Single bubble sonoluminescence and stable cavitation

A single bubble trapped at an antinode of an acoustic standing wave field in water can emit 50ps-140ps light pulses, called “single bubble sonoluminescence” (SBSL). It arouses much interest in physical acoustics because of its highly non-linear characteristics, high concentration of energy, and stable cavitation behavior. In this paper, bubble stability, the dynamic behavior of bubbles, non-invasive measurement of driving acoustic pressure and Mie scattering method are introduced.

Cavitation Bubble Luminescence in Cone-Type Throttle Valve

In light of the light emission from cavitation bubbles under certain conditions, the phenomena of the cavitation bubble luminescence in the hydraulic cone-type throttle valve is focused in this paper. Firstly,the software of automatic dynamic incremental nonlinear analysis( ADINA) is applied to studying the flow field of the flow channel of the cone-type throttle valve. And the pressure distribution of the valve flow channel is obtained. The easyhappening area of cavitation in the cone-type throttle valve is also found out by ADINA. Then,the experimental research on the conetype throttle valve is carried out in this paper. The changing law of the hydraulic oil temperature in the corresponding region under different system pressure and the backpressure condition are experimentally researched. The relationship between the luminescence intensity and the cavitation intensity,the pressure,and the temperature are also studied. Finally,a summary of the causal relationship between the luminescence and cavitation in the cone-type throttle valve,the cavitation effect on the hydraulic oil temperature,and the method for the inhibition of cavitation bubble luminescence are presented. The results show that the light intensity increases with the increase of the cavitation intensity,and the luminescence can be inhibited by the increase of backpressure.

Experimental research of the cavitation bubble dynamics during the second oscillation period near a spherical particle

In this paper,the dynamic behaviors of the cavitation bubble near a fixed spherical particle during the second oscillation period are analyzed based on the high-speed photographic system.The deformation and motion of the bubble during the second period are investigated by changing the distance between the particle and the bubble and the maximum radius of the bubble.Meanwhile,the variation of the equivalent radius and the centroid motions are analyzed,and the dynamic behaviors of the bubble are categorized according to the bubble morphological characteristics during the second period.Through this research,it is found that(1)The dynamic behaviors of the bubble during the second oscillation period could be divided into three typical cases:For case 1,a bulge would exist on the bubble interface away from the particle,and for case 2,a bulge would appear on the bubble interface and evolve towards the particle,while for case 3,the bubble would be divided into two parts.(2)The larger the dimensionless distance between the particle and the bubble,the smaller the maximum bubble equivalent radius in the second period,and the shorter the second oscillation period.(3)When the bubble is close to the particle,a counter-jet appears at the bubble interface away from the particle during the rebound stage.

A Coupled Cavitation Model in an Oscillatory Oil Squeeze Film

In this paper,the oscillatory oil squeeze film is taken as a research object,and a coupled cavitation model based on the theory of bubble dynamics and hydrodynamic lubrication is used and a specific method of numerical calculation is given.Then the parallel-plate squeeze film test apparatus is used to validate the coupled cavitation model.The pictures of the cavitation were captured by a high-speed camera and then processed to obtain the variation of the cavitation area in experiments.Compared with the experimental results,the model can successfully predict the process of generation and development of cavitation.At the same time,the pressure variation calculated by the model is in good agreement with the experimental data and the value of negative pressure is close to the experiment.On this basis,three new parameters that are related to bubble dynamics are studied by comparing the pressure and cavitation area variation in an oscillating squeeze oil film.The results show that surface dilatational viscosity,initial radii of cavitation nuclei and number of bubbles per unit area have a significant influence on the generation time of the cavitation,the maximum tensile stress that the oil film can withstand,and the speed of the cavitation collapse respectively.

Dynamics and measurement of cavitation bubble

Based on the introduction of international progress, our investigations on acoustic cavitation have been reported. Firstly we considered the cavity's dynamics under the drive of the asymmetrical acoustic pressure. An aspheric dynamical model was proposed and a new stable and aspheric solution was found in numerical simulation of the theoretical framework of the aspheric model. Then, a dual Mie-scattering technique was developed to measure the cavity's aspheric pulsation. A significant asynchronous pulsation signal between two Mie-scattering channels was caught in the case of large cavity driven by low acoustic pressure. As a direct deduction, we observed an evidence of cavity's aspheric pulsation. Furthermore, we studied the dependency of the asynchronous pulsation signal on the various parameters, such as the amplitude and frequency of the driving acoustic pressure, and the surface tension, viscosity and gas concentration of the liquid. Finally, we introduced a new numeric imaging technique to measure the shapes of the periodic pulsation cavities. The time-resolution was in the order of 20 ns, one order of magnitude lower than that in the previous work, say, 200 ns.

