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This paper presents an experimental study focused on the annular secondary cavitation caused by the interaction between a high-voltage spark bubble and a cylinder. A 2000 V high-voltage underwater electric discharge was used to create the bubble, which reached a maximum diameter of approximately 31 mm. High-speed photography was employed to capture the behavior of the spark bubble and the induced secondary cavitation bubbles. The formation and evolution of the annular secondary cavitation bubbles near the cylinder's edge were captured and analyzed. The study classified the morphology of the annular secondary cavitation bubbles into three different types based on various bubble distances and cylinder diameters. Notably, unique bubble morphologies, such as the terraced bubble shape, were identified, resulting from the coupling between the spark bubble, the annular secondary cavitation bubble, and the cylinder. The interaction between the annular secondary cavitation bubble and the spark bubble, including coalescence, and the resulting effects on bubble shape and period were investigated. The changes of secondary cavitation bubbles are compared when the cylinder has different chamfer angles. Moreover, the study examined the dynamic characteristics of the bubbles, including their shape evolution, jetting behavior, period length, and pressure pulses upon collapse, at various distances from the top-surface of cylinders with different diameters. The experiment also introduced flow visualization with dye into spark bubble experiments for the first time to trace the vortex around the annular secondary bubble. Additionally, the transparency of bubbles in the images was improved through multi-source lighting techniques. [ABSTRACT FROM AUTHOR]

Particle propulsion by an attached acoustic cavitation bubble under strong ultrasonic wave excitation occupies the core of many applications, including ultrasonic cleaning, ultrasonography, targeted therapy, and microbubble motors. However, the driving capacity and mode of bubbles in the field of ultrasonics are far from being well understood, which severely limits its applicability in a variety of fields. In this study, a fluid–structure interaction model based on the boundary integral method is proposed to simulate complex interactions between a suspended spherical particle and an attached cavitation bubble. A one-to-one comparison between the numerical results and experimental data demonstrates the distinct advantage of our model over conventional approaches. Thereafter, we systematically investigate the dependence of bubble–particle interactions on the governing parameters, including the amplitude and phase of the ultrasonic wave, particle density, and particle-to-bubble size ratio. We also document different types of bubble dynamic behaviors under various governing parameters. Finally, we obtain scaling laws for the maximum displacement of the particle with respect to the governing parameters. [ABSTRACT FROM AUTHOR]

For a bubble initiated in oil, the bubble migrates towards the interface and gradually passes through the interface, leading to a stronger bubble-interface interaction. Summary and conclusions In summary, we have studied the complicated interaction between cavitation bubbles and a water-oil interface on multiple time scales via experimental, numerical and theoretical means, including the bubble dynamics on a short time scale Graph ${\sim }10^0$ ms and the water-oil interface evolution on a much longer time scale Graph ${\sim }10^2$ ms. When a bubble is initiated in water, the bubble migrates away from the interface and thus the subsequent bubble-interface interaction is weakened. More specifically, the interface is pushed away by the expanding bubble and attracted by the collapsing bubble, which is different from that in bubble interactions with a water-air interface (Blake & Gibson 1987; Koukouvinis I et al. i 2016; Kang & Cho 2019). Here Graph $S$ refers to the fluid-fluid interface and the bubble surface when Graph $i = 1$ (flow domain 1), while it refers to the fluid-fluid interface only when Graph $i = 2$ (flow domain 2). [Extracted from the article]

