Level set method for numerical simulation of a cavitation bubble,its growth, collapse and rebound near a rigid wall

A level set method of non-uniform grids is used to simulate the whole evolution of a cavitation bubble, including its growth, collapse and rebound near a rigid wall. Single-phase Navier-Stokes equation in the liquid region is solved by MAC projection algorithm combined with second-order ENO scheme for the advection terms. The moving inter-face is captured by the level set function, and the interface velocity is resolved by ＂one-side＂ velocity extension from the liquid region to the bubble region, complementing the second-order weighted least squares method across the interface and projection inside bubble. The use of non-uniform grid overcomes the difficulty caused by the large computational domain and very small bubble size. The computation is very stable without suffering from large flow-field gradients, and the results are in good agreements with other studies. The bubble interface kinematics, dynamics and its effect on the wall are highlighted, which shows that the code can effectively capture the ＂shock wave＂-like pressure and velocity at jet impact, toroidal bubble, and complicated pressure structure with peak, plateau and valley in the later stage of bubble oscillating.

The Collapse Intensity of Cavities and the Concentration of Free Hydroxyl Radical Released in Cavitation Flow

Enhancing the chemical reaction processes by means of the energy released in the collapse of micro bubbles or cavities in the cavitation flow is a new research area. In the previous work, a new approach of measuring concentration of free hydroxyl radicals induced in cavitation flow by using methylene blue as the indicator was developed and used to study concentration of free radical induced in Venturi cavitation flow under various experimental conditions. In the present research, the radial evolution of a cavity bubble and the corresponding collapse pressure in sonic cavitation field are obtained by solving three different bubble dynamics equations： Rayleigh equation, Rayleigh-Plesset equation and Gilmore equation. By comparing with the experimental data on the radial evolution of a cavity bubble in the literature, it is found that the predicted results by the Gilmore equation, which takes account of the compressibility of fluid in addition to the viscosity and interfacial tension, agree with the experimental ones better than those by other two equations. Moreover, the theoretically predicted collapse pressures are consistent with the concentration of the free hydroxyl radical induced in the experimental venture. Thus, the concentration of the liberated free hydroxyl radical not only influences the reaction rate but also is used as an available parameter for measuring collapse intensity of cavities.

Effect of cavitation bubble collapse on hydraulic oil temperature

Cavitation bubble collapse has a great influence on the temperature of hydraulic oil. Herein, cone-type throttle valve experiments are carried out to study the thermodynamic processes of cavitation. First, the processes of growth and collapse are analysed, and the relationships between the hydraulic oil temperature and bubble growth and collapse are deduced. The effect of temperature is then considered on the hydraulic oil viscosity and saturated vapour pressure. Additionally, an improved form of the Rayleigh–Plesset equation is developed. The effect of cavitation on the hydraulic oil temperature is experimentally studied and the effects of cavitation bubble collapse in the hydraulic system are summarised. Using the cone-type throttle valve as an example, a method to suppress cavitation is proposed.

Bubble dynamics characteristics and influencing factors on the cavitation collapse intensity for self-resonating cavitating jets

Based on bubble dynamics theory, a mathematic model describing the cavitation bubble size variation in the flow field of self-resonating cavitating jet was developed considering the pressure field and mass and heat exchange between cavitation bubble and ambient fluid. With this model, the influence factors on the cavitation intensity are investigated. The results show that the destructiveness of cavitating jet in breaking rocks depends on the bubble's first collapse, with decreasing intensity in the subsequent collapses. The self-resonating effect significantly enhances the cavitation intensity by promoting the collapse pressure and elongating its duration. Hydraulic parameters are proven to be the dominating factors influencing cavitation intensity: while collapse intensity monotonously increases with jet velocity, there exists an optimum ambient pressure where highest collapse intensity can be achieved. Conversely, the fluid properties show minor influences: cavitation intensity only slightly decreases with the increasing of fluid's density and barely changes with the variation of viscosity and surface tension. The results from this investigation help to uncover the mechanism of the enhanced erosion potential of self-resonating cavitating jet. The conclusions can be used to further improve the performance of self-resonating cavitating jet in field applications.

