边界条件对水下爆炸气泡运动特性的影响分析

在近距爆炸条件下,受到边界的影响,水下爆炸气泡会出现非球状运动情况。为了得到球状气泡假设的适用条件,对深水自由场、近自由液面、近刚性壁和弹性边界等不同边界条件下水下爆炸气泡的动态特性进行了仿真计算。通过对气泡最大半径和脉动周期仿真结果的对比分析,可以得到如下结论:当爆心距离边界约等于气泡最大半径时,自由表面会使气泡的最大半径略有减小,周期缩短,刚性壁的影响与之相反,弹性边界的影响介于二者之间;在近自由边界条件下爆距大于2倍气泡半径,近弹性结构和刚性壁边界条件下爆距大于3倍气泡半径时,水下爆炸气泡可近似认为球状脉动。     

刚性壁面附近多空化泡辐射噪声的研究

对空化泡在刚性壁面附近溃灭过程中的辐射声场进行研究,并得出刚性壁面附近单空泡、双空泡的辐射声压分布。研究表明由于刚性壁面的影响使得空化泡的辐射声场表现出一定的指向性。双空泡辐射声压分布还与空泡崩溃时间、空泡之间距离以及空化泡初始半径有关。在考虑到声压相位叠加时,待测点处的辐射声压依赖于空泡的声压幅值[p]和两空泡到观察点的声程差[Δ1=l12sinθ1]和[Δ2=l22sinθ2]。计算过程及结果为刚性壁面附近的空化计算提供依据,并为多泡空化的辐射声场分析提供理论依据,有助于修正空化泡的运动方程。     

酒精-水溶液中空化泡的动力学研究

给出了不同浓度的酒精水溶液中空化泡的动力学测量结果。在最优化驱动参数的情况下,随着酒精浓度的增大,空化泡的可忍受声压随之减小,进而减弱了空化泡的径向脉动;在特定驱动参数情况下,气泡的最大半径和半径压缩比随酒精浓度的变化情况都会因驱动参数点的选取而发生变化。总的来说,酒精的加入减弱了空化泡的运动,改变了空化泡的参数空间,因而改变了空化泡的动力学行为。     

单泡声致发光中声压与气泡最大半径的关系

1引言: 当一定幅度的超声波作用于水中时,水中的微小气泡会在声压的驱动下膨胀和塌缩,并不断成长为肉眼可见的气泡,这种现象就是声空化.当超声波的幅度继续增大,水中气泡的膨胀、塌缩更加激烈,会产生短暂发光,这就是声致发光.早期的观察到的声致发光都是来自大量空化泡的集体效应,所以称为多泡声致发光.1992年,Gaitan等人将超声波作用于除气水,实现了单个气泡在水中的悬浮、膨胀、塌缩和发光,发明了单泡声致发光[1],使得人们可以对发光的单个气泡的参数进行具体的研究.     

水下近距爆炸作用下弹性钢板处的空化特性研究

弹性钢板在水下近距爆炸作用下,冲击波会使其附近流体形成局部空化,脉动气泡会使流体形成锥形空化。利用平面冲击波理论对局部空化的形成特性进行了研究,理论分析了结构目标尺度的变化对空化区域形成的影响,并通过具体试验对局部空化理论进行了验证,两者符合较好;通过试验和数值仿真方法研究了气泡脉动引起的锥形空化的形成特点,初步分析了锥形空化的形成原因。结果表明,锥形空化对结构具有较大的冲击作用。     

箱形梁在水下近距非接触爆炸作用下的整体毁伤研究

为了研究舰船在水下近场爆炸作用下的整体毁伤模式,根据相似原则设计了两种箱形梁模型,将TNT炸药置于模型中部正下方爆炸,通过改变爆距和药量来研究试验模型在不同爆炸工况下的整体运动特性,并利用高速摄影仪观察模型的动态响应过程.试验研究发现:在梁模型一阶湿频率与气泡脉动频率相近的条件下,气泡脉动会激起结构明显的整体弯曲运动;当爆距与最大气泡半径之比为1～1.5时,梁结构会发生中垂破坏,形成一个或两个塑性绞;当爆距与最大气泡半径之比为2～4时,梁结构会发生鞭状运动响应;水下爆炸气泡对水面结构的整体毁伤作用大于初始冲击波。     