This study presents an experimental investigation of the dynamic properties of underwater explosion (UNDEX) bubble pairs produced with a range of phase differences Δθ, defined as 2π(t1−t2)/Tosc, where ti (i = 1,2) represents the bubble inception moment and Tosc is the experimentally obtained first period of a single UNDEX bubble. Each bubble was generated by a spherical hexogen explosive charge detonated in a cubical tank and observed via high-speed photography. The phase difference was adjusted by setting different delays between the two detonations, with an accuracy of 1.0 ms. Experiments were conducted with both horizontally and vertically positioned bubble pairs and with single bubbles as well. UNDEX bubble pairs are subject to a larger buoyancy effect than cavitation or spark-generated bubble pairs. The resultant bubble behavior in the bubble–bubble interaction is more complex and is yet to be understood. In our experiments, various bubble parameters, including bubble pulsation periods, bubble elongation ratios, and collapse-induced shock wave pressures bubble, were measured and studied. Dependence of the bubble dynamics on Δθ was found, demonstrating the significant influence of Δθ on the morphology and shock wave pressure of bubble pairs. The findings suggest a method of strengthening or weakening the damage potential of an UNDEX bubble pair based on the proper adjustment of the delay between two detonations. It may also lead to a better understanding of the dynamics of interacting bubbles with buoyancy effects. [ABSTRACT FROM AUTHOR]

The physics of cavitation bubbles in water has been extensively studied for over a century. However, our understanding of bubble dynamics in binary immiscible fluids, such as a water-oil system, is limited. These systems have recently gained a lot of interest due to their relevance in modern medical treatment, emulsification, and the food industry. In this study, we establish an accurate and inexpensive three-dimensional boundary integral (BI) model for inertial cavitation bubble dynamics in binary immiscible fluid systems. Our novel scheme for solving velocities on the flow boundaries, expressed in a matrix form, can be easily extended to complex situations where multiple bubbles are generated in different phases. Additionally, we employ a density potential method and a weighted moving least-squares method to maintain a high level of mesh regularity. Our results demonstrate that the proposed 3D model is comparable in accuracy to a 3D axisymmetric model by comparing it against analytical solutions and the results obtained from an axisymmetric model. Furthermore, we compare our numerical simulations against a purposely conducted experiment for the bubble-droplet interaction, and excellent agreement is achieved. For the first time, we present simulation results of two-bubble interactions with an initially flat fluid-fluid interface and inside a droplet surrounded by a second fluid. We also reveal the dependences of the two-bubble morphologies, flow patterns, and jet velocities on the density ratio between the two phases. Finally, we find a significant difference in the mechanism of fluid mixing by multiple bubbles compared to that of a single bubble case. • We established a 3D model for inertial bubbles in binary immiscible fluids. • Our 3D model has shown comparable accuracy to an axisymmetric model. • We reproduce the essential physics in an experiment of bubble-droplet interaction. • We unveil novel fluid-mixing mechanisms with multiple bubbles. • We explore the dependencies of two-bubble interactions on the density ratio. [ABSTRACT FROM AUTHOR]

In this work ice breaking caused by a pair of interacting collapsing bubbles was studied by an experimental approach. The bubbles were generated by an underwater electric discharge simultaneously, positioned either horizontally or vertically below a floating ice plate and observed via high-speed photography. The bubble-induced shock waves, which turn out to be crucial to the fracturing of the ice, were visualized using a shadowgraph method and also measured using pressure transduces. Unique bubble behaviour was observed, including bubble coalescence, bubble splitting, inclined counter-jets and asymmetric toroidal bubble collapse. Bubble dynamic properties, such as jet speed, jet energy and bubble centre displacement, were measured. Shock wave emission and ice breaking capability of the two bubbles were investigated over a range of inter-bubble and bubble–boundary distances. Regions where the damaging potential of the bubble pair are strengthened or weakened were summarized and possible reasons for the variation in the ice breaking capability were analysed based on bubble morphology, jet characteristics and shock wave pressure. The findings may contribute to more efficient ice breaking and also inspire new ways to manipulate cavitation bubble damage. [ABSTRACT FROM AUTHOR]