Cavitation Bubble Collapse near a Curved Wall by the Multiple-Relaxation-Time Shan-Chen Lattice Boltzmann Model

The cavitation bubble collapse near a cell can cause damage to the cell wall. This effect has received increasing attention in biomedical supersonics. Based on the lattice Boltzmann method, a multiple-relaxation-time Shan–Chen model is built to study the cavitation bubble collapse. Using this model, the cavitation phenomena induced by density perturbation are simulated to obtain the coexistence densities at certain temperature and to demonstrate the Young–Laplace equation. Then, the cavitation bubble collapse near a curved rigid wall and the consequent high-speed jet towards the wall are simulated. Moreover, the influences of initial pressure difference and bubble-wall distance on the cavitation bubble collapse are investigated.

The Numerical Simulation of Collapse Pressure and Boundary of the Cavity Cloud in Venturi

The idea that the collapse proceeds from the outer boundary of the cavity cloud towards its center for the ultrasonic cavitation proposed by Hasson and Morch in 1980s is further developed for calculating the collapse pressure and boundaries of cavity cloud at the collapse stage of bubbles for hydraulic cavitation flow in Venturi in present research. The numerical simulation is carried out based on Gilmore＇s eouations of bubble dynamics, which take account of the compressibility of fluid besides the viscosity and interfacial tension. The collapse of the cavity cloud is considered to proceed layer by layer from the outer cloud towards its inner part. The simulation results indicate that thepredicted boundaries of the cavity cloudat the collapse stage agree.well with the exPerimental ones.It is also found that the maximum collapse pressure of the cavity cloud is several times as high as the collapse pressure of outside boundary, and it is located at a point in the axis, where the cavity cloud disappears completely. This means that a cavity cloud has higher collapse pressure or strength than that of a single bubble due to the interactions of the bubbles. The effects of operation and structural parameters on the collapse pressure are also analyzed in detail.

Modeling for Collapsing Cavitation Bubble near Rough Solid Wall by Mulit-Relaxation-Time Pseudopotential Lattice Boltzmann Model

Cavitation bubble collapse near rough solid wall is modeled by the multi-relaxation-time (MRT) pseudopotential lattice Boltzmann (LB) model. The modified forcing scheme, which can achieve LB model’s thermodynamic consistency by tuning a parameter related with the particle interaction range, is adopted to achieve desired stability and density ratio. The bubble collapse near rough solid wall was simulated by the improved MRT pseudopotential LB model. The mechanism of bubble collapse is studied by investigating the bubble profiles, pressure field and velocity field evolution. The eroding effects of collapsing bubble are analyzed in details. It is found that the process and the effect of the interaction between bubble collapse and rough solid wall are affected seriously by the geometry of solid boundary. At the same time, it demonstrates that the MRT pseudopotential LB model is a potential tool for the investigation of the interaction mechanism between the collapsing bubble and complex geometry boundary.

A numerical model for cloud cavitation based on bubble cluster

The cavitation cloud of different internal structures results in different collapse pressures owing to the interaction among bubbles. The internal structure of cloud cavitation is required to accurately predict collapse pressure. A cavitation model was developed through dimensional analysis and direct numerical simulation of collapse of bubble cluster. Bubble number density was included in proposed model to characterize the internal structure of bubble cloud. Implemented on flows over a projectile, the proposed model predicts a higher collapse pressure compared with Singhal model. Results indicate that the collapse pressure of detached cavitation cloud is affected by bubble number density.

BUBBLE COLLAPSE IN SOLID-LIQUID TWO-PHASE FLUID

Equations of motion for bubble collapse in solid-liquid two-phase fluid have been derived, in which the resistance coupling effects between the liquid and solid particles have been considered. The motion of particles during the bubble collapse and the effects of particles on bubble collapse have been calculated and discussed. Qualitative relations between the concentration and the size of the particles and the rate of bubble collapse have been obtaind.