Effect of liquid viscoelasticity on the interactions between cavitation bubbles and free surfaces

The influence of liquid viscoelasticity on the interaction between cavitation bubbles and free surfaces is of great practical significance in understanding bubble dynamics in biological systems. A series of millimeter cavitation bubbles were induced by laser near the free surfaces of the water and viscoelastic polyacrylamide (PAM) solutions with different concentrations. The effects of liquid viscoelasticity on the interactions of cavitation bubbles with free surfaces are analyzed from the perspectives of the evolution of free surface and bubble dynamics. The experimental results show that as the dimensionless standoff distance increases, the evolutions of free surface behaviors in all experimental fluids can be divided into six types of water mounds, i.e., breaking wrinkles, spraying water film, crown, swallowed water spike, hillock, and slight bulge. All the critical values of the dimensionless distance dividing different types decrease with increasing concentration. The evolutions of first four types of water mounds in PAM solutions differ from those in the water. Water droplets splashing in different directions are produced around the breaking wrinkles in the water. Meanwhile, the breaking wrinkles in PAM solution move with the “liquid filaments” towards the central axis. The water spike in the pattern of spraying water film in PAM solution is more stable than that in the water. As the solution concentration increases, the water skirt in the pattern of crown contracts earlier and faster, and the rate of increase in the height of the water skirt decreases. For swallowed water spike in PAM solution, the upper part of the newly formed water spike is not significantly thicker than the middle part, and thus the water waist structure does not form. Liquid viscoelasticity inhibits the bubble growth and collapse, and the bubble migration as well, especially in the second period. Shorter and thicker cavities are formed in PAM solutions with higher concentration, while slender and stable cavities formed in the water at the same dimensionless distance. The velocity and displacement of the tip of bullet jet both decrease as the solution concentration increases.

Dynamic behaviors of cavitation bubble for the steady cavitating flow

In this paper, by introducing the flow velocity item into the classical Rayleigh-Plesset dynamic equation, a newequation, which does not involve the time term and can describe the motion of cavitation bubble in the steadycavitating flow, has been obtained. By solving the new motion equation using Runge-Kutta fourth order methodwith adaptive step size control, the dynamic behaviors of cavitation bubble driven by the varying pressure fielddownstream of a venturi cavitation reactor are numerically simulated. The effects of liquid temperature (correspondingto the saturated vapor pressure of liquid), cavitation number and inlet pressure of venturi on radial motionof bubble and pressure pulse due to the radial motion are analyzed and discussed in detail. Some dynamicbehaviors of bubble different from those in previous papers are displayed. In addition, the internal relationshipbetween bubble dynamics and process intensification is also discussed. The simulation results reported in thiswork reveal the variation laws of cavitation intensity with the flow conditions of liquid, and will lay a foundationfor the practical application of hydrodynamic cavitation technology.

Interaction Mechanism of Double Bubbles in Hydrodynamic Cavitation

Bubble-bubble interaction is an important factor in cavitation bubble dynamics. In this paper, the dynamic behaviors of double cavitation bubbles driven by varying pressure field downstream of an orifice plate in hydrodynamic cavitation reactor are examined. The bubble-bubble interaction between two bubbles with different radii is considered. We have shown the different dynamic behaviors between double cavitation bubbles and a single bubble by solving two coupling nonlinear equations using the Runge-Kutta fourth order method with adaptive step size control. The simulation results indicate that, when considering the role of the neighbor smaller bubble, the oscil-lation of the bigger bubble gradually exhibits a lag in comparison with the single-bubble case, and the extent of the lag becomes much more obvious as time goes by. This phenomenon is more easily observed with the increase of the initial radius of the smaller bubble. In comparison with the single-bubble case, the oscillation of the bigger bubble is enhanced by the neighbor smaller bubble. Especially, the pressure pulse of the bigger bubble rises intensely when the sizes of two bubbles approach, and a series of peak values for different initial radii are acquired when the initial radius ratio of two bubbles is in the range of 0.9～1.0. Although the increase of the center distance between two bubbles can weaken the mutual interaction, it has no significant influence on the enhancement trend. On the one hand, the interaction between two bubbles with different radii can suppress the growth of the smaller bubble; on the other hand, it also can enhance the growth of the bigger one at the same time. The significant en-hancement effect due to the interaction of multi-bubbles should be paid more attention because it can be used to reinforce the cavitation intensity for various potential applications in future.

