水下爆炸异相气泡动力学特性的Euler有限元数值模拟研究北大核心CSCD

该研究基于Euler有限元方法建立了水下爆炸异相气泡动力学模型,将计算结果和气泡统一理论以及异相爆炸试验进行对比验证计算模型的有效性.通过和自由场单气泡进行对比分析,发现异相爆炸冲击波对气泡做功是异相爆炸气泡总能量增加的原因,相位差绝对值越接近π、距离参数越小气泡总能量损失越少,后产生的气泡会使先产生的气泡提前坍缩.气泡的射流方向受相位差影响,相位差为零时产生对向射流,其他相位差产生反向射流.     

Numerical simulation of trajectory and deformation of bubble in tip vortex

According to the behaviors of a bubble in the ship wake flow,the numerical simulation is divided into two stages,quasi-spherical motion and non-spherical motion,based on whether the bubble is captured by the vortex or not.The one-way coupled particle tracking method(PTM) and the boundary element method(BEM) are adopted to simulate these two stages,respectively.Meanwhile,the initial condition of the second stage is taken as the output of the first one,and the entire simulation is connected and completed.Based on the numerical results and the published experimental data,the cavitation inception is studied,and the wake bubble is tracked.Besides,the split of the bubble captured by the vortex and the following sub-bubbles are simulated,including motion,deformation,and collapse.The results provide some insights into the control on wake bubbles and optimization of the wake flow.     

水中空气隔层衰减冲击波性能研究

 根据鱼雷对舰船毁伤的特点,建立了多介质水下爆炸数值模型,提出了交界面处理及广义光滑长度计算方法,编制了多介质水下爆炸计算程序,并对水下爆炸空气隔层衰减冲击波的性能进行了定量分析。结果表明：无论是接触爆炸还是非接触爆炸,空气隔层均可有效衰减冲击波,并且接触爆炸中空气隔层衰减冲击波的效果更好,最大可使冲击压力峰值降低约55%;空气隔层厚度与爆炸厚度之比为1时便可达到较好的衰减冲击波效果,继续增加空气隔层厚度对衰减冲击波效果影响不大。研究结果可为舰船结构防护及防雷舱的结构设计提供参考。     

三维频域格林函数的高效率计算方法

将高斯积分引入频域格林函数及其导数的数值计算,计算结果与已有文献进行对比,证明该方法在满足足够精度的基础上,使计算过程简化,减少分区,提高计算效率.     

近壁面气泡的运动规律研究

基于势流假设，建立气泡与壁面耦合数值模型，运用边界积分法求解，并开发三维计算程序，计算值与实验值符合很好.从气泡与壁面相互作用的基本现象入手，基于开发的程序系统地研究了刚性壁面附近气泡的动力学特性，其中包括水平壁面及倾斜壁面，研究壁面的Bjerknes效应与各特征参数之间的关系，并将各种工况的计算结果与基于Kelvin-impulse理论的Blake准则进行对比分析讨论，得出偏射流方向及壁面压力与气泡的特征参数有密切的关系，同时给出了Blake准则的适用范围.旨在为相关的近壁面气泡动态特性研究提供参考. 关键词： 气泡 壁面 边界积分 Bjerknes效应     

自由场水中爆炸气泡的物理特性

将水中爆炸气泡运动阶段周围流场假设为无粘、无旋、不可压缩的理想流体，运用边界元法模拟自由场中气泡的运动，在气泡运动模拟过程中引入数值光顺技术及弹性网格技术，避免因网格扭曲而导致的数值发散，并开发计算程序。计算值与实验值吻合良好，误差小于10%。从自由场水中爆炸气泡的基本现象入手，基于本文中开发的程序系统地研究了自由场中气泡的动力学特性。对流场中不同方位的压力进行分析，得出气泡中心的迁移方向及射流的攻击方向压力载荷比其他方向均大，说明气泡射流的攻击方向压力载荷最大，对水中结构造成严重毁伤，表明了气泡载荷的不对称性。计算了流场中不同位置的速度变化曲线，结果表明随着距气泡中心距离的增大，气泡运动引起的滞后流的速度迅速减小，且随着气泡的膨胀和坍塌，滞后流的方向逆转，总结了滞后流的衰减及变化规律。     