Modeling and Simulation of a Gas-Liquid Coupling Excitation-Induced Cavitation Bubble

A gas-liquid coupling excitation mode is proposed and the gas-liquid excitation experimental system is developed. Air from pulse generator is mixed with liquid,through which the generated cavitation bubbles can strip contaminants adhered to the pipe inner wall rapidly. The kinematics equation of the bubble inside the hydraulic oil is established and the numerical simulations are carried out. The influential factors such as gas pressure, excitation frequency,initial bubble radius and fluid viscosity are analyzed.The results show that the cavitation will evolve from steady state to transient state with the increasing gas pressure and initial bubble radius. The pulse generator frequency has a slightly effect on the growth of the bubble radius,and the breakup time of the bubble is shortened with the rising frequency. Similarly, the increasing viscosity of liquid has minimal impact on cavitation effect,which can weaken the growth and the collapse of the bubble. Moreover,the temperature inside the cavitation bubble is investigated,indicating that the instantaneous temperature inside the bubble increases with the rising gas pressure. Once the gas pressure is raised to a certain value greater than the fluid static pressure, the instantaneous temperature inside the bubble will rise sharply. So, it can be concluded that the gas-liquid coupling excitation-induced cavitation process is controllable, and some theoretical basis of the new excitation mode is presented,which is expected to be applied in the online cleaning of the complex hydraulic system.

Effect of inertial acoustic cavitation on antibiotic efficacy in biofilms

Bacterial biofilms can lead to chronic infections,increase tolerance to antibiotics and disinfectants,resistance to phagocytosis,and other components of the body’s immune system.Biofilm formation is implicated in the persistence of staphylococcal infections and chronic Pseudomonas aeruginosa lung infections in cystic fibrosis(CF)patients(which can result from biofilm-growing mucoid strains).Conventional treatments utilize aggressive antibiotic prophylaxis/therapy to prevent/eliminate biofilms,followed by chronic suppressive therapy.Recently,the use of enzymes to dissolve the biofilm matrix was investigated,in addition to quorum sensing inhibitors to increase biofilm susceptibility to antibiotics.Here,we propose a novel strategy,utilizing ultrasound-induced inertial cavitation,to increase antibiotic efficacy.The wall shear stress at the biofilm interface is calculated,and viscoplastic constitutive equations are used to examine the biofilm response to the mechanical stress.Our simulations suggest that the maximum biofilm detachment occurs at high pressure/low frequency,and the mechanical disruption can affect the biochemical processes inside the biofilm resulting in vulnerability to antibiotics.

Investigation of cavitation bubble collapse near rigid boundary by lattice Boltzmann method

The dynamics of the bubble collapse near a rigid boundary is a fundamental issue for the bubble collapse application and prevention. In this paper, the bubble collapse is modeled by adopting the lattice Boltzmann method （LBM） and is verified, and then the dynamic characteristics of the collapsing bubble with the second collapse is investigated. The widely used Shan-Chen model in the LBM multiphase community is modified by coupling with the Carnahan-Starling equation of state （C-S EOS） and the exact difference method （EDM） for the forcing term treatment. The simulation results of the bubble profile evolution by the LBM are in excellent agreements with the theoretical and experimental results. From the two-dimensional pressure field evolution, the dynamic characteristics of the different parts during the bubble collapse stage are studied. The role of the second collapse in the rigid boundary damage is discussed, and the impeding effect between two collapses is demonstrated.

