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This paper presents an experimental study on the pulsation characteristics of bubbles under a damaged boundary. Bubble phenomena in the fluid domain, such as bubble shape, pulsation period, water jet, and motion, are observed, and...
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This paper presents an experimental study on the pulsation characteristics of bubbles under a damaged boundary. Bubble phenomena in the fluid domain, such as bubble shape, pulsation period, water jet, and motion, are observed, and the influence of distance parameters on these phenomena is analyzed in detail with the help of high-speed photography technology. We also discuss the relationship between bubble pulsation, the water spike form, and characteristic parameters under different boundary conditions, such as different hole shapes. It is found that the broken boundary weakens the effect of the wall on bubble motion, while the shape and area of the hole exert noticeable effects on the water spike. In addition, the rising part of the free surface consisting of three parts, the water spike, water skirt I, and water skirt II, along with their evolutionary forms at the gas-liquid interface, are experimentally studied.
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This paper is concerned with the dynamics of a spark-generated bubble inside a long, rigid, circular tube with two open ends submerged horizontally in a tank filled with water. The behaviour of the bubble was found to be sensitive...
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This paper is concerned with the dynamics of a spark-generated bubble inside a long, rigid, circular tube with two open ends submerged horizontally in a tank filled with water. The behaviour of the bubble was found to be sensitive to two geometrical parameters: the dimensionless tube radius alpha = R-T/R-Bmax and the dimensionless eccentricity epsilon = E/R-Bmax, where R-T is the inner radius of the tube, E is the distance from the bubble center at inception to the axis of symmetry of the tube, and R-Bmax is the maximum equivalent bubble radius (similar to 10 mm). The expansion, collapse and rebound of the bubble were captured using a high-speed camera both for the case where alpha > 1 and alpha < 1, with and without eccentricity, respectively. Some new features of the bubble dynamics were observed. In particular, a bubble initiated with eccentricity in a tube for which alpha < 1 was seen to migrate to the distal part of the tube at the end of collapse with formation of a jet also in that direction. This is distinct from the case of a bubble collapsing near a flat surface. A similar phenomenon has been observed previously in the case of a microbubble collapsing in a blood vessel under ultrasound excitation, but was attributed to the elasticity of the vessel wall. The present study suggests that it may in fact be due simply to the geometry of the system. A cloud of microbubbles was observed shortly after the start of rebound of the bubble. Our analysis shows that the microbubbles should be generated from nuclei in tap water with radii in the range of 10(-8) -10(-6) m. (C) 2019 Elsevier Ltd. All rights reserved.
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The dynamics of a bubble near a corner formed by two flat rigid boundaries (walls), is studied experimentally using a spark-generated bubble. The expansion, collapse, rebound, re-collapse and migration of the bubble, along with je...
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The dynamics of a bubble near a corner formed by two flat rigid boundaries (walls), is studied experimentally using a spark-generated bubble. The expansion, collapse, rebound, re-collapse and migration of the bubble, along with jetting and protrusion, are captured using a high-speed camera. Our experimental observations reveal the behaviour of the bubble in terms of the corner angle and the dimensionless standoff distances to the near and far walls in terms of the maximum bubble radius. The bubble remains approximately spherical during expansion except for its surface becoming flattened when in close proximity to a wall. When a bubble is initiated at the bisector of the two walls, the bubble becomes oblate along the bisector during the late stages of collapse. A jet forms towards the end of collapse, pointing to the corner. The closer the bubble to the two walls, the more oblate along the bisector the bubble becomes, and the wider the jet. A bubble initiated near one of the two walls is mainly influenced by the nearer wall. The jet formed is pointing to the near wall but inclined towards the corner. After the jet penetrates through the bubble surface, the bubble becomes a bubble ring, and a bubble protrusion forms following the jet. The bubble ring collapses and subsequently disappears, while the protrusion firstly expands, and then collapses and migrates to the corner.
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The strong coupling effect between an air bubble attached to a flexible plate and a nearby spark-generated bubble is investigated experimentally while also considering the structural deformation of the plate. Numerous interesting ...
