Interactions between a central bubble and a surrounding bubble cluster

The interaction of multiple bubbles is a complex physical problem.A simplified case of multiple bubbles is studied theoretically with a bubble located at the center of a circular bubble cluster.All bubbles in the cluster are equally spaced and own the same initial conditions as the central bubble.The unified theory for bubble dynamics[35]is applied to model the interaction between the central bubble and the circular bubble cluster.To account for the effect of the propagation time of pressure waves,the emission source of the wave is obtained by interpolating the physical information on the time axis.An underwater explosion experiment with two bubbles of different scales is used to validate the theoretical model.The effect of the bubble cluster with a variation in scale on the pulsation characteristics of the central bubble is studied.

Experimental and numerical study on the growth and collapse of a bubble in a narrow tube

The growth, expansion and collapse of a bubble in a narrow tube are studied using both experiments and numerical simulations. In experiment, the bubble is generated by an electric spark in a water tank and recorded by a highspeed camera system. In numerical simulation, the evolution of the bubble is solved by adopting axisymmetric boundary integral equation, considering the surface tension effect. The results of experiments and numerical simulations are compared and good agreements are achieved. Both of them show that a counter-jet forms and penetrates the bubble at the end of the collapse stage, before a ring type bubble forms. Under the attraction of the tube wall due to Bjerknes force, a ring jet is generated, pointing towards the tube. On the basis of this, some physical quantities like the pressure on the tube wall and kinetic energy are calculated in a case study. The effects of tube diameters and tube lengths on the bubble＇s behaviors are also investigated.

On the interaction between bubbles and the free surface with high density ratio 3D lattice Boltzmann method

The bubbles rise up and burst at the free surface is a complex two-phase process.A free energy lattice Boltzmann method(LBM)model is adopted in this paper to study this phenomenon.The interface capturing technique[Zheng et al.,2006]is used to deal with the high density ratio problem.The Laplace law and the air-water interface capturing ability are validated for the multiphase model.The interaction between the single bubble or multiple bubbles and the free surface are studied by the multiphase model.The force acting on the bubble and the evolution of the free surface is studied.Meanwhile,effect of the initial distance between two adjacent bubbles on interaction effects of multiple bubbles is investigated as well.

Prolonged simulation of near-free surface underwater explosion based on Eulerian finite element method

n the area of naval architecture and ocean engineering,the research about the underwater xplosion problem is of great significance.To achieve prolonged simulation of near-free surface underwater explosion,the underwater explosion transient numerical model is established in this paper based on compressible Eulerian finite element method(EFEM).Compared with Geers Hunter formula,EFEM is availably validated by simulating the free-field underwater xplosion case.Then,the bubble pulsation and flow field dynamic characteristics of the cases with different underwater explosive depth are compared in this work.Lastly,the height of the water hump and the pressure of flow flied are analyzed quantitatively through the simulation results.

A robust shell element in meshfree SPH method

With the incorporation of total Lagrangian smoothed particle hydrodynamics（SPH） method equation and moving least square（MLS） function,the traditional SPH method is improved regarding the stability and consistency.Based on Mindlin-Ressiner plate theory,the SPH method simulating dynamic behavior via one layer of particles is applied to plate＇s mid-plane,i.e.,a SPH shell model is constructed.Finally,through comparative analyses on the dynamic response of square,stiffened shells and cylindrical shells under various strong impact loads with common finite element software,the feasibility,validity and numerical accuracy of the SPH shell method are verified.Consequently,further researches on SPH shell may well pave the way towards solving problems involving dynamic plastic damage,tearing or even crushing.

Effect of a rigid structure on the dynamics of a bubble beneath the free surface

The coupling effect between a pulsating bubble and a free surface near a rigid structure is a complicated physical process.In this study,the evolution of an underwater explosion bubble and the free surface near a rigid structure is modeled by the boundary integral method.An approach of“double-node method”is used to maintain the stability of fluid-structure junction in the simulations,and meshes on the free surface and the structure are transformed to an open domain to ensure the calculation accuracy and efficiency.Validations are conducted against an underwater explosion experiment near a rigid structure.As a result,the simulations trace the jetting behavior of the bubble and the rise of the free surface.Finally,the bubble migration and the height of the free surface for different structure draughts are analyzed.

