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2002, JSME International Journal Series B
The present paper reports observation results of collapsing cavity bubbles on a two-dimensional foil section by a highspeed video camera, together with impulsive force measurement. Results of numerical simulations of the behavior of bubble cluster corresponding to the above condition are also shown. With these materials the authors discuss the mechanism of generation of the impulsive force due to cavitation collapse.
EPJ Web of Conferences
Cavitation bubbles generated via laser-induced breakdown are investigated experimentally. The present work focuses on the direction of the first bubble collapse near a solid surface in distilled water. The solid surface is placed first to the right side in a cuvette filled with distilled water and then placed to the top of the cuvette. In this experiment, it is observed in which direction the cavitation bubble collapses. The cavitation bubble is visualized by a high-speed camera of frequency 68kHz.
Physics of Fluids
This paper presents a numerical study of the dynamics of an initially spherical bubble collapse near an oscillating rigid wall with a large amplitude; the wall oscillating amplitude is greater than 1% of the initial maximum bubble radius. Numerical simulations were conducted using a compressible two-phase flow model and the volume of fluid (VOF) interphase-sharpening technique on a general curvilinear moving grid. The numerical results for bubbles in the free field and near a wall were computed and compared with published experimental data. To study the effects of the oscillating wall on bubble collapse, a sinusoidal function was used for wall oscillation. The initial bubble conditions were set as a Rayleigh bubble located above the rigid wall at a dimensionless bubble-boundary distance with initial phases of 0{degree sign} and 180{degree sign}. During bubble collapse, the interface deformation, jetting behavior, bubble collapse time, and bubble migration were determined. Violent co...
Journal of Fluid Mechanics, 2002
High-speed observations (for example, clearly show that though a collapsing cavitation bubble approaches its minimum size as a coherent single volume, it usually reappears in the first rebounding frame as a cloud of much smaller bubbles or as a highly distorted single volume (see, for example, ). This paper explores two mechanisms that may be responsible for that bubble fission process, one invoking a Rayleigh-Taylor stability analysis and the other utilizing the so-called microjet mechanism. Both approaches are shown to lead to qualitatively similar values for the number of fission fragments and the paper investigates the flow parameters that effect that number. Finally, we explore the effective damping of the Rayleigh-Plesset single bubble calculation which that fission process implies and show that it is consistent with the number of collapses and rebounds which are observed to occur in experiments.
Journal of Fluid Mechanics, 1993
During the collapse of an initially spherical cavitation bubble near a rigid wall, a reentrant jet forms from the side of the bubble farthest from the wall. This re-entrant jet impacts and penetrates the bubble surface closest to the wall during the final stage of the collapse. In the present paper, this phenomenon is modelled with potential flow theory, and a numerical approach based on conventional and hypersingular boundary integral equations is presented. The method allows for the continuous simulation of the bubble motion from growth to collapse and the impact and penetration of the reentrant jet. The numerical investigations show that during penetration the bubble surface is transformed to a ring bubble that is smoothly attached to a vortex sheet. The velocity of the tip of the re-entrant jet is always directed toward the wall during penetration with a speed less than its speed before impact. A high-pressure region is created around the penetration interface. Theoretical analysis and numerical results show that the liquid-liquid impact causes a loss in the kinetic energy of the flow field. Variations in the initial distance from the bubble centre to the wall are found to cause large changes in the details of the flow field. No existing experimental data are available to make a direct comparison with the numerical predictions. However, the results obtained in this study agree qualitatively with experimental observations.
Recent observations of the geometries of growing and collapsing bubbles in typical cavitating Aows , Kuhn de Chizelle et al. 1992 have revealed the complexity of the "microfluidmechanics" associated with these flows. Clearly the interaction of individual bubbles with the nearby solid surface and its boundary layer produce features in the dynamics of growth and collapse which were not present in experiments on bubbles in a quiescent liquid. These include several mechanisms for bubble fission prior to collapse and the role played by the concentration of accumulated vorticity in producing a hybrid vortex/bubble during collapse.
Aiche Journal, 1997
Harnessing the energy associated with cavitation for a number of physical and chemical changes has been pursued enthusiastically by sonochemists using ultrasound equipment of various sizes, shapes and forms. The effect of cavitation as observed in ultrasonic equipment has been attributed to the transient form of cavitation. Hydrodynamically generated cavities are believed to behave in a stable cavitation mode and, hence, are not very useful for the desired sonochemical effects.
European Journal of Mechanics - B/Fluids, 2011
In the present study the effects of surface tension on the growth and collapse stages of cavitation bubbles are studied individually for both spherical and nonspherical bubbles. The Gilmore equation is used to simulate the spherical bubble dynamics by considering mass diffusion and heat transfer. For the collapse stage near a rigid boundary, the Navier-Stokes and energy equations are used to simulate the flow domain, and the VOF method is adopted to track the interface between the gas and the liquid phases. Simulations are divided into two cases. In the first case, the collapse stage alone is considered in both spherical and nonspherical situations with different conditions of bubble radius and surface tension. According to the results, surface tension has no significant effects on the flow pattern and collapse rate. In the second case, both the growth and collapse stages of bubbles with different initial radii and surface tensions are considered. In this case surface tension affects the growth stage considerably and, as a result, the jet velocity and collapse time decrease with increasing surface tension coefficient. This effect is more significant for bubbles with smaller radii.