Interaction of underwater explosion bubble with complex elastic-plastic structure

Considering the elastic-plasticity of the structure, the combination of boundary element method and finite element method （FEM） is employed to present the calculation method for solving the complex coupled dynamic problem of bubble, elastic-plastic structure and the free surface, and the complete three-dimensional calculation program is developed as well. The error between the calculated result and the experimental result is within 10%. Taking a surface ship for example, the three-dimensional calculation program is extended to engineering filed. By employing the program, the response of the ship under the bubble loading is analyzed. From the stress-time history curves of typical elements of the structure, it can be seen that the pressure reaches its maximum when the bubble collapses and this validates that the pressure generated by the bubble collapse and the jet can cause serious damage on the ship structure. From the dynamic process of the interaction between the three-dimensional bubble and the ship, the low order vertical mode of the ship is provoked and the ship presents whip-shaped motion. And the ship does elevation and subsidence movement with the expansion and shrinkage of the bubble. Some rules and conclusions which can be applied to the engineering problems are obtained from the analysis in this paper.     

水中上浮气泡动态特性研究

针对水中的上浮气泡,计入表面张力的作用,然后将气泡边界层内黏性效应分为法向和切向两部分,其中附加法向黏性应力通过Young-Laplace关系考虑;附加切向黏性力是基于黏性耗散能量等效原理,引入黏性修正压力代替.首先建立了轴对称和三维上浮气泡边界元模型,将数值结果与理论值和实验值进行对比分析,有良好的符合度,验证了数值模型的有效性;然后针对毫米量级上浮气泡的平衡速度与形态,讨论了气泡初始条件、表面张力和黏性对气泡上浮过程中动力学行为的影响;最后,提出了一种处理三维上浮气泡融合的数值方法,计算结果与实验现象符合良好,并且能够反映气泡融合后的复杂现象细节.     

相同尺度下气泡与复杂壁面的耦合特性研究

采用格子Boltzmann方法(LBM)建立了气液固三相耦合的动力学模型,研究了相同尺度下上浮气泡与复杂壁面的相互耦合作用.首先,基于黏性流体理论,通过构建一组格子Boltzmann(LB)方程来描述气液两相的运动,并以LB离散体积力的形式计入了黏性力、表面张力和重力.同时,采用LBM中的Half-way反弹模型与有限差分格式相结合的方式进行固壁边界的处理.然后,利用本文建立的模型,对不同特征尺寸比条件下,气泡与考虑边缘效应的平面固壁和曲面固壁的耦合特性进行了研究.研究发现固壁边界条件以及特征尺寸比对气泡的运动和拓扑结构的变化都具有明显的非线性影响.最后,研究了流体属性对气泡与复杂壁面耦合规律的影响.     

Three-dimensional numerical simulation of crown spike due to coupling effect between bubbles and free surface

The motion of gas bubbles beneath a free surface will lead to a spike of fluid on the free surface. The distance of the bubbles to the free surface is the key factor to different phenomena. When the inception distance varies in some range, crown phenomenon would happen after the impact of weak buoyancy bubbles, so this kind of spike is defined as crown spike in the present paper. Based on potential flow theory, a three-dimensional numerical model is established to simulate the motion of the free-surface spike generated by one bubble or a horizontal line of two in-phase bubbles. After the downward jet formed near the end of the collapse phase, the simulation of the free surface is performed to study the crown spike without regard to the toroidal bubble＇s effect. Calculations about the interaction between one bubble and free surface agree well with the experimental results conducted with a high-speed camera, and relative error is within 15%. Crown spike in both single- and two-bubble cases are simulated numerically. Different features and laws of the motion of crown spike, depending on the bubble-boundary distances and the inter-bubble distances, have been investigated.