Coupling bubble and material dynamics to model cavitation peening and pitting

Non-spherical bubble collapses near solid boundaries, generating water hammer pressures and shock waves, were recognized as key mechanisms for cavitation erosion. However, there is no agreement on local erosion patterns, and cavitation erosion damage lacks quantitative analysis. In our experiments, five distinct local erosion patterns were identified on aluminium sample surfaces, resulting from the collapse of laser-induced cavitation bubbles at moderate stand-off distances of $0.4\leqslant \gamma \leqslant 2.2$ , namely bipolar, monopolar, annular, solar-halo and central. Among them, the bipolar and monopolar patterns exhibit the most severe cavitation erosion when the toroidal bubbles undergo asymmetrical collapse along the circumferential direction during the second cycle. Shadowgraphy visualization revealed that asymmetrical collapse caused shockwave focusing through head-on collision and oblique superposition of wavefronts. This led to the variations in toroidal bubble radii and the positions of maximum erosion depth not matching at certain stand-off distances. Both initial plasma asymmetry and bubble–wall stand-off distance were critical in determining circumferential asymmetrical collapse behaviours. At large initial aspect ratios, the elliptical jet tips form during the contraction process, resulting in the toroidal bubble collapsing from regions with smaller curvature radii, ultimately converging to the colliding point along the circumferential direction. Our three-dimensional simulations using OpenFOAM successfully reproduce the key features of circumferentially asymmetrical bubble collapse. This study provides new insights into the non-spherical near-wall bubble collapse dynamics and provides a foundation for developing predictive models for cavitation erosion.

The introduction of the cavitation effect in ultrasonic surface rolling can further improve the strengthening effect. However, the mechanism of the time-space bubble collapse in cavitation ultrasonic surface rolling on the material wall is still uncertain. Therefore, in this study, both the shock wave produced by spherical bubble collapse and the microjet generated by non-spherical bubble collapse in cavitation ultrasonic surface rolling were investigated. A dynamic model of spherical bubble collapse in the ultrasonic surface rolling area was established, and the shock wave pressure on the wall surface produced by the collapse was analyzed numerically. The numerical results reveal that the shock wave pressure is significantly influenced by the ultrasonic amplitude. As the ultrasonic amplitude increased from 2 to 5 μm, the maximum collapse pressure of the bubble rose from 4700 to 38690 MPa, while the wall pressure increased from 75.07 to 416.07 MPa. Additionally, the pressure distribution on the wall surface caused by near-wall non-spherical bubble collapse was determined using the computational fluid dynamics (CFD) analysis method. The wall pressure generated by microjets peaked with a slight delay compared to that generated by shock waves, reaching a maximum value of 20.25 MPa when the normalized standoff distance was 0.6. Finally, cavitation erosion experiments were conducted. The results showed that the wall pressure was approximately between 340 and 382 MPa when the amplitude was 5 μm, which is generally consistent with previous numerical calculation results. This indicates that the shock wave generated by the collapse of the spherical bubble plays a dominant role in cavitation ultrasonic surface rolling. These research findings are crucial for further studies on the formation of residual stresses and microstructure evolution in cavitation ultrasonic surface rolling.

Mechanism of material deformation during cavitation bubble collapse

The present study deals with the collapse of nonspherical bubbles in a compressible liquid by taking the thermal diffusion into account. The ghost fluid method (GFM) is modified so as to consider the thermal diffusion through the bubble surface. The boundary condition for the temperature continuity at the interface is discussed for determining the values of the ghost fluids. The improved GFM is applied to the collapse of a single spherical bubble. The present results are in good agreement with those obtained from the equation of motion for a single bubble (Keller equation) coupling with the energy equation. The improved multigrid GFM is also applied to the interaction of a gas bubble with a strong shock wave. The non-spherical bubble collapse is simulated successfully by taking the thermal diffusion into account. The thermal boundary layers both inside and outside the bubble are captured with the present method although the thermal boundary layer in liquid is very thin. The bubble collapse due to the incident shock wave accompanies the formation of the liquid jets and shock waves leading to the high temperature field. The influence of thermal diffusion becomes more prominent when the initial bubble radius is small. It is shown that a large amount of heat outflows from the interior of the bubble to the liquid when the liquid jet hits the downstream surface of the bubble and the bubble rebounds. The increased thermal diffusion causes the decrease of the internal pressure and temperature in the bubble leading to more violent collapse.