Multiscale Science and Engineering
In this paper, we studied the nature of shockwave induced cavitation bubble collapse using molecular dynamics (MD) simulation. A major objective of this study is to determine how the variation of nanoscale bubble size and entrapped gas density affects the bubble dynamics. It is known that cavitation bubble collapse/implosion is a robust dynamic event that occurs when the potential energy of the acting pressure performs work in a rapid and violent manner. Usually, the energy converges into a significantly smaller region compared to the original cavitation bubble. While the mechanisms of cavitation have been studied for more than a century, recently, it has received renewed interests due to its connection with biomedical applications and traumatic brain injury (TBI). One of the common causes of TBI is blast-induced shock exposure. A shockwave, as a moving discontinuity of pressure, can lead to the formation and collapse of cavitation bubble and produce an impactful jet stream. The jet stream may cause a localized mechanical/thermal damage to the structures that come on its path. We have considered two different bubble sizes (10 and 20 nm). We found that different gas densities inside the cavitation bubble largely change the intensity of the aftermath. We also found the peak temperature during the collapse is linked to the bubble size but unrelated with the peak pressure.
Ultrasonics Sonochemistry, 2021
Cavitation events create extreme conditions in a localized 'bubble collapse' region, leading to the formation of hydroxyl radicals, shockwaves and microscopic high-speed jets, which are useful for many chemical and physical transformation processes. Single bubble dynamics equations have been used previously to investigate the chemical and physical effects of cavitation. In the present study, the state of the art of the single bubble dynamics equations was reviewed and certain noteworthy modifications were implemented. Simulations reaffirmed previously reported collapse temperatures of the order ~5,000 K and collapse pressures well over ~1,000 bar under varying operating conditions. The chemical effects were assessed in terms of the hydroxyl radical generation rate (OHG), calculated by applying the minimization of the Gibb's Free Energy method using simulated collapse conditions. OHG values as high as 1x10 12 • OH molecules per collapse event were found under certain operating conditions. A new equation was proposed to assess the physical effects, in terms of the impact pressure of the water jet-termed as the jet hammer pressure (JHP), formed due to the asymmetrical collapse of a bubble near a wall. The predicted JHP were found to be within a range of ~100 to 1000 bar under varying operating conditions. Important issues such as the onset of cavitation and chaotic solutions, for a cavitating single bubble dynamics were discussed. The Blake threshold pressure was found to be a sufficient criterion to capture the onset of cavitation. The impact of key operating parameters on the chemical and physical effects of cavitation were investigated exhaustively through simulations, over the parameter ranges relevant to acoustic and hydrodynamic cavitation processes. Presented methodology and results will be useful for optimisation and further investigations of a broad range of acoustic and hydrodynamic cavitation-based applications.
Ultrasonics Sonochemistry, 2005
In the present work, a cavity cluster of predetermined size has been considered to study the bubble dynamics in the hydrodynamic cavitation reactor. The effect of different operating and system parameters on the cavitational intensity has been numerically investigated. The yield of any cavitationally induced physical/chemical transformations depends not only on the collapse pressure of the cavities but also on the active volume of cavitation within the reactor. Empirical correlations have been developed to predict the collapse pressure and the active volume of cavitation as a function of different operating parameters based on the bubble dynamics studies. Recommendations are made for designing a cavitational reactor on the basis of the proposed empirical correlations. This work is a first step towards the designing and optimization of hydrodynamic cavitational reactor with cluster approach.
2003
In the present paper, we focus on a specific type of bubble cavitation over a lifting hydrofoil generated in a periodic way, which turns into attached spot cavitation for high generation frequency. The aim of our work is a better understanding of the inception mechanism of such cavitation as well as its interaction with the liquid flow. Tests are conducted in EPFL cavitation tunnel on a 2-D Naca0009 hydrofoil equipped with miniature pressure sensors. Flow visualisation and pressure transient are recorded in synchronous way for several test conditions. For a given hydrodynamic conditions, the frequency of bubble generation is found to be different for two neighboring bubble sources. Generation frequencies, as high as 5 kHz were measured. We have shown that periodic bubble cavitation originates from a local vaporization process, which takes place within the surface roughness in the minimum pressure area. The pressure transient caused by a traveling bubble passage over the hydrofoil was analyzed. It turns out that the pressure beneath the bubble is always negative and well below the expected vapor pressure. Therefore, a thin film of liquid, which could correspond to the boundary layer, stands between the bubble and the solid surface. Moreover, we have shown that a moving 3-D boundary layer separation is produced behind the bubble.