Experimental investigation of the interaction between a pulsating bubble and a rigid cylinder

In this paper, the behavior of a bubble near a rigid cylinder is studied experimentally as the positions of bubble induction change, and several cylinders with different diam- eters are used in the experiment. The main results are as follows. The behavior of a bubble near a rigid cylinder is distinct from that near a rigid plate. When the cylinders are laid in deep water, there will occur three kinds of typical bubble shapes as the distance between bubble and cylinder increases. And the bubble shapes are different as the diam- eter of cylinder varies. When the cylinders are laid near a free surface, the behaviors of bubble near cylinders with dif- ferent diameters are similar. For a certain distance between bubble and free surface, as the distance between bubble and cylinder increases, ＂double jet＂, ＂inclined jet＂ and ＂down- ward jet＂ will take place successively.

Numerical simulation of the self-propulsive motion of a fishlike swimming foil using the δ^+-SPH model

The present work is dedicated to the application of the recently developed （δ＋ -SPH） scheme to the self-propulsive fishlike swimming hydrodynamics. In the numerical method, a particle shifting technique （PST） is implemented in the framework of δ-SPH, combining with an adaptive particle refinement （APR） which is a numerical technique adopted to refine the particle resolution in the local region and de-refine particles outside that region. This comes into being the so-called δ＋- SPH scheme which contributes to higher numerical accuracy and efficiency. In the fishlike swimming modeling, a NACA0012 profile is controlled to perform a wavy motion mimicking the fish swimming in water. Thanks to the mesh-free characteristic of SPH method, the NACA0012 profile can undergo a wavy motion with large amplitude and move forward freely, avoiding the problem of mesh distortion. A parallel staggered algorithm is adopted to perform the fluidstructure interaction between the foil and the surrounding fluid. Two different approaches are adopted for the fishlike swimming problem. In Approach 1, the foil is fixed and flaps in a free stream and in Approach 2, the wavy foil can move forward under the self-driving force. The numerical results clearly demonstrate the capability of the δ＋ -SPH scheme in modeling such kind of self-propulsive fishlike swimming problems.

Theoretical study on bubble dynamics under hybrid-boundary and multi-bubble conditions using the unified equation

This paper aims to use the unified bubble dynamics equation to investigate bubble behavior in complex scenarios involving hybrid free surface/wall boundaries and interactions between multiple bubbles.The effect of singularity movement on the unified equation’s form is analyzed after deriving the bubble pulsation equation using a moving point source and a dipole,followed by discussions on the effect of migration compressibility-related terms on the bubble dynamics.In addition,the present study accounts for the impact of hybrid boundaries,including crossed and parallel boundaries,by introducing a finite number of mirror bubbles for the former and an infinite number of mirror bubbles for the latter.Spark bubble experiments and numerical simulation are conducted to validate the present theory.The application of the unified equation in multi-bubble interactions is exemplified by computing a spherical bubble array containing more than 100 uniformly distributed cavitation bubbles under different boundary conditions.The bubble cluster-induced pressure peak can reach nearly two times or even higher than that of an individual bubble,highlighting the damage potential caused by cavitation bubble clusters.

Transient interactions between bubbles and a high-speed cylinder in underwater launches: An experimental and numerical study

The underwater launch of high-speed vehicles involves complex bubble-structure interactions,which are not currently well understood.In this study,two small-scale experiments are carried out involving transient bubble-cylinder interactions.We adopt the underwater electric discharge method to generate a high-pressure bubble that drives a cylinder to a maximum velocity of∼25 m/s within 1 ms.A tail bubble forms as the cylinder is ejected from the launch tube.Moreover,we observe a shoulder cavity around the head of the cylinder due to the pressure reduction in the flow.To better understand the complex interaction between bubbles and the high-speed cylinder,we use the boundary element method to establish a bubble—structure interaction model.Our numerical model reproduces the experimental observations quite well,including the cylinder motion and the transient evolution of the bubbles.Thereafter,a systematic study is carried out to reveal the dependence of the bubble-cylinder interactions on the initial pressure of the tail bubble p0.We obtain a scaling law for the maximum velocity of the cylinder v_(m) with respect to p_(0),namely,v_(m) ∝ p_(0)^(0.45).The findings from this study may provide a reference for subsequent research into underwater launches.