Experimental and numerical investigations of the collapse of a laser-induced cavitation bubble near a solid wall

With the collapse of cavitation bubbles near the wall,micro-jets and shock waves will be formed,to generate a high-pressure load and to cause the cavitation damage on the surface of the hydraulic machinery.Due to the rapid development of the cavitation bubble collapse process(in the time scale of hundred nanoseconds),the time resolution of the conventional high-speed cameras should reach more than one million frames per second,which will limit the spatial resolution,and obscure the details of the cavitation bubble shape near the cavitation bubble collapse moment.In this paper,with the help of the laser cavitation bubble photogrammetry system with nanosecond-micron space-time resolution,the experiment is carried out for the cavitation bubble collapse morphology evolution near the wall.The morphological characteristics of the cavitation bubble collapse at specific times are analyzed.With the help of the OpenFOAM code,the collapse process of the cavitation bubble near the solid wall is calculated.It is shown that the cavitation bubble near the wall collapses in an axial symmetric heart shape and the micro-jet directed to the wall will pull the cavitation bubble towards the wall.The counter-jet generated in the rebound stage will drive the cavitation bubble to move away from the wall.The numerical simulation of the cavitation bubble shape in the collapse period is well consistent with the experimental results,but the ability to capture the shock wavefront needs to be improved.Under the conditions studied in this paper,the cavitation bubble collapse micro-jet velocity can reach up to a hundred meters per second both in the experiment and the numerical simulation.

Cavitation bubbles collapse characteristics behind a convex body

Cavitation bubbles behind a convex body were experimentally studied by a high speed camera and a hydrophone synchronously. The experiments were conducted in a circulating water tunnel with five various contraction ratios： β = 0.497, β= 0.6, β= 0.697, β= 0.751, and β= 0.799. The distributions of the cavitation bubble collapse positions behind the five different convex bodies were obtained by combining the images taken by the high speed camera. According to the collapse positions, it was found that no cavitation bubble was collapsed in the region near the wall until the ratio of the water head loss over the convex body height was larger than 20, which can be used to predict if the cavitation damage would occur in the tunnel with orifice energy dissipaters.

Investigation of cavitation erosion caused by laser-induced single bubble collapse near alloy coating surface

To investigate the mechanism of cavitation erosion caused by laser-induced single bubble near the surface coating alloy coating material, we utilized a nanosecond resolution photography system based on a Q-switched Nd: YAG laser and conventional industrial camera to carefully observe the transient process of bubble collapse under different conditions. We analyzed the generation of collapse microjets and the emission of collapse shock waves and explored the cavitation erosion characteristics caused by laser-induced single bubble collapse. We discovered that even on surfaces of highly hard and corrosion-resistant alloy coatings, severe cavitation erosion occurred, and there was a phenomenon of mismatch between the cavitation erosion location and the bubble projection position. The intensity of cavitation erosion depended on the energy self-focusing effect of the collapse shockwaves. In the experiments, we also observed the self-focusing phenomenon of collapse shockwaves under different conditions. The self-focusing effect of collapse shockwaves weakened as the distance between the bubble and the material surface increased, which may be the cause of cavitation erosion induced by a laser-induced single bubble.

Hydrodynamic mechanisms of aggressive collapse events in leading edge cavitation

Transient cavities generated from unsteady leading-edge cavitation may undergo aggressive collapses which are responsible for cavitation erosion.In this paper,we studied the hydrodynamic mechanisms of these events in the leading edge cavitation fonned over a modified NACA0009 hydrofoil using experimental and numerical methods.In the experimental investigation,high-speed visualization(HSV)and paint test are employed to study the behavior of the cavitating flow at σ=1.25,α=5°,U∞=20 m/s.In the numerical part,the same cavitating flow is simulated using an inviscid density-based compressible solver with a barotropic cavitation model.The numerical results are first compared with the experimental HSV to show that the simulation is able to reproduce the main features of the cavitating flow.Then,as the compressible solver is capable of capturing the shock wave upon the collapse of cavities,the location of collapse events with high erosion potential are determined.The location of these collapse events are compared with the paint test results with a qualitatively good agreement.It is clearly observed,in both the experiments and the numerical simulation,that there exists four distinct regions along the hydrofoil with higher risks of erosion:(1)A very narrow strip at the leading edge,(2)an area of accumulated collapses at around 60 percent of the sheet cavity maximum length,(3)an area around the closure line of the sheet cavity with the highest erosion damage,and(4)a wide area close to the trailing edge with dispersed collapse events.A combined analysis of the experimental and numerical results reveals that the small-scale structures generated by secondary shedding are more aggressive than the large-scale cloud cavities(primary shedding).It is also observed that the high risk of cavitation erosion in regions 2 and 3 is mainly due to the collapses of the small cavity structures that are formed around the sheet cavity closure line or the rolling cloud cavity.