In this paper, we establish a fluid-structure interaction (FSI) model to investigate the dynamic interactions between the underwater explosion (UNDEX) shock wave and a movable sphere near the free surface. We utilize the local discontinuous Galerkin (LDG) method to capture the propagation of the shock wave in the fluid domain and employ the pressure cutoff model to calculate cavitation effects. The fluid elements at the fluid-structure interface are directly coupled to the structural dynamic model, and the structural transient dynamic responses are coupled with fluid acoustic pressure at the fluid-structure interface in the governing equation. The validity of the present FSI model is verified by comparing with the continuous Galerkin method. Due to the advantage of the LDG method in capturing the discontinuous wave, the present model shows better properties than the traditional coupled acoustic-structural model. With the present FSI model, we investigate the interaction between the UNDEX shock wave and a submerged and floated sphere. Under the combined effects of the free surface and structure, the UNDEX shock and cavitation loading characteristics are analyzed, and the influences of complicated cavitation effects on dynamic responses of the sphere are discussed. [ABSTRACT FROM AUTHOR]

The Boundary Integral Method (BIM) has been widely and successfully applied to cavitation bubble dynamics; however, the physical complexities involved in the coalescence of multiple bubbles are still challenging for numerical modeling. In this study, an improved three-dimensional (3D) BIM model is developed to simulate the coalescence of multiple cavitation bubbles near a rigid wall, including an extreme situation when cavitation bubbles are in contact with the rigid wall. As the first highlight of the present model, a universal topological treatment for arbitrary coalescence is proposed for 3D cases, combined with a density potential method and an adaptive remesh scheme to maintain a stable and high-accuracy calculation. Modeling for the multiple bubbles attached to the rigid boundary is the second challenging task of the present study. The effects of the rigid wall are modeled using the method of image; thus, the boundary value problem is transformed to the coalescence of real bubbles and their images across the boundary. Additionally, the numerical difficulties associated with the splitting of a toroidal bubble and self-coalescence due to the self-film-thinning process of a coalesced bubble are successfully overcome. The present 3D model is verified through convergence studies and further validated by the purposely conducted experiments. Finally, representative simulations are carried out to elucidate the main features of a coalesced bubble near a rigid boundary and the flow fields are provided to reveal the underlying physical mechanisms. [ABSTRACT FROM AUTHOR]

According to the behaviors of a bubble in the ship wake flow, the numerical simulation is divided into two stages, quasi-spherical motion and non-spherical motion, based on whether the bubble is captured by the vortex or not. The one-way coupled particle tracking method (PTM) and the boundary element method (BEM) are adopted to simulate these two stages, respectively. Meanwhile, the initial condition of the second stage is taken as the output of the first one, and the entire simulation is connected and completed. Based on the numerical results and the published experimental data, the cavitation inception is studied, and the wake bubble is tracked. Besides, the split of the bubble captured by the vortex and the following sub-bubbles are simulated, including motion, deformation, and collapse. The results provide some insights into the control on wake bubbles and optimization of the wake flow. [ABSTRACT FROM AUTHOR]

Experiments on oblique water entry of high-speed projectiles are performed to investigate cavity evolution and impact loads. The nose shape determines the cavity behavior and consequently affects the load characteristics. Each projectile used in the experiments has a truncated-cone head and a cylindrical body. The geometry of the truncated cone is defined by two dimensionless parameters. We adopt 40 kinds of head shape, while the impact velocity (41 m/s) and impact angle (30°) are kept the same. The cavity and load characteristics are investigated using a high-speed camera and an acceleration sensor, respectively. We divide the cavity dynamics into two types. In one type, the first cavity develops and coalesces with the second cavity, leading to the formation of a single continuous cavity. In the second type, the closure of the first leading cavity occurs on the truncated-cone head or the cavity may collapse onto the head in later stages, causing the two cavities develop independently without coalescence. We also find that the transient impact load and the cavity dynamics are closely related, which has not been discussed in detail in previous studies. The quantitative relationship between the load peaks and the two dimensionless parameters is revealed. [ABSTRACT FROM AUTHOR]