一种改进的适用于多相流SPH模拟的粒子位移修正算法

由于光滑粒子流体动力学(smoothed particle hydrodynamics,SPH)方法具有天然的拉格朗日特性,粒子常常沿着流线运动,易产生不均匀的粒子分布,导致计算精度和稳定性下降.作为一种粒子分布均匀化技术,粒子位移修正(shifting)算法凭借其原理简单、效果显著等优点在SPH水动力学模拟中得到广泛应用.但同时,应用于多相流动中的shifting算法的计算过程较为复杂.针对这一问题,本文提出了一种改进的适用于多相流模拟的粒子shifting算法.与文献中已有的多相流shifting算法相比,它在处理多相交界面时能够维持更加均匀的粒子分布,同时可以保持清晰的异相界面,且实施相对简单高效.数值算例表明,该shifting算法在多相流模拟中具有更高的精度和更好的能量守恒特性.

Bubble dynamics and its applications

Bubbles have very important applications in many fields such as shipbuilding engineering, ocean engineering, mechanical engineering, environmental engineering, chemical engineering, medical science and so on. In this paper, the research status and the development of the bubble dynamics in terms of theory, numerical simulation and experimental technique are reviewed, which cover the underwater explosion bubble, airgun bubble, spark bubble, laser bubble, rising bubble, propeller cavitation bubble, water entry/exit cavitation bubble and bubble dynamics in other fields. Former researchers have done a lot of researches on bubble dynamics and gained fruitful achievements. However, due to the complexity of the bubble motion, many tough mechanical problems remain to be solved. Based on the research progress of bubble dynamics, this paper gives the future research direction of bubble dynamics, aiming to provide references for researches related to bubble dynamics.

基于多酸制备的新型氮化钼基复合材料作为锂离子电池负极材料

锂离子电池作为新一代储能装置,已经在生产和生活中得到了广泛应用.多酸有着多电子转移、化学结构稳定、氧化还原性可逆等诸多优点,具有作为电池电极材料的良好潜力.但易溶于电解液、导电性差且易发生团聚等问题阻碍了多酸用于电池电极材料的应用研究.本工作为解决上述问题,充分利用磷钼酸良好的水溶性、优异的氧化还原性和独特的酸性,通过简单的一锅法,在温和的条件下制备了磷钼酸/聚吡咯/石墨烯前驱体(PMo12/PPy/RGO,简称PCG),并通过高温氨化处理制备了新型氮化钼基复合材料(NPC@Mo2N/NPRGO),磷钼酸、聚吡咯在提供金属源和碳源的同时也提供了P,N杂原子掺杂到多孔碳和石墨烯中,该材料作为锂离子电池负极材料表现出较好的循环性能和倍率性能.在电流密度为100m A/g时,NPC@Mo2N/NPRGO的首周放电比容量可以达到1446 mAh/g,循环200周后仍然可以达到771 mAh/g.在电流密度为100, 200, 500, 1000, 2000 mA/g时,循环比容量分别为797,725,630,545,460m Ah/g.尤其是在大电流密度(1000m A/g)循环300周后容量仍能达到554mAh/g.

Numerical study on the bubble dynamics in a broken confined domain

In this paper,the dynamic characteristics of the bubble in a broken confined domain are studied.The broken confined domain is composed of a solid wall and a plate that has a hole.The axisymmetric numerical model is established by combining the Eulerian finite-element method with volume of fluid(VOF)method,and is validated by comparing the results with those from an experiment.Then the influences of the wall distance,plate distance and size of the hole are analyzed.The results show that cavity-attraction jet caused by the hole and annular jet caused by the upper solid wall compete with each other to dominate the bubble dynamics.The cavity-attraction jet develops earlier,but slower.Thus,jet load in the bubble stage is mainly generated by the annular jet with a higher impact speed.Within a certain range,the closer the hole is to the bubble or the smaller the hole,the longer the pulsation period of the bubble will be.

