The ultrasonic cavitation threshold is a significant area of research in therapeutic ultrasound. This study conducts a numerical simulation of the ultrasound cavitation threshold by solving the dynamic equations of bubble clusters composed of bubbles with varying sizes. The effects of different criteria for cavitation threshold, bubble dynamics models, medium types, viscoelasticity and number of bubbles on the cavitation threshold are analyzed. Moreover, a comparison is made between calculation outcomes and those from previous experimental research. The results show that the relationship between the cavitation threshold P and frequency f can be expressed as P = Af^α + B, where A, B and α depend on the properties of the medium and different criteria of cavitation threshold. As the number of bubbles increases, the cavitation threshold initially rises and then falls, indicating a non-monotonic trend. Furthermore, the influence of the medium's shear modulus on the cavitation threshold is more intricate, and the cavitation threshold in viscoelastic medium is not consistently higher than that in water. Among all parameters in this study, the cavitation threshold is more sensitive to the change of frequency.

Acoustic levitation is widely used in non-container measurement and non-contact manipulation. Particles in the single-axis acoustic levitation are easily unstable in the radial direction under external perturbations. In order to explore the instability in the acoustic levitation during radial perturbations, a nonlinear acoustic levitation model considering the coupling of radial and axial vibration is proposed to analyze the dominant factors influencing the levitation stability, an acoustic levitation system consisting of a transducer and a plane reflector is established, and high-speed photography is used to observe the vibration behavior of the particle with large radial vibration and the levitation stability. The simulation results are compared and verified with the experiments, which indicate that the reduction in axial trapping stiffness due to radial vibration plays a vital role in the levitation instability. The present model can characterize the radial anti-interference ability of different levitators as well as predict the movement trajectories of levitated particles after being disturbed, which is helpful to optimize the design of acoustic levitators and provide guidance for acoustic manipulation.

The phase relation of harmonics in high-intensity focused ultrasound is investigated numerically and experimentally. The nonlinear Westervelt equation is solved to model nonlinear focused sound field by using the finite difference time domain method. Experimental waveforms are measured by a robust needle hydrophone. Then the relative phase quantity is introduced and obtained by using the zero-phase filter. The results show that the nth harmonic relative phase quantity is approximately (n — 1)π/3 at geometric center and increases along the axial direction. Moreover, the relative phase quantity decreases with the increase of source amplitude. This phase relation gives an explanation of some nonlinear phenomena such as the discrepancy of positive and negative pressure.