Mechanisms of single bubble cleaning

SUMMARYSingle cavitation bubbles exhibit severe modeling and simulation difficulties. This is due to the small scales of time and space as well as due to the involvement of different phenomena in the dynamics of the bubble. For example, the compressibility, phase transition, and the existence of a noncondensable gas inside the bubble have strong effects on the dynamics of the bubble. Moreover, the collapse of the bubble involves the occurrence of critical conditions for the pressure and temperature. This adds extra difficulties to the choice of equations of state. Even though several models and simulations have been used to study the dynamics of the cavitation bubbles, many details are still not clearly accounted for.Here, we present a numerical investigation for the collapse and rebound of a laser‐induced cavitation bubble in liquid water. The compressibility of the liquid and vapor are involved. In addition, great focus is devoted to study the effects of phase transition and the existence of a noncondensable gas on the dynamics of the collapsing bubble. If the bubble contains vapor only, we use the six‐equation model for two‐phase flows that was modified in our previous work [A. Zein, M. Hantke, and G. Warnecke,J. Comput. Phys., 229(8):2964‐2998, 2010]. This model is an extension to the six‐equation model with a single velocity of Kapilaet al.(Phys. Fluid, 13:3002‐3024, 2001) taking into account the heat and mass transfer.To study the effect of a noncondensable gas inside the bubble, we add a third phase to the original model. In this case, the phase transition is considered only at interfaces that separate the liquid and its vapor. The stiffened gas equations of state are used as closure relations. We use our own method to determine the parameters to obtain reasonable equations of state for a wide range of temperatures and make them suitable for the phase transition effects. We compare our results with experimental ones. Also our results confirm some expected physical phenomena. Copyright © 2013 John Wiley & Sons, Ltd.

In the case of extracorporeal shock wave lithotripsy (ESWL), a shock wave–bubble interaction inevitably occurs near the focusing point of stones, resulting in stone fragmentation and subsequent tissue damage. Because shock wave–bubble interactions are high-speed phenomena occurring in tissue consisting of various media with different acoustic impedance values, numerical analysis is an effective method for elucidating the mechanism of these interactions. However, the mechanism has not been examined in detail because, at present, numerical simulations capable of incorporating the acoustic impedance of various tissues do not exist. Here, we show that the improved ghost fluid method (IGFM) can treat shock wave–bubble interactions in various media. Nonspherical bubble collapse near a rigid or soft tissue boundary (stone, liver, gelatin and fat) was analyzed. The reflection wave of an incident shock wave at a tissue boundary was the primary cause for the acceleration or deceleration of bubble collapse. The impulse that was obtained from the temporal evolution of pressure created by the bubble collapse increased the downward velocity of the boundary and caused subsequent boundary deformation. Results of this study showed that the IGFM is a useful method for analyzing the shock wave–bubble interaction near various tissues with different acoustic impedance.

The non-spherical collapse of a cavitation bubble near a rigid wall generates a high-speed liquid jet directed toward the surface, which has significant implications for fluid–solid interactions. The jet's direction can be controlled by introducing an in-phase bubble above the near-wall bubble. In this study, numerical simulations are employed with the compressibleInterFoam solver within the OpenFOAM framework to investigate the parametric conditions and formation mechanisms of transferred jets of near-wall bubbles that are directed away from the rigid wall. Transferred jets are classified as thin jets or regular jets based on the presence or absence of neck formation during near-wall bubble contraction. Thin jets, which develop a neck through curvature reversal at the bubble equator, exhibit significantly higher velocities owing to the high-pressure region generated by flow focusing. In contrast, regular jets, which do not form a neck, demonstrate relatively low jet velocities. Furthermore, we investigate the time integral of the forces acting on the surface of the near-wall bubble during its formation and collapse, i.e., the Kelvin impulse. By applying the momentum theorem, we establish a correlation between the Kelvin impulse and the transferred jet velocity, deriving a scaling law: Ujet*∼η/ζ, where ζ(ζ=(Rc/Rmax)3) is the dimensionless volume and η(η=1/γ22−1/(2γ1)2−1/(2γ1+γ2)2) is the distance factor. This scaling law is verified using numerical simulations. This study's findings enhance our understanding of the dynamics of non-spherical cavitation bubbles under the combined influence of multiple factors, which is of great importance.

Nd:YAG laser photodisruption with nanosecond (ns) pulses is an established method for intraocular surgery. In order to assess whether an increased precision can be achieved by the use of picosecond (ps) pulses, the plasma size, the shock wave characteristics, and the cavitation bubble expansion after optical breakdown with ps- and ns-laser pulses were investigated by time-resolved photography and acoustic measurements. Nd:YAG laser pulses with a duration of 30 ps and 6 ns, respectively, were focused into a water-filled glass cuvette. Frequency doubled light from the same laser pulses was optically delayed between 2 ns and 136 ns and used as illumination light source for photography. Since the individual events were well reproducible, the shock wave and bubble wall position could be determined as a function of time. From the slope of these r(t) curves, the shock wave and bubble wall velocities were determined, and the shock wave pressure was calculated from the shock velocity. The plasma size at various laser pulse energies was measured from photographs of the plasma radiation. The breakdown thresholds at 30 ps and 6 ns pulse duration were found to be 15 microJ and 200 microJ, respectively. At threshold, ps-plasmas are shorter than ns-plasmas, but at the same pulse energy they are always approximately 2.5 times longer. The initial shock pressures were 17 kbar after ps-pulses with an energy of 50 microJ, and 21 kbar after 1 mJ ns-pulses. The pressure amplitude decayed much faster after the ps-pulses. The maximum expansion velocity of the cavitation bubble was 350 m/s after a 50 microJ ps-pulse, but 1,600 m/s after a 1 mJ ns-pulse. The side effects of intraocular microsurgery associated with shock wave emission and cavitation bubble expansion can be considerably reduced by the use of ps-pulses, and new applications of photodisruption may become possible.