Simulation of Heat Transfer with the Growth and Collapse of a Cavitation Bubble Near the Heated Wall

The growth and collapse behaviors of a single cavitation bubble near a heated wail and its effect on the heat transfer are numerically investigated. The present study is designed to reveal the mechanism of cavitation enhanced heat transfer from a microscopic perspective. In the simulation, the time-dependent Navier-Stokes equations are solved in an axisymmetric two-dimensional domain. The volume of fluid （VOF） method is employed to track the liquid-gas interface. It is assumed that the gas inside the bubble is compressible vapor, and the sur- rounding liquid is incompressible water. Mass transfer between two phases is ignored. The eaiculated bubble pro-files were compared to the available experimental data, and a good agreement was obtained. Then, the relationship among bubble motion, flow field and surface heat transfer coefficient was analyzed. On this basis, the effects of such factors as the initial distance between the bubble and the wall, the initial vapor pressure and the initial bubble nucleus size on the heat transfer enhancement are discussed. The present study is helpful to understand the heat transfer phenomenon in presence of cavitation bubble in liquid.

航行体出水过程空化结构演变与溃灭载荷特性研究

高速航行体肩部附着空泡穿越自由液面过程中发生溃灭现象,将产生巨大的冲击载荷,严重影响航行体出水姿态和结构安全性。该研究采用改进型分离涡模型,建立了水下发射非定常空化特性数值模拟方法。结果表明:航行体附着空泡出水过程自上而下发生溃灭现象,在溃灭末期收缩为较小的孤立空泡,溃灭产生的高速射流冲击结构表面;在出水过程中,水面附近的“反向旋转涡对”导致附着空泡发生了快速溃灭现象,以发卡涡为代表的壁面涡旋结构发展明显受到附着空泡的抑制;空泡溃灭末期的孤立泡产生了巨大的冲击压力峰值,对航行体结构安全性将造成巨大破坏。初期航行体迎流侧空泡溃灭行为也会导致较大的压力峰值出现。

通孔平板附近空化泡动力学特性数值模拟研究

在空化清洗过程中,清洗的程度与空化泡到孔板的距离和空化泡最大半径之间的比值息息相关。然而对通孔平板附近的空化泡动力学研究大多局限于空化泡形态和射流方向,对于射流强度以及演变周期的关注很少。本文基于开源流体动力学软件OpenFOAM平台,利用已建立的两相可压缩空化泡动力学模型对带有通孔平板附近的空化泡多周期演变动力学特性进行了深入地研究。通过分析发现,空化泡溃灭形成的液体射流到达孔心时的最大速度随着γ(特征距离)的增加而减小;随着ε(相对大小)的减小,液体射流达到孔心的最大速度增加;空化泡第一次溃灭时的最小半径均随着γ和ε的减小而逐渐增大,空化泡溃灭的剧烈程度有所减缓,第一次溃灭周期有所延长。

振动及加热壁面条件下的近壁空泡溃灭可视化研究

采用低压电火花生成空泡的方式系统研究了复杂壁面条件下的近壁空泡溃灭行为。通过不同类型振动壁面及不同温度壁面附近的空泡溃灭行为的可视化试验,发现壁面振动对空泡溃灭有促进作用,空泡溃缩过程更快,体积变化更显著。不同类型壁面对空泡溃灭过程的影响不同,柱形壁面相较于球形壁面,近壁的空泡形态较不规则,溃灭呈现整体塌陷的趋势,在反弹阶段会形成不规则的空泡云。壁面升温会使得近壁空泡产生明显趋向于壁面的溃灭行为,并伴随多次空化的现象,这会对壁面造成严重损伤。