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The strong coupling effect between an air bubble attached to a flexible plate and a nearby spark-generated bubble is investigated experimentally while also considering the structural deformation of the plate. Numerous interesting and complex bubble behaviors that depend strongly on the attached air bubble and deformation effect of the structure are observed with a high-speed camera. A multitude of bubble jet behaviors are observed during the two bubble pulsations, such as bubble splitting, jets away from the plate, jets toward the plate, upward and downward movement after splitting, and splitting after the coalescence of the two bubbles. Moreover, the attached air bubble can appear hemispherical, mushroom-shaped, or cup-shaped. To gain a better understanding of the interaction between the bubble and the flexible boundary, the displacement time history curves at the center point of the flexible structure and at the top and bottom vertices of the bubble are obtained. The motion response mode of the flexible plate under bubble loading is explored to summarize the deformation pattern of the flexible structure and the motion characteristics of the bubble. An air bubble attached to the flexible structure can change the motion characteristics of the spark-generated bubble to some extent.
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The interaction between a spark-generated bubble and an elastic sphere is investigated. A spark-generated bubble is created at various distances horizontally away from a suspended elastic sphere made of silicone rubber or super ab...
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The interaction between a spark-generated bubble and an elastic sphere is investigated. A spark-generated bubble is created at various distances horizontally away from a suspended elastic sphere made of silicone rubber or super absorbent polymer (of shear modulus of elasticity G of between 5 and 312 kPa), using a low-voltage spark discharge method. We observe pronounced deformation and elongation of the elastic sphere when the spark-bubble is generated very close to a sphere. This happens when the elastic sphere has a small modulus of elasticity and a small size ratio R' between the bubble and the elastic sphere (i.e. the bubble and the sphere have similar radii). Numerical simulations are also conducted using a Boundary Element Method (BEM) model coupled with a Finite Element Method (FEM) solver. The simulation results compare well with the experimental data. The numerical model is then extended to study the effects of elasticity and experimental parameters, such as the dimensionless stand-off distance H', and size ratio R', on the degree of deformation of the elastic cell and the dynamics of the bubble. (C) 2016 Elsevier Ltd. All rights reserved.
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The effect of cavitation bubbles on the removal of juvenile barnacles was documented using high speed photography. Using spark generated bubbles, the interaction between barnacle and cavitation bubble was examined in detail. The l...
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The effect of cavitation bubbles on the removal of juvenile barnacles was documented using high speed photography. Using spark generated bubbles, the interaction between barnacle and cavitation bubble was examined in detail. The liquid jet generated by the bubble collapse was observed to be directed towards barnacle at different impact intensities, which is related to the dimensionless distance H' (H'=H/R_m), where H is the distance between bubble formation point and the top of barnacle, and Rm is the maximum bubble radius. At lower values of H', higher speed liquid jet was produced; consequently a larger impact pressure was generated. In general, barnacles are more easily removed at a younger stage. In older barnacles, the liquid jet impact was only able to remove the barnacle shells, leaving the base plate attached to the surface. This study indicates that cavitation can be used to remove attached barnacles, and it would be more efficient if it is applied during early stages of fouling, before the formation of hard calcareous structures.
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摘要 :
The effect of cavitation bubbles on the removal of juvenile barnacles was documented using high speed photography. Using spark generated bubbles, the interaction between barnacle and cavitation bubble was examined in detail. The l...
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The effect of cavitation bubbles on the removal of juvenile barnacles was documented using high speed photography. Using spark generated bubbles, the interaction between barnacle and cavitation bubble was examined in detail. The liquid jet generated by the bubble collapse was observed to be directed towards barnacle at different impact intensities, which is related to the dimensionless distance H' (H'=H/R_m), where H is the distance between bubble formation point and the top of barnacle, and Rm is the maximum bubble radius. At lower values of H', higher speed liquid jet was produced; consequently a larger impact pressure was generated. In general, barnacles are more easily removed at a younger stage. In older barnacles, the liquid jet impact was only able to remove the barnacle shells, leaving the base plate attached to the surface. This study indicates that cavitation can be used to remove attached barnacles, and it would be more efficient if it is applied during early stages of fouling, before the formation of hard calcareous structures.