Laser-induced cavitation bubble near boundaries

Laser-induced cavitation bubble has been widely used to investigate the mechanisms of hydraulic machinery cavitation erosion and to explore applications in atomization, alloy strengthening, ultrasonic chemistry, biomedicine, surface cleaning and materials processing. This paper consolidates existing research findings on the cavitation bubble dynamics near different boundaries and provides insights for future research work. Firstly, the dynamics of a single cavitation bubble in an infinite field is presented. Subsequently, the focus shifts to the dynamics of cavitation bubble near a rigid wall, angular walls, particles and hydrofoil. Lastly, the paper delves into the dynamics of cavitation bubble within a droplet, revealing the microscopic mechanism of droplet breakup induced by cavitation bubble.

High-speed experimental photography of collapsing cavitation bubble between a spherical particle and a rigid wall

The synergetic effects between cavitation bubbles and silt particles on the damages of materials are essential problems in fluid machineries. For studying the underlying microscopic mechanisms, in the present paper, the dynamic behaviors of a single cavitation bubble between a spherical particle and a rigid wall are experimentally investigated with a high-speed camera. The results indicate that the existence of the particle can affect the bubble shape during collapse and significantly accelerate the collapse velocity of the bubble. The influences of the particle size, the distance between the bubble and the particle and the distance between the bubble and the rigid wall on the phenomena are qualitatively and quantitatively analyzed. These parameters can prominently affect the collapse velocity of the bubble(especially its maximum value).

Interaction of Two Cavitation Bubbles in a Tube and its Effects on Heat Transfer

When two cavitation bubbles exist in a confined space, the interaction between the bubbles significantly affects the characteristics of bubble dynamic behaviors. In this paper, a three-dimensional(3D) model is established to study the growth and collapse of two cavitation bubbles in a heated tube and its effects on heat transfer. The liquid and gas phases throughout the calculation domain are solved by a set of Navier-Stokes equations. It is assumed that the gas inside the bubble is compressible vapor, and the surrounding liquid is incompressible water. The mass transfer between two phases is ignored. The calculated bubble profiles were compared to the available experimental data, and a good agreement has been achieved. Then, the relationship among the bubble motion, flow field and pressure distributions was analyzed. On this basis, the effects of bubble interaction on the heat transfer between the wall surface and sounding liquid were discussed. It is found that heat transfer in the centre wall region is enhanced owing to the vortex flow and micro-jet induced by the bubble contraction and collapse. In contrast, the highest surface temperature appears in the surrounding region, which is mainly attributed to the thermal resistance induced by the bubble. The present study is helpful to understand the heat transfer phenomenon with cavitation in the liquid.

Simulation of 2D Cavitation Bubble Growth Under Shear Flow by Lattice Boltzmann Model

Natural cavitation is defined as the phenomenon of the formation of vaporbubbles in a flow due to the pressure falls below the liquid’s vapor pressure. The in-ception of the cavitation bubble is influenced by many factors, such as impurities, tur-bulence, liquid thermal properties etc. In this paper, we simulate a 2D cavitation "bub-ble" growth under shear flow in the inception stage by Single-Component-MultiphaseLattice Boltzmann Model (SCMP LBM). An empirical boundary condition sensitive 2Dbubble growth rate, R ∼ et, is postulated. Furthermore, the comparison is conductedfor bubble behavior under different shear rates. The results show that the cavitationbubble deformation is coincident with prior droplet theories and the bubble growthdecreases slightly with the flow shear rate.