The dynamics of cavitation bubbles near the wedge region between the free surface and solid wall were experimentally studied using a high-speed camera. In the experiments, the wedge region had different angles θ depending on the solid surface's hydrophobicity. Additionally, the standoff distances of the bubble-solid wall (λw) and bubble-free surface (λs) were varied. Several intriguing results were obtained. First, a competitive mechanism between the free surface and the solid wall during the collapse was observed: the influence of the free surface on the bubble occurred earlier than that of the solid wall. As the collapse progressed, the influence of the solid wall increased. The ratio of the free surface-induced pressure gradient (FPG) to the solid wall-induced pressure gradient (SPG) at the end of the collapse determined the jet direction. Second, altering θ appeared to have minimal influence on the relative ratio (s) between FPG and SPG during bubble collapse. In contrast, variations in λw and λs significantly affected the changing of s, leading to distinct bubble collapse dynamics at varying standoff distances. Finally, the synergistic effects of θ and standoff distances on the jet direction were quantitatively and systematically studied. The distribution of different scale jets was divided based on the value of ζ (Kelvin impulse), and an overall distribution map was provided. This fundamental understanding of the interaction between bubble dynamics and complex boundaries may offer strategies for improving ultrasonic cleaning and reducing cavitation erosion.

This study introduces total internal reflection fluorescence (TIRF) as a method for investigating the dynamics of a single laser-induced cavitation bubble in water within a thin layer near a solid surface. TIRF, supported by a planar waveguide, was employed to image cavitation bubble collapses near the liquid–solid interface at 50 thousand frames per second (kfps). Simultaneously, traditional side-view background illumination at 150 kfps captured the bubble dynamics. Dilution series and thickness estimation were conducted to determine interfering fluorescence signal contributions. The results confirm that most fluorescence is excited by the evanescent field rather than surface scattering or bulk scattering. The lifecycle of cavitation bubbles with moderate non-dimensional stand-off distances, γ = 1.15, 1.35, and 1.6, are discussed. We find that a thin liquid film persists between the bubble and the solid surface throughout the entire collapse process, except for randomly distributed micron-sized bubbles attached to the surface between the first and second collapse. Shock waves were captured with TIRF during the second collapse, centered around regions of the toroidal cavity exhibiting locally stronger dynamics. These regions, termed gas-filled strong collapse areas (SCAs), were also where microcracks formed during the second collapse of a single cavitation bubble. While not all SCAs resulted in microcracks, every observed microcrack could be traced back to a preceding SCA. To our knowledge, this is the first application of TIRF to observe cavitation bubble collapses with sub-millisecond time resolution.

Dynamics of laser-induced plasma and cavitation bubble at high pressures and the impacts on underwater LIBS signals

Cavitation bubble dynamics

This study investigates the behaviors of a cavitation bubble in the vicinity of a hemisphere on a flat wall using a numerical solver of the OpenFOAM platform. First, the typical collapse behavior of the cavitation bubble is analyzed by examining the evolution of the pressure and temperature fields. Second, the spatial–temporal evolution of temperature along the symmetry axis of the cavitation bubble and the variations in temperature and pressure at the hemisphere vertex are analyzed. Finally, the influence of the stand-off distance and hemisphere radius on collapse behavior is discussed. From the numerical results, four distinct cases of collapse behavior are identified, and these cases are associated with the hemisphere radius and the stand-off distance. As the jet impinges on the hemisphere vertex, the temperature at the vertex initially rises due to the high temperature generated when the jet pierces the bubble and then decreases because of the low-temperature liquid within the jet. For a hemisphere with a small radius, the jet undergoes deflection upon impact, which then causes the cavitation bubble to fragment into two or three annular bubbles.

ABSTRACT: ltrasonic cavitation, particularly its mechanism and bubble dynamic behavior, has attracted considerable interest in ultrasound applications. However, the thermal damage of cavitation bubble collapse is difficult to study because of its complicated mechanism. In this work, the formation mechanism of cavitation bubbles in ultrasonic vibration honing (UVH) was analyzed. A dynamic model was established on the basis of the UVH mechanism, and bubble collapse temperature was presented under the law of thermodynamic equilibrium. Six honing parameters (i.e. rotation speed of the honing head, ultrasonic wave vibration amplitude, reciprocation speed of oilstone, initial bubble radius, ultrasonic wave stress amplitude, and honing pressure) were studied to explore the influence of bubble movement radius and collapse temperature. For the given parameters, the bubble movement radius and crack temperature were simulated by changing the honing parameter conditions. The bubble collapse radius ranged from 100 µm to 200 µm, and the highest and the lowest crack temperatures were 1,400 K and 300 K, respectively. The cavitation erosion on an aluminum foil surface corresponded to the thermal damage caused by the bubble collapse. The conclusions obtained in this study provide significant reference values for the mechanism of cavitation collapse and can be applied in ultrasonic cleaning, particularly in descaling oxide layers. Keywords: ultrasonic vibration honing, cavitation bubble, collapse temperature, cavitation erosion