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The characteristics of synchronous and phase difference bubble pairs in axisymmetric configuration near a boundary are investigated experimentally by the spark discharge method. Their destructive forces on nearby boundaries are me...
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The characteristics of synchronous and phase difference bubble pairs in axisymmetric configuration near a boundary are investigated experimentally by the spark discharge method. Their destructive forces on nearby boundaries are measured using a polyvinylidene fluoride sensor. The bubble pair interactions and deformed features in the boundary vicinity are dissimilar to those in bulk water. Moreover, significant discrepancies between in-phase and out-of-phase pair interactions and their intensities of impulses are also witnessed. The interbubble distance (η), stand-off distance from the boundary (γ), and phase offset (τ) are crucial parameters affecting the shape evolutions and impulsive forces. From the qualitative analysis of sensor acquisition and high-speed imaging, it is observed that bubble periods are either prolonged or shorter than their corresponding isolated single cavity according to different parameters and arrangements. Additionally, the strongest impingements are produced by in-phase pairs. The impulses of phase difference bubble pairs are remarkably lower than in-phase pairs and even lower than a single bubble in some arrangements.
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Machine components operating in a fluid under conditions of cavitation and hard particle erosion can be severely affected by wear, which may reduce the lifespans of the components. To understand this synergic behaviour, in this wo...
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Machine components operating in a fluid under conditions of cavitation and hard particle erosion can be severely affected by wear, which may reduce the lifespans of the components. To understand this synergic behaviour, in this work, experimental and numerical approximations of the damage caused by a particle interacting with a spark-generated bubble were developed. The effects of particle size, particle material, bubble position, surface material and bubble size on the damage of a surface impacted by a particle propelled by the spark-generated bubble were evaluated. The experimental results show that under the tested conditions, the heaviest particles and larger bubbles caused more considerable damage, while the initial position of the bubble did not exert a significant influence. It was found that the relationship between the increase in the bubble size and the increase in damage was quadratic. Numerical simulations involving computational fluid dynamics (CFD) and explicit finite element analysis (FEA) of a particle interacting with bubble of several sizes were conducted. The findings exhibited good correlation with the experimental data which validated the proposed numerical models. Additionally, the simulation indicated that the damage on the surface was linearly related to the kinetic energy of a particle. Furthermore, it was identified that particles closer to the bubble nucleation point had higher velocities and could thus lead to more considerable damage to the surface; however, when the pressure inside the initial bubble was high (which produced larger bubbles), the bubble interface moved faster than the particle, and the particle was trapped by the bubble, which decelerated the particle and reduced the velocity of impact on the surface. The obtained results could help to explain the mechanism of interaction between the particle and the bubble and its correlation with solid surface damage.
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In the present work, the oscillation of a spark-created bubble near a confined water-air interface and the ensuing droplet generation and ejection are studied numerically using the boundary element method. The interface is accorde...
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In the present work, the oscillation of a spark-created bubble near a confined water-air interface and the ensuing droplet generation and ejection are studied numerically using the boundary element method. The interface is accorded by the top opening of either one of the following symmetrical configurations, which are distinguished by the value of angle between their vertical symmetry axis and lateral wall (i.e., θ): (i) a centrally perforated horizontal flat plate (θ = 90°) and (ii) vertically placed cylinder (θ = 0°), nozzle (θ > 0°) and diffuser (θ < 0°). Furthermore, the influences of the effective parameters such as the strength parameter (i.e., the intensity of local energy input), the bubble-free surface distance (standoff distance) and the nozzle size on the bubble dynamics and droplet formation and ejection processes are investigated. It was found that the moment at which the bubble attained its maximum volume was advanced as θ increased. In addition, by decreasing θ the attraction of the bubble toward the free surface during its expansion phase and its migration from the free surface during its contraction phase became stronger. Furthermore, for the nozzle case, by increasing θ, the volume of the droplet was increased. It was also found that by increasing the strength parameter, the volume of the droplet increased and its pinch-off happened earlier. Finally, as the standoff distance was increased, the volume of the droplet increased and its pinch-off was delayed.
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