Effects of adjacent bubble on spatiotemporal evolutions of mechanical stresses surrounding bubbles oscillating in tissues

The cavitation dynamics and mechanical stress in viscoelastic tissues, as the primary mechanisms of some ultrasound therapies, are extremely complex due to the interactions of cavitation bubble with adjacent bubbles and surrounding tissues.Therefore, the cavitation dynamics and resultant mechanical stress of two-interacting bubbles in the viscoelastic tissues are numerically investigated, especially focusing on the effects of the adjacent bubble. The results demonstrate that the mechanical stress is highly dependent on the bubble dynamics. The compressive stress and tensile stress are generated at the stage of bubble expansion and collapse stage, respectively. Furthermore, within the initial parameters examined in this paper, the effects of the adjacent bubble will distinctly suppress the radial expansion of the small bubble and consequently lead its associated stresses to decrease. Owing to the superimposition of two stress fields, the mechanical stresses surrounding the small bubble in the direction of the neighboring bubble are smaller than those in other directions. For two interacting cavitation bubbles, the suppression effects of the nearby bubble on both the cavitation dynamics and the stresses surrounding the small bubble increase as the ultrasound amplitude and the initial radius of the large bubble increase, whereas they decrease with the inter-bubble distance increasing. Moreover, increasing the tissue viscoelasticity will reduce the suppression effects of the nearby bubble, except in instances where the compressive stress and tensile stress first increase and then decrease with the tissue elasticity and viscosity increasing respectively. This study can provide a further understanding of the mechanisms of cavitation-associated mechanical damage to the adjacent tissues or cells.

Dynamics of Vapor Bubble in a Variable Pressure Field

This paper presents analytical and numerical results of vapor bubble dynamics and acoustics in a variable pressure field.First,a classical model problem of bubble collapse due to sudden pressure increase is introduced.In this problem,the Rayleigh–Plesset equation is treated considering gas content,surface tension,and viscosity,displaying possible multiple expansion–compression cycles.Second,a similar investigation is conducted for the case when the bubble originates near the rounded leading edge of a thin and slightly curved foil at a small angle of attack.Mathematically the flow field around the foil is constructed using the method of matched asymptotic expansions.The outer flow past the hydrofoil is described by linear(small perturbations)theory,which furnishes closed-form solutions for any analytical foil.By stretching local coordinates inversely proportionally to the radius of curvature of the rounded leading edge,the inner flow problem is derived as that past a semi-infinite osculating parabola for any analytical foil with a rounded leading edge.Assuming that the pressure outside the bubble at any moment of time is equal to that at the corresponding point of the streamline,the dynamics problem of a vapor bubble is reduced to solving the Rayleigh-Plesset equation for the spherical bubble evolution in a time-dependent pressure field.For the case of bubble collapse in an adverse pressure field,the spectral parameters of the induced acoustic pressure impulses are determined similarly to equivalent triangular ones.The present analysis can be extended to 3D flows around wings and screw propellers.In this case,the outer expansion of the solution corresponds to a linear lifting surface theory,and the local inner flow remains quasi-2D in the planes normal to the planform contour of the leading edge of the wing(or screw propeller blade).Note that a typical bubble contraction time,ending up with its collapse,is very small compared to typical time of any variation in the flow.Therefore,the approach can also be applied to unsteady flow problems.

Numerical analysis of cavitation cloud shedding in a submerged water jet

Focused on the unsteady behavior of high-speed water jets with intensive cavitation a numerical analysis is performed by applying a practical compressible mixture flow bubble cavitation model with a simplified estimation of bubble radius. The mean flow of two-phase mixture is calculated by unsteady Reynolds averaged Navier-Stokes （URANS） for compressible flow and the intensity of cavitation in a local field is evaluated by the volume fraction of gas bubbles whose radius is estimated with a simplified Rayleigh- Plesset equation according to pressure variation of the mean flow field. High-speed submerged water jet issuing from a sheathed sharp-edge orifice nozzle is treated. The periodically shedding of cavitation clouds is captured in a certain reliability compared to experiment data of visualization observation and the capability to capture the unsteadily shedding of cavitation clouds is demonstrated. The results demonstrate that cavitation takes place near the entrance of nozzle throat and cavitation cloud expands consequentially while flowing downstream. Developed bubble clouds break up near the nozzle exit and shed downstream periodically along the shear layer. Under the effect of cavitation bubbles the decay of core velocity is delayed compared to the case of no-cavitation jet.