水下爆炸气泡脉动的数值研究磁

利用AUTODYN软件模拟PETN药柱和TNT球形装药水下爆炸气泡脉动的过程,分析了脉动周期与最大半径随装药量、爆炸深度的变化规律。结果表明：当炸药所处的爆炸深度一定时,气泡膨胀最大半径与初始半径的比值、气泡脉动周期与初始半径的比值均为一个定值;一定质量的TNT装药水下爆炸时,气泡最大半径随爆炸深度呈指数形式变化且该变化规律与装药量无关;在不考虑重力的影响下,装药的爆炸深度一定时,当装药的初始半径成倍增加时,气泡脉动最大半径亦成相同倍数增加。     

近场气泡载荷下柔性结构耦合响应实验研究

通过电火花气泡装置和高速摄像设备对气泡与附近刚性/柔性边界间的耦合现象进行实验观测和研究。针对不同材质平板和不同距离参数下气泡脉动实验数据进行整理,对气泡动力学特性如撕裂现象、射流方向特性及气泡迁移特性等进行机理分析;并借助MATLAB开发图片数字化识别程序,分析总结了整个气泡脉动载荷下柔性板的整体运动响应模式和结构中心测点的响应特征。实验结果表明:气泡在柔性板附近收缩时出现“棒槌”状或气泡撕裂及远离对向射流现象;探究气泡载荷下柔性结构响应模式,发现气泡膨胀时柔性板呈一阶响应,收缩时呈现三阶响应及双峰值迁移现象。     

不同空化程度下离心泵流固耦合特性研究

针对离心泵空化下流固耦合问题,采用完全空化和气液两相模型,对离心泵空化进行了数值模拟计算,结合单向耦合计算方法求解了不同空化程度下转子系统的变形。分析了空化时叶轮上气泡、静压、液体相对速度分布情况以及蜗壳内部压力脉动和叶轮径向力特性,研究空化对离心泵内部流场和对转子系统变形的影响。结果表明,叶片吸力面较压力面的气泡体积和气泡区更大;随着空化的发展叶轮静压分布越不均匀;严重空化时叶轮上的气泡会堵塞流道,引起脱流现象,生成漩涡;空化导致压力脉动增强,径向力分布不规律;空化影响了离心泵转子系统的变形,空化引起的漩涡造成叶轮非轴对称变形。     

对称边界条件下气泡脉动特性实验研究

采用铜丝电极低电压条件下打火生成气泡方法,配合高速摄影技术研究两平行板间与筒状区域构成的对称边界附近气泡脉动特性。气泡生成位置位于两平行板与筒状区域中心。气泡在两平行板间运动时,气泡会在中间撕裂成两小气泡,形成的两小气泡射流方向相反,且指向平行板。随两平行板间无量纲距离的减小,依次出现回弹撕裂、相背射流及连通射流等不同坍塌运动形式。气泡在圆、方筒中运动时会形成"红细胞"型及对射流,对射流穿透气泡形成环形气泡继续坍塌,射流方向指向筒壁。通过对称边界条件下气泡运动特性研究,旨在为不同边界条件下气泡坍塌形式提供参考。     

Jets in bubbles

Different types of jet formation in collapsing cavitation and gas bubbles near a rigid boundary are explored by using an advanced boundary-integral technique which incorporates the transition from simply connected to multiply connected bubbles (i.e. toroidal bubbles). Physical interpretation and understanding is facilitated by the calculation of the evolving bubble shape, fluid velocities and pressures, the partitioning of kinetic, potential and gravitational energies, the circulation around the bubble and the Kelvin impulse associated with both the complete bubble and the high-speed liquid jet. In the most vigorous jet formation examples considered it is found that upto 31% of the total energy and upto 53% of the Kelvin impulse is associated with the jet. Practical implications of this study beyond the usual damage mechanisms imply that the level of bubble compression will be signiffcantly lessened leading to lower bubble gas temperatures and thence the corresponding change in the chemical reactivity of its contents or the emission of light. Calculations also suggest interesting phenomena around a standoff distance of 1?2 maximum bubble radii where the circulation around the bubble and the kinetic energy of the jet appear to have maximum values. The practical implications and experimental confirmation of this are yet to be explored.     

A modified Rayleigh–Plesset model for a non-spherically symmetric oscillating bubble with applications to boundary integral methods

In this article the analytical solution to the Rayleigh–Plesset equation for a spherically symmetric oscillating bubble is extended to apply to the much more general (non-spherically symmetric) bubble configuration. An equivalent bubble radius and an equivalent bubble wall velocity are introduced in order to do so. The influence of gravity, surface tension, nearby solid walls, vapor bubbles, bubbles filled with adiabatic or isothermal gas have been considered in the model. An interesting outcome is that the equivalent bubble wall velocity is no longer the time derivative of the equivalent bubble radius. This observation can possibly explain why in various numerical and experimental observations the oscillation time of a bubble changes when compared to that of a standalone bubble; near a solid surface it increases while it decreases when the bubble is placed near a free surface. The current developed theory can be further employed to ascertain the accuracy of a numerical scheme simulating bubble dynamics in an incompressible surrounding flow approximation. An often used numerical technique to simulate such bubble dynamics is the boundary integral method (BIM).     

