This paper presents an experimental study on the mechanism of interaction between a cavitation bubble and an air bubble. The cavitation bubble was generated by means of the low-voltage discharge method, and the combination of high-speed photography and a pressure measurement system allowed for simultaneous observation and measurement of the evolution of the shock wave and the change in shock wave strength with the presence of the air bubble in the vicinity. The high-speed imaging revealed the predominant roles of the relative distance φ and relative size ε between the cavitation and air bubbles in the determination of the stratification effect that the air bubble exerted on the shock wave produced from the first collapse of the cavitation bubble. The pressure measurement indicated that, when the air bubble did not merge with the cavitation bubble, the aforementioned factors, together with the angle , where the values of the coefficients a and b depended on whether the shock wave was stratified or not. When the cavitation bubble and the air bubble merged, the energy and the pressure peak of the shock wave decreased to less than 40 % of the values in the absence of the air bubble. With the new insight into bubble–bubble interaction mechanisms, the findings will facilitate a better understanding and development of cavitation utilization and prevention technology in water--air two phase systems.

As a hydrodynamic phenomenon, cavitation is a main concern in many industries such as water conservancy, the chemical industry and medical care. There are many studies on the generation, development and collapse of cavitation bubbles, but there are few studies on the variation of the cyclic impact strength on walls from the collapse of cavitation bubbles. In this paper, a high-speed dynamic acquisition and analysis system and a pressure measuring system are combined to study the impact of a cavitation bubble generated near a wall for various distances between the cavitation bubble and the wall. The results show that (1) with the discriminating criteria of the impact pressure borne by the wall, the critical conditions for the generation of a micro-jet in the collapse process of the cavitation bubbles are obtained, and therefore collapses of cavitation bubbles near the wall are mainly divided into primary impact area collapses, secondary impact area collapses and slow release area collapses; (2) it can be seen from the impact strength of the cavitation bubble collapse on the wall surface that the impact of cavitation bubbles on the wall surface during the first collapse decreases as γ (the dimensionless distance between the cavitation bubble and the wall) increases, but the impact of the second collapse on the wall surface increases first and then decreases sharply. When γ is less than 1.33, the impact on the wall surface is mainly from the first collapse. When γ is between 1.33 and 2.37, the impact on the wall surface is mainly from the second collapse. These conclusions have potential theoretical value for the utilization or prevention and control technologies for cavitation erosion.

Cavitation is a very important hydrodynamic phenomenon in many scientific and engineering fields, such as acoustics, medicine, and hydraulics. The relationship between the physical characteristics of liquid media and the erosion status of materials in a cavitation field is a crucial concern for many researchers. In this study, we adopted underwater low-voltage discharge technology to generate cavitation bubbles. We studied the effect of the viscosity of liquid media on cavitation bubble dynamics at the mesoscale level by using high-speed photography and a transient stress test system to illustrate the mechanism and the characteristics of material erosion under viscosity changes at the macroscopic level. It was found that high liquid viscosity delays shrinkage of the cavitation bubble and increases the minimum volume to which the cavitation bubble shrinks. The shrinkage characteristics of a cavitation bubble in a solution with high viscosity can reduce the speed of micro-jet formation during the collapse of the cavitation bubble near a wall. In addition, they can delay the impact of the micro-jet on the wall surface and reduce the impact strength when the cavitation bubble collapses. The effect of viscosity on cavitation bubble dynamics at the mesoscale level can explain the erosion law of solid-wall surfaces in a cavitation field from a mechanical point of view. These conclusions present significant potential for cavitation application and prevention in the fields of ultrasonic cleaning, medicine, and hydraulic machinery.

How to precisely control and efficiently utilize the physical processes such as high temperature, high pressure, and shockwaves during the collapse of cavitation bubbles is a focal concern in the field of cavitation applications. The viscosity change of the liquid will affect the bubble dynamics in turn, and further affect the precise control of intensity of cavitation field. This study used high-speed photography technology and schlieren optical path system to observe the spatiotemporal evolution of shockwaves in liquid with different viscosities. It was found that as the viscosity of the liquid increased, the wave front of the collapse shockwave of the cavitation bubble gradually thickened. Furthermore, a high-frequency pressure testing system was used to quantitatively analyze the influence of viscosity on the intensity of the shockwave. It was found that the pressure peak of the shockwave in different viscous liquid was proportional to L^b (L represented the distance between the center of bubble and the sensor measuring point), and the larger the viscosity was, the smaller the value of b was. Through in-depth analysis, it was found that as the viscosity of the liquid increased, the proportion of the shockwave energy of first bubble collapse to the maximal mechanical energy of bubble gradually decreased. The proportion of the mechanical energy of rebounding bubble to the maximal mechanical energy of bubble gradually increased. These new findings have an important theoretical significance for the efficient utilization of ultrasonic cavitation.

The bubble dynamics under the influence of particles is an unavoidable issue in many cavitation applications, with a fundamental aspect being the shockwave affected by particles during bubble collapse. In our experiments, the method of spark-induced bubbles was used, while a high-speed camera and a piezoresistive pressure sensor were utilized to investigate how particle shape affects the evolution of shockwaves. Through the high-speed photography, we found that the presence of the particle altered the consistency of the liquid medium around the bubble, which result in the emitting of water hammer shockwave and implosion shockwave respectively during the collapse of the bubble. This stratification effect was closely related to the bubble-particle relative distance φ and particle shape δ. Specifically, when the bubble-particle relative distance φ < 1.34 e^-0.10δ, particles disrupted the medium consistency around the bubbles and led to a nonspherical collapse and the consequent stratification of the shockwave. By measuring the stratified shockwave intensity affected by different particle shapes, we found that the stratified shockwave intensity experienced varying degrees of attenuation. Furthermore, as the particle shape δ increased, the attenuation of the particle on shockwave intensity gradually reduced. These new findings hold significant theoretical implications for elucidating cavitation erosion mechanisms in liquid-solid two-phase flows and applications and prevention strategies in liquid-solid two-phase cavitation fields.

A three-dimensional T-shaped flexible beam deformation was investigated using model experiments and numerical simulations. In the experiment, a beam was placed in a recirculating water channel with a steady uniform flow in the inlet. A high-speed camera system (HSC) was utilized to record the T-shaped flexible beam deformation in the cross-flow direction. In addition, a two-way fluid-structure interaction (FSI) numerical method was employed to simulate the deformation of the T-shaped flexible beam. A system coupling was used for conjoining the fluid and solid domain. The dynamic mesh method was used for recreating the mesh. After the validation of the three-dimensional numerical T-shaped flexible solid beam with the HSC results, deformation and stress were calculated for different Reynolds numbers. This study exhibited that the deformation of the T-shaped flexible beam increases by nearly 90% when the velocity is changed from 0.25 to 0.35 m/s, whereas deformation of the T-shaped flexible beam decreases by nearly 63% when the velocity is varied from 0.25 to 0.15 m/s.