show good qualitative agreement with SCL results in terms of antinode predictions. The empirical phase speed calculated from SCL measurements was found to be important to circumvent uncertainties in bubble parameter specifications which reduces error in the simulations. Additionally, simulation results also highlighted the importance of geometry in the context of optimising the standing wave magnitudes for each working frequency due to the effects of constructive and destructive interference.