Numerical simulation of water entry problems considering air effect using a multiphase Riemann-SPH model

The water entry is a classic fluid-structure interaction problem in ocean engineering.The prediction of impact loads on structure during the water entry is critical to some engineering applications.In this paper,a multiphase Riemann-SPH model is developed to investigate water entry problems.In this model,a special treatment,a cut-off value for the particle density,is arranged to avoid the occurrence of negative pressure.A remarkable advantage of the present multiphase SPH model is that the real speed of sound in air can be allowed when simulating water-air flows.In the present work,considering the air effect,several typical water entry problems are studied,and the evolution of multiphase interface,the motion characteristic of structure and complex fluid-structure interactions during the water entry are analyzed.Compared with the experimental data,the present multiphase SPH model can obtain satisfactory results,and it can be considered as a reliable tool in reproducing some fluid-structure interaction problems.

Rupture of a rubber sheet by a cavitation bubble:an experimental study

Dynamics of spark-generated cavitation bubbles near a deformable rubber sheet are experimentally investigated in this study.The transient bubble–rubber interaction is captured by high-speed photography,including the expansion and collapse of the bubble,jetting behaviors,large deformation,and rupture of the rubber sheet.The dependence of bubble behaviors and rubber response on two governing parameters(the bubble–rubber stand-off distanceγand the rubber thickness T)are systematically studied.Firstly,the bubble collapse patterns or jetting directions are categorized into three regimes.If the bubble is initially located at a largeγfrom the rubber sheet,the jet is directed towards the boundary(regime I).Unexpectedly,when the generated bubble is close to the rubber sheet,it develops a jet away from the boundary(regime III).Bubble splitting and the formation of two axial liquid jets directing in opposite directions can only be observed with an intermediateγ(regime II).A phase diagram of the bubble behaviors is given in theγ–T space.For a large rubber sheet thickness(e.g.,T=5 mm),only a downward jet is observed.From the perspective of jet direction,the effect of such a thick rubber sheet on the bubble dynamics is similar to that of a rigid wall.Note that the jet velocity and jet width may be slightly different.Secondly,the results reveal that bubble expansion can cause the boundary to deform.The strong axial jet towards the rubber sheet can damage and even rupture the rubber boundary.A parameter range within which the bubble jetting can rupture the rubber boundary is obtained.Besides,for a thin rubber sheet(T≤4 mm),the rupture is more likely to occur in regime II.For a thick rubber sheet(e.g.,T=5 mm),the rupture can be seen when the jet directly impacts the rubber sheet(regime I).

Publisher Correction: Numerical simulation of water entry problems considering air effect using a multiphase Riemann-SPH model

Correction to:Adv Aerodyn 3,13(2021)https://doi.org/10.1186/s42774-021-00066-x After publication of this article[1],it is noticed the article contained some errors.The details are listed below:(1)Page 3:In the sentence‘ca and cb denote the sound of speed of particles a and b,respectively.’,‘sound of speed’should be corrected to‘speed of sound’.(2)Page 4:In the sentence‘whereρ0 and c0 denote the initial density and the artificial sound of speed,respectively.’,‘sound of speed’should be corrected to‘speed of sound’。

On the comparison of particle regeneration technique and volume adaptive scheme in the compressible flow based on smoothed particle hydrodynamics

The collapse of a cavitation bubble is an interesting topic and it has many applications in the engineering fields.Due to its compressible nature,the modelling of a cavitation bubble is not easy by the Lagrangian method,like the smoothed particle hydrodynamics(SPH),as there is large variation of particle volume.Currently,there are two kinds of method that have been proposed to deal with this problem:the particle regeneration technique(PRT)and the volume adaptive scheme(VAS).They have all been validated via several numerical tests of compressible flow in the past studies.As is based on totally different concept,the ultimate simulation results and properties may differ.Here,we intend to compare these two methods based on the Riemann-based SPH solver with monotone upwind-centered scheme for conservation laws(MUSCL)reconstruction via several numerical tests.The characteristics of these two methods are discussed and the applicable scope for them are also commented for further usage.