Abstract The bubble-sphere interaction beneath a free surface is investigated experimentally and numerically, which is associated with underwater explosion and cavitation. We use the electric discharge technique to generate an oscillating bubble that is set above a movable sphere and below a free surface. The transient fluid-structure interaction (FSI) is recorded by high-speed imaging. We identify three distinctive bubble jet impact patterns, i.e., (i) jet impacts on the sphere top with water layer between them; (ii) the bubble partly envelops the sphere at first and then the jet impacts on the sphere top without water layer; (iii) the jet impacts on an annular ring, splitting the bubble into a toroidal bubble and a new singly-connected bubble. Such transient FSI events are modeled by the boundary integral method and the foregoing bubble splitting is modeled by an improved vortex ring model. The bubble, free surface, and sphere dynamics in experiments are found to be captured extremely well by numerical simulations. The bubble dynamic behaviors are associated with three dimensionless parameters and a systematic study of these parameters are provided. Lastly, the two types of cavitation phenomena on sphere surface are discussed and the detailed physical mechanisms are analyzed. Highlights • The bubble-sphere interaction beneath a free surface is investigated experimentally and numerically. • We identify three distinctive bubble jet impact patterns, which are classified in a graph according to two parameters. • A full coupling model based on boundary integral method reproduces the experimental observations well. • An improved vortex ring model is established to study the splitting of a bubble into a torus and a singly-connected bubble. • Two types of cavitation phenomena on sphere surface during the rebounding phase of the bubble. [ABSTRACT FROM AUTHOR]

The underwater explosion (UNDEX) phenomenon involves hydrodynamic behaviors with a wide range of spatial scales, from macroscopic cavitation regions to refined areas with fluid-structure interactions around the shock wavefront. Correct description of the refined shock wavefront is important for determining the shock wave position and pressure peak. In this work, the local discontinuous Galerkin (LDG) method for the two-dimensional wave equation, which allows for the local triangular meshes refinement and merging during the shock wave propagation process, is presented to model the UNDEX shock hydrodynamics. This article is an extension of our previous work, where we discretize the UNDEX total wave formulation in the LDG framework and simulate the UNDEX shock wave and cavitation phenomenon. In order to save computational cost (including CPU time and computational storage) and capture the shock wavefront more accurately, the LDG method is extended to be in combination with the mesh adaption technique. The troubled-cell indicator, depending on the local jump of the pressure, is adopted to identify elements which require to be refined or coarsened. Typical numerical examples are tested to assess the capability of the present h -adaptive LDG method for capturing the UNDEX shock wave. The numerical results clearly demonstrate that the mesh adaption technique enables us to dynamically increase the mesh resolution in close proximity to the flow discontinuities with a large pressure jump. The local increase of the mesh resolution results in a substantial increase in the accuracy of the pressure peak. In terms of the computational efficiency, the h -adaptive LDG method can save nearly half of the calculation time in comparison to the non-adaptive LDG method on uniform meshes, whose mesh resolution is the same as that of the maximum level of refinement of the h -adaptive LDG method. • An h -adaptive LDG method is presented for simulating the UNDEX shock hydrodynamics. • The mesh resolution is adaptive with the evolution of the UNDEX shock wavefront. • The present h -adaptive LDG method can well capture the UNDEX pressure peak at a relative small computational cost. [ABSTRACT FROM AUTHOR]

Cavitation effects play a substantial role in the dynamic response of marine structures. In the previous numerical studies, it's assumed that the incident explosive loading is reflected by the horizontal free surface. But the cavitation phenomenon near the water wave has rarely been investigated. In this paper, we propose a novel 2D underwater explosion (UNDEX) model, which can be applied to study UNDEX cavitation characteristics near the water wave. On the basis of the local discontinuous Galerkin method, the 2D far-field UNDEX model is established to simulate the propagation of incident shock loading in fluid domain. In order to consider the water wave effects in the numerical model, the UNDEX model is combined with the nonlinear Stokes wave theory. The effects of Stokes wave pressure are introduced into the cavitation model, and the influences of the water wave profile on UNDEX loading characteristics are included in the present model. By comparing with an underwater explosion experiment, the present model is verified in simulating the shock wave propagation, and exhibits better properties than the acoustic finite element method. By means of the present model, the effects of the water wave phase angle and wave length on the cavitation characteristics are investigated. It's found that the cavitation duration of the region below wave crests is prolonged, while the cavitation duration of the region below wave troughs is shortened. The present model can be further extended to study the influence of irregular waves on the UNDEX loading characteristics. • Based on the LDG method and Stokes wave theory, the UNDEX cavitation characteristics near water wave are studied. • By comparing with the experimental result, the present LDG model is verified and exhibits better properties than the AFEM. • The curvature of the gravity wave surface is shown to play a profound role in UNDEX cavitation characteristics. • The cavitation phenomena near the gravity wave surface appear different scenarios with different wave lengths. [ABSTRACT FROM AUTHOR]

