This paper presents the simulation results of the acoustic field around an underwater supercavitation vehicle under various operating conditions and analyzes the cavitation phenomenon and the hydrodynamic noise spectrum. Regarding the ventilated cavitation phenomenon, the simulation shows that low vehicle speed can reduce the threshold of the ventilated supercavitation, and high background pressure can enhance the stability of the supercavitation structure. As for hydrodynamic noise, firstly, the simulation results reveal that when cavitation occurs, the noise spectrum exhibits several characteristic peaks near 1 kHz and between 3 and 10 kHz. The overall noise amplitude demonstrates a descending trend between 10 and 40 kHz. Further, under natural cavitation conditions, a characteristic peak is detectable between 40 and 80 kHz. The influence of the operating conditions on the noise is essentially achieved by altering the scale of the cavitation flow: with the growth of the bubble flow scale, the noise between 3 and 10 kHz first increases and then decreases due to its own pulsation and the masking effect, while the noise between 10 to 40 kHz substantially reduces. On the other hand, if the scale expansion of bubble flow is related to the increase of ventilation flow, the noise amplitude near 1 kHz will increase significantly. These results provide theoretical support for studying the supercavitation vehicles’ noise and applying the ventilated supercavitation technology.

This study employs the VOF method to conduct the direct numerical simulation of the collapse progress of the near-wall bubble cluster. Factors such as viscosity, compressibility, and surface tension are taken into account, with an emphasis on the flow field energy evolution. Firstly, the collapse of a cubic bubble cluster comprising 64 bubbles is simulated, validating previous research regarding the morphological evolution and energy release mechanisms during cluster collapse. Overall, the cubic bubble cluster collapse exhibits a layer-by-layer phenomenon, where the outer layer bubbles collapse first, converting a portion of bubble potential energy into fluid kinetic energy, which then contributes to the inner layer bubble collapse. The pressure wave energy is primarily released when the whole bubble cluster completely collapses. Secondly, we investigate the collapse process of columnar bubble clusters, which closely resemble realistic cloud cavitation. By comparing the collapse behavior of bubble clusters with different heights, we reveal the non-linear delay effect of the cluster height on the collapse time. Additionally, we consolidate our long-term research on the bubble cluster and conclude that both the scale and shape of the bubble clusters have a limited impact on the conversion rate $\eta$ of bubble potential energy to pressure wave energy $\eta$. For instance, when the stand-off distance $\eta =1.5$ and the inter-bubble distance $D=2.5$, the conversion rate $\eta$ remains consistently 9–15% for various bubble clusters of different scales and shapes.

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.

Complex nonlinear interactions exist between oscillating bubbles and sound waves. Understanding the modulation mechanism of sound waves on bubble oscillation noise is crucial for acoustic detection and noise control in equipment. The direct numerical simulations approach is used to study the coupling of the oscillating bubble to the acoustic field, and the bubble release acoustic pressure is calculated by the Ffowcs Williams–Hawkings equation. The results reveal that the input sound waves can amplify bubble oscillation, reduce the oscillation period, and induce non-spherical deformation. Further analysis revealed the acoustic characteristics of the bubble oscillation under various conditions. There is a cutoff frequency, which makes the bubble release acoustic pressure amplitude follow a two-stage pattern: an initial increase followed by an exponential decay. And there is a frequency threshold beyond which the bubble release acoustic pressure remains stable. When the input sound wave frequency f is held constant, the acoustic release from the bubble increases linearly with increasing pa, and the maximum bubble acoustic release rate k is approximately 46.7. However, when the fixed f value exceeds the frequency threshold, the bubble's acoustic pressure no longer varies significantly with increasing pa, and the value of k remains stable. Altering the bubble's initial oscillation conditions demonstrates that the sound wave primarily governs the frequency response trend of its oscillatory acoustic pressure, while the frequency threshold is affected by the initial oscillation conditions.

In this paper, the dynamic pressure characteristics induced by an internal solitary wave (ISW) in a two-layer fluid of finite depth are studied. A method for detecting ISWs by dynamic pressure is proposed. First, an accurate controllable ISW numerical flume is established based on the applicability conditions of three kinds of ISW theories. Then the relationship between the maximum dynamic pressure and the amplitude of the ISW under four density stratifications is studied. The results show that the dynamic pressure variation induced by the ISW is synchronous with that of interface displacement. Under the same density stratification, the maximum dynamic pressure has a strong linear relationship with the corresponding amplitude. Furthermore, the nonlinear error is less than 7%, and the relationship is not affected by the ocean structure. According to the above characteristics of dynamic pressure, a series of dynamic pressure measurement experiments were carried out in a large internal wave flume (IWF) using a very low frequency (VLF) piezoelectric sensor. The experimental results show that the dynamic pressure measurement results are in good agreement with the numerical simulation results. Last, the Miyata-Choi-Camassa (MCC) theory is used to invert the measured maximum dynamic pressure to the amplitude of ISW, and compared with the measured amplitude, the maximum error is less than 6%, which verifies the feasibility of the inversion method of ISW amplitude by using dynamic pressure measurement proposed in this paper, and provides a new method and idea for detecting ISW at sea in the future.