Physics of Fluids, 2006
We report experimental and numerical investigations on the dynamics of the cavitation of bubbles on a solid surface and the interaction between them with the help of controlled cavitation nuclei: hemispherical bubbles are nucleated from hydrophobic microcavities that act as gas traps when the substrate is immersed in water. The expansion of these nuclei is triggered by an impulsive lowering of the liquid pressure. The patterning of the substrate allows us to control the number of bubbles and the distance between them. Each hemispherical bubble experiences the effect of its mirror image. Correspondingly, an isolated hemispherical bubble together with its mirror image behaves like a free spherical bubble, i.e., its dynamics is well described by the Rayleigh-Plesset equation. We employ the setup to study the dynamics of two and more bubbles in a row at controlled and fixed distances from each other. For weak interaction, namely when the maximum size of the bubbles is smaller than the bubble distance, the dynamics of the system is well captured by an extended Rayleigh-Plesset equation, where mutual pressure coupling through sound emission is included. Bubble pairs last longer than an isolated bubble as neighboring bubbles modify the surrounding pressure and screen each other. For strong interaction, obtained by increasing the tensile stress or decreasing the bubble distance, the bubbles eventually flatten and form a liquid film between each other which can rupture, leading to coalescence. The film thinning is inertia dominated. A potential flow boundary integral simulation captures the overall shape evolution of the bubbles, including the formation of jets horizontal to the wall. These horizontal jets are caused by symmetry breaking due to the neighboring bubbles.
EPJ Web of Conferences, 2012
The article describes experimental setup for investigation of the impact load from collapsing cavitation bubble on a solid wall. A vapour bubble is generated inside a cubic chamber by local heating of water inside a thin channel in a button. The bubble collapse is initiated by a piezoelectric actuator attached to the flexible wall of the chamber. A laser diode with a linear CCD sensor are used to detect the bubble position during its buoyancy-driven rise to the upper wall of the chamber. The bubble collapse impact load is measured using a PVDF piezoelectric transducer glued to the upper wall of the chamber and recorded by high-speed CCD camera illuminated by a high-power LED diode. The pressure inside the chamber is measured by the dynamic pressure transducer. All the system components are controlled and synchronized by an oscilloscope and pulse generator using the LabView software.
Computers & Fluids, 2012
In this paper, we investigate the high-speed dynamics of symmetric and asymmetric cavitation bubble-collapse. For this purpose, a sharp-interface numerical model is employed, that includes a numerically effi-cient evaporation/condensation model. The underlying assumption is that phase change occurs in thermal non-equilibrium and that the associated timescale is much larger than that of the wave-dynamics described by the interfacial Riemann problem. The sharp-interface model allows for an accurate tracking of the interface evolution throughout collapse and rebound. With a first set of simulations, we investigate the influence of the non-equilibrium on the relaxation behaviour of an oscillating vapour bubble. We observe that a good prediction of the phase-change rate is essential. Of high practical interest is the col-lapse of cavitation bubbles near walls under high ambient-pressure conditions. We investigate the differ-ences in collapse evolution for detached and attached bubbles. It is shown that the maximum wall pressure strongly depends on the symmetry of the collapse mechanisms, and regions with a high proba-bility of bubble rebound are identified. Asymmetric attached bubbles lead to significantly different topol-ogy changes during collapse than symmetric bubbles but exhibit roughly the same range of maximum pressures.
Several recent experimental and allalytical investigations of cavitating flows have revealed new phenomena which clearly affect how we should view cavitation growth arid collapse and the strategies used t o ameliorate its adverse effects.
Atomization and Sprays, 2013
Journal of Fluid Mechanics, 1995
Recent observations of growing and collapsing bubbles in flows over axisymmetric headforms have revealed the complexity of the 'micro-fluid-mechanics ' associated with these bubbles Brianqon-Marjollet et al. 1990;. Among the complex features observed were the bubble-tobubble and bubble-to-boundary-layer interactions which leads to the shearing of the underside of the bubble and alters the collapsing process. All of these previous tests, though, were performed on small headform sizes. The focus of this research is to analyse the scaling effects of these phenomena due to variations in model size, Reynolds number and cavitation number. For this purpose, cavitating flows over Schiebe headforms of different sizes (5.08, 25.4 and 50.8 cm in diameter) were studied in the David Taylor Large Cavitation Channel (LCC). The bubble dynamics captured using high-speed film and electrode sensors are presented along with the noise signals generated during the collapse of the cavities.
EPJ Web of Conferences, 2018
There have been tried many types of liquids with different ranges of viscosity values that have been tested to form a single cavitation bubble. The purpose of these experiments was to observe the behaviour of cavitation bubbles in media with different ranges of absorbance. The most of the method was based on spark to induced superheat limit of liquid. Here we used arrangement of the laser-induced breakdown (LIB) method. There were described the set cavitation setting that affects the size bubble in media with different absorbance. We visualized the cavitation bubble with a 60 kHz high speed camera. We used here shadowgraphy setup for the bubble visualization. There were observed time development and bubble extinction in various media, where the size of the bubble in the silicone oil was extremely small, due to the absorbance size of silicon oil.
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