In the present study, the nonlinear bubble–plate interaction is investigated via boundary integral (BI) simulations with a particular focus on the effect of fluid–structure interaction (FSI). A series of experiments of electric discharge cavitation bubbles are also conducted to gain more physical insights. Good agreement between BI simulations and high-speed recordings is obtained. The effects of two geometrical parameters including the normalized stand-off distance γ and the plate-bubble size ratio θ (both scaled by the maximum bubble radius) are investigated systematically. We find four types of distinct bubble collapse patterns, namely, contacting jet that impacts the plate directly, non-contacting jet that impacts a layer of liquid, collision of two axial jets and no evident jet. A detailed phase diagram for the bubble collapse pattern is given in a large parameter space. Through the comparison of bubble dynamics near a movable and a fixed plate, the FSI effect is found to induce a significant increase in the jet impact velocity and the corresponding impact pressure. We also give a criterion condition for neglecting the FSI effect. Finally, we find two scaling laws for the plate motion with respect to γ , which can be explained using a semi-analytical model. • The transient fluid–structure interaction between a pulsating bubble and a movable plate is investigated. • Four types of distinct bubble collapse patterns are identified in a large parameter space. • The effect of FSI on the jet impact velocity and the structural pressure load is unveiled. • A criterion condition for neglecting FSI effect is given. • We find two scaling laws for the plate motion. [ABSTRACT FROM AUTHOR]

In this study, a two-phase flow solver with phase change is improved with considering the compressibility and is implemented in the open source software OpenFOAM. The thermal effect between water and vapor is ignored and the barotropic fluid hypothesis is used to close the system of Navier-Stokes equations. The finite volume method (FVM) is utilized for discretization and the volume of fluid (VOF) method is employed to capture the water/vapor interface. The improved solver is applied to the cavitating flow around a 2D CLARK-Y hydrofoil. The numerical results show good agreement with the experimental results. The mechanism of the cavitation instability induced by the re-entrant jet can be well reproduced. Pressure waves propagation induced by the collapse of the local shedding cloud cavity are discussed, which might lead to the shrinkage of the surrounding cavitation and high pressure pulse to the hydrofoil surface. The different roles played by the re-entrant jet and the local cavity collapse at different instants over one period in the compressible fluids are analyzed. Finally, analyses of the interaction between the cavity and the vortex structures are conducted. [ABSTRACT FROM AUTHOR]

• A modified BEM model considering the viscous effect is used to study the micrometre-sized bubble pair in ultrasonic waves. • Nonlinear bubble deformation results in the shift of the bubbles' first order frequency. • The excitation frequency which is lower than the natural frequency induces the stronger collapse of the bubbles. • The vertical distribution of the bubble pair may result in better surface cleaning effect. The dynamics of a micrometer-sized bubble pair in water near a rigid boundary under standing ultrasonic wave excitation is investigated in this study. The viscous effect in the boundary layer at the air-water interface is considered following the viscous correction model. The evolution of the bubble surface at the collapsing stage of the bubble pair is presented for different parameter sets. The field pressure near the rigid boundary, which is induced by the oscillating bubble pair, and the high-speed water jet at the collapse stage, form the main focus of the analysis. This reveals that a horizontal configuration of the bubble pair retards the strength of the bubble jet towards the boundary, whilst a vertical configuration, especially with differently-sized bubbles, can enhance the bubble collapse. This study may help to understand the interaction of multiple bubbles in an acoustic field and its application to surface cleaning. [ABSTRACT FROM AUTHOR]