Experimental investigation of thermal effect on cavitation characteristics in a liquid rocket engine turbopump inducer

The objective of this paper is to investigate the effect of water temperature on cavitation characteristics in a turbopump inducer,a series of experiments at different temperatures have been conducted in a newly developed visualization test facility.It is found that higher temperature shows little influence on the non-cavitation performance and breakdown characteristic in the investigated range.The relationship between cavitation development and pressure fluctuation has been discussed in detail.Higher temperature displays a remarkable stabilization effect on the cavitation excited pressure.In particular,the inception cavitation numbers of both the super-synchronous rotating cavitation and synchronous rotating cavitation are decreased at higher temperatures,and the corresponding frequencies are not affected,while the amplitudes are distinctly reduced,and the occurrence range of synchronous rotating cavitation is significantly narrowed.A generalized RayleighPlesset equation has been employed to account for the thermal effect on the bubble development,which may provide a deep insight in understanding the experimental results.Thermal effect is found to act as a remarkable dissipation mechanism to suppress the bubble growth,smooth the collapse.In particular,the excited pressure during collapse is smaller at higher temperatures,which may lead to the stabilization effect of high temperature in this study.

Study of acoustic bubble cluster dynamics using a lattice Boltzmann model

The search for the development of a reliable mathematical model for understanding bubble dynamics behavior is an ongoing endeavor.A long list of complex phenomena underlies the physics of this problem.In the past decades,the lattice Boltzmann method has emerged as a promising tool to address such complexities.In this regard,we have applied a 121-velocity multiphase lattice Boltzmann model to an asymmetric cluster of bubbles in an acoustic field.A problem as a benchmark is studied to check the consistency and applicability of the model.The problem of interest is to study the deformation and coalescence phenomena in bubble cluster dynamics,as well as the screening effect on an acoustic multibubble medium.It has been observed that the LB model is able to simulate the combination of the three aforementioned phenomena for a bubble cluster as a whole and for every individual bubble in the cluster.

Quantitative calculation of reaction performance in sonochemical reactor by bubble dynamics

In order to design a sonochemical reactor with high reaction efficiency, it is important to clarify the size and intensity of the sonochemical reaction field. In this study, the reaction field in a sonochemical reactor is estimated from the distribution of pressure above the threshold for cavitation. The quantitation of hydroxide radical in a sonochemical reactor is obtained from the calculation of bubble dynamics and reaction equations. The distribution of the reaction field of the numerical simulation is consistent with that of the sonochemical luminescence. The sound absorption coefficient of liquid in the sonochemical reactor is much larger than that attributed to classical contributions which are heat conduction and shear viscosity. Under the dual irradiation, the reaction field becomes extensive and intensive because the acoustic pressure amplitude is intensified by the interference of two ultrasonic waves.

AN EXPERIMENTAL STUDY ON INCIPIENT CAVITATION EROSION OF FLOOD DISCHARGE TUNNEL WITH ORIFICE

Serial grades of cement mortar specimens have been tested on the model of con- crete-lining flood discharge tunnel with orifice,The results show that incipient cavitation erosion number is much smaller than incipient cavitation number.Based on the principle of bubble dy- namics,the flow speed effect on incipient cavitation erosion number for cement mortar is pre- sented.

Theoretical estimation of sonochemical yield in bubble cluster in acoustic field

In order to learn more about the physical phenomena occurring in cloud cavitation,the nonlinear dynamics of a spherical cluster of cavitation bubbles and cavitation bubbles in cluster in an acoustic field excited by a square pressure wave are numerically investigated by considering viscosity,surface tension,and the weak compressibility of the liquid.The theoretical prediction of the yield of oxidants produced inside bubbles during the strong collapse stage of cavitation bubbles is also investigated.The effects of acoustic frequency,acoustic pressure amplitude,and the number of bubbles in cluster on bubble temperature and the quantity of oxidants produced inside bubbles are analyzed.The results show that the change of acoustic frequency,acoustic pressure amplitude,and the number of bubbles in cluster have an effect not only on temperature and the quantity of oxidants inside the bubble,but also on the degradation types of pollutants,which provides a guidance in improving the sonochemical degradation of organic pollutants.