模拟不同海拔水下爆炸气泡动态特性研究

不同海拔高度进行水下爆破工程时,因水体表面气压随海拔的升高而线性降低,其装药爆炸产生的气泡脉动过程及气泡大小将会发生一定的变化。通过爆炸载荷的测试以及高速摄影2种测试方法对气泡脉动进行了研究。结果表明:在海拔0～4 500 m范围内随着气压降低,气泡脉动周期按二阶多项式规律显著增大,2种方法所得数值误差小于3.8%,数据一致性较好;在海拔0～3 500 m范围内气泡最大半径随海拔升高而线性变大,而4 000 m和4 500 m有突跃变化。将研究结果与水下爆炸气泡周期理论公式进行了比较,并对理论公式在低气压这一特定条件下进行修正,得出的气泡脉动周期系数由原来的2.11修正为1.995,而气泡最大半径系数是一个与海拔有关的一次函数。     

Velocity field and pressure distribution around a collapsing cavitation bubble during necking and splitting

The hydrodynamic behavior of the fluid around a cavitation bubble located above a rigid boundary is investigated numerically. The liquid around the cavitation bubble is assumed to be incompressible, inviscid and irrotatational and surface tension is assumed to be negligible. Boundary-integral-equation and finite-difference methods are employed to study the problem. Three cases are investigated: (1) when the Bjerknes force is negligible in comparison with the buoyancy force, (2) when the buoyancy force is negligible in comparison with the Bjerknes force, (3) when the Bjerknes attraction force through the rigid surface and the buoyancy force are comparable. It is shown that during the collapse phase in the third case, an annular liquid jet develops around the bubble, causing it to take the shape of an hour-glass. This phenomenon is called necking which is followed by splitting the bubble into two parts. Features to note are the large lateral pressures and the high relative velocities of the fluid particles near the annular liquid jet of the bubble. This large lateral pressure may be the cause of bubble collapse. The velocity field of the liquid domain around the two parts of the cavitation bubble after splitting shows that a stagnation point exists between the two parts of the bubble. Because of the unsteady nature of the problem, the stagnation point and the point of maximum pressure do not coincide.     

A modified model for simplified description of evolution of a single bubble upon increase in the pressure of the surrounding fluid

The problem of evolution of a single vapor bubble after an increase in the pressure of the surrounding fluid is considered. In contrast to the classical formulation of this problem, the fluid pressure is set at a finite distance from the bubble. For the case when this distance is large, an analytical solution is found, which relates the bubble compression rate with its current radius. On the basis of the solution obtained, the regimes of bubble compression and fluid pressure upon bubble compression in the collapse regime are investigated.     

一种考虑时延的双气泡耦合振荡模型

基于单气泡Keller-Miksis振荡方程,在考虑时延的情况下,建立了一种双气泡耦合振荡计算模型。该模型将气泡振荡的周期分成若干份,初始扰动引起第一个气泡的半径在极短时间内变化而产生振荡并辐射声压,声压在传播一定时间后作用到第二个气泡,第二个气泡同样在短时间内做耦合振荡并反馈到第一个气泡,然后重复此过程。利用数值仿真在此模型的基础上分别研究了气泡振幅、半径、间距等参数对耦合振荡的影响。结果表明:初始扰动越大、两个气泡半径越接近,气泡耦合效应越明显;初始半径和平衡半径较大的气泡对耦合振荡有显著影响,振荡的频率向低频移动;气泡间距越大,耦合效应越弱;在某个特定距离处,气泡耦合效应的阻尼会异常减小或者增大。     

Bubble Motion near a Wall Under Microgravity: Existence of Attractive and Repulsive Forces

A numerical simulation for a bubble motion near a wall under microgravity, relevant to material processing such as crystal growth in space, is presented based on a mass conservation level set algorithm to predict the bubble behavior affected by the near wall. The simulation for the wall effect on the bubble driven by an external acceleration parallel with the near wall referred to as g-jitter confirms for the first time the existence of the wall attractive force to the bubble near the wall under certain conditions such as the initial distance between the bubble and the wall, density and viscosity ratios between the bubble and surrounding liquid under microgravity. The wall effect mechanism is explained, and the results show that the wall attractive force increases with the increasing of density ratio. Moreover, the simulation for the wall repulsive effect on the bubble near the wall under microgravity has been carried out as well.