水中爆炸气泡脉动现象的实验研究

在实验水箱中采用高速摄影技术获得了炸药水中爆炸气泡脉动过程图像。实验结果表明:短长径比（1.05）PETN炸药水中爆炸产生的气泡形状近似球形,气泡脉动周期和半径随炸药质量的增加而增大。在膨胀阶段,气泡中心位置保持不变,随着体积的不断缩小,气泡上浮越来越明显。受重力影响,气泡下表面收缩速度高于上表面,在收缩至最小时下表面向上冲顶,气泡溃灭形成水射流。     

Observations on braided thin wire nucleate boiling in microgravity

A microgravity experiment was conducted on the Space Shuttle Endeavor (STS-108) to observe sustained nucleate boiling of water. Subcooled water was boiled with a single strand and a braid of three 0.16 mm diameter and 80 mm long Nichrome resistive wires. A CCD video camera recorded the experiment while six thermistors recorded the temperature of the fluid at various distances from the heating element. This paper reports experimental results in observations, measurements, and data analysis. Bubble explosions were found to take place shortly after the onset of boiling for both the single and braid of wires. The explosion may produce a high heat transfer rate, as it generates a cloud of microbubbles. The number, size, and departure rate of the bubbles from the heater wire were measured and compared with theoretical models as a function of time. The temperature measurements revealed a complex temperature distribution in the fluid chamber due to bubbles ejected from the wire that carried thermal energy close to the temperature sensors. Drag forces on departing bubbles were calculated based on bubble movement and used to predict bubble propagation. Results from this experiment provided further understanding of nucleate boiling dynamics in microgravity for the eventual design and implementation of two-phase heat transfer systems in space applications.     

Robust numerical analysis of the dynamic bubble formation process in a viscous liquid

The dynamics of bubble formation from a submerged nozzle in a highly viscous liquid with relatively fast inflow gas velocity is studied numerically. The numerical simulations are carried out using a sharp interface coupled level set/volume-of-fluid (CLSVOF) method and the governing equations are solved through a hydrodynamic scheme with formal second-order accuracy. Numerical results agree well with experimental results and it is shown that the sharp interface CLSVOF method enables one to reproduce the bubble formation process for a wide range of inflow gas velocities. From numerical results, one can improve their understanding of the mechanisms regarding the dynamics of bubble formation. For example, it is found that for some sets of parameters that the bubble formation process reaches steady state after several bubbles are released from the nozzle. At steady state, bubbles uniformly rise freely in the viscous liquid. It is observed that the fluid flow around a formed bubble has a significant role in determining the overall dynamic process of bubble formation; e.g. the effect of the fluid flow from the preceding bubble can be seen on newly formed bubbles.     

近场水下爆炸气泡脉动及水射流的实验与数值模拟研究

海上作战时,近场水下爆炸形成的水射流能造成水面舰船结构的严重局部毁伤。为了研究近场爆炸时舰船底部水射流的形成机理及规律,开展了TNT当量2.5 g的炸药在固支方板底部不同爆距下起爆的水下爆炸实验。结果表明,气泡坍塌形成水射流的过程随着爆距的增加由吸附式向非吸附式转化。接着,基于ABAQUS软件采用CEL方法开展了系列数值模拟,结果表明:爆距在0.821～0.867倍最大气泡半径时,存在吸附式射流向非吸附式射流转化的临界点;固支方板加快了气泡坍塌的进程,炸药与钢板间的距离越小则射流形成的时间越早;射流形成过程中最大速度和射流击中钢板时速度均随着爆距的增大先增大后减小,并在临界点附近达到最大值,射流速度最大可达621 m/s,射流击中钢板时速度最大可达269 m/s。最后,给出了射流开始形成时间、射流最大速度、射流最大速度出现时间、射流击中钢板速度和射流击中钢板时间与距离参数的函数关系式。     

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.     

不同环境下气泡脉动特性实验研究

水下爆炸气泡对舰船结构造成严重的毁伤, 海上实船爆炸实验是考核水下爆炸气泡对舰船毁伤威力最直接、最有效的方法, 然而真实情况的舰船水下爆炸实验难以进行.为此, 在已有研究成果的基础上, 设计实验电路, 利用电容在相对较低的电压下放电打火产生的电火花气泡来模拟水下爆炸气泡, 设计多组实验工况来模拟气泡在不同环境下的运动特性. 研究气泡在不同环境下的脉动特性、射流特征, 揭示一些特殊的实验现象, 总结环境对气泡运动的影响规律.      

水下爆炸对沉箱重力式码头毁伤效应

通过开展不同爆距下水下爆炸对沉箱重力式码头模型毁伤效应试验,对水下荷载进行了采集分析,对毁伤因素、毁伤模式和毁伤机理开展了研究,初步讨论了爆距的影响。结果表明:试验中未形成完整的气泡脉动过程,荷载超压主要出现在冲击波传播阶段,爆炸冲击波、水底反射波和侧壁反射波是主要的毁伤因素;水下爆炸对沉箱重力式码头造成的毁伤程度大、模式多、机理复杂,主要毁伤部位为迎爆面外墙、迎爆侧管沟、封仓板和面板;爆距越近码头毁伤越严重;当爆距过近时,爆炸能量被迎爆面结构变形大量吸收,迎爆面毁伤程度大幅增大,码头其他部位毁伤程度增幅放缓。     

水中爆炸气泡与水底边界相互作用的水射流现象

应用开尔文冲量原理,分析了水中爆炸产生的气泡与水底边界相互作用产生水射流的主要影响因 素,开展了气泡与水平放置的钢板相互作用水射流现象的实验研究工作。通过改变炸药到钢板的距离,得到 不同爆距条件下,气泡与钢板相互作用形成水射流的动态过程图像和气泡2次脉动压力。理论分析和实验结 果均表明:随着爆距的减小,Bjerknes力迅速增加,气泡与水平放置钢板相互作用产生水射流的临界条件为爆 距小于气泡最大半径。水射流具有非常明显的方向性,对目标毁伤作用明显。     

气泡及其破碎兴波对浮动冲击平台影响探究

基于LS-DYNA软件中的ALE算法,对近水面水下爆炸气泡脉动过程进行数值模拟并与实验结果进行对比,验证了近水面近壁面混合边界有限元模型和参数设置的正确性。设置不同爆炸工况,对气泡及其破碎兴波对浮动冲击平台影响进行探究,结果表明:在水下爆炸过程中,气泡、自由面、浮动冲击平台会发生强烈的耦合作用,在气泡脉动阶段,气泡会诱导出涌流和水冢效应,影响浮动冲击平台的安全性和使用性;冲击波是影响浮动冲击平台冲击环境的主要因素,由于气泡的低频性,气泡脉动及水冢对浮动冲击平台的直接冲击作用,会小幅度增加浮动冲击平台冲击环境的谱速度值、谱位移值,对谱加速度值几乎无影响;水冢抨击水面所形成的波浪和气泡破碎兴波,对浮动冲击平台造成的激励载荷呈周期性,其周期与波浪周期相同。波浪的激励载荷仅通过激励其对应频率的浮动冲击平台共振来改变平台的冲击环境。波浪载荷很小,对浮动冲击平台的冲击环境影响较小。     

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

In the area of naval architecture and ocean engineering, the research about the underwater explosion 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 explosion 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.     

舱段结构在气泡射流作用下的毁伤效果

为研究水下爆炸气泡载荷对船舶局部结构的毁伤效果,设计建造了实尺度舱段模型,并进行水下 爆炸实验,测量了水中压力、气泡脉动周期、舱段外板结构的动应变与塑性变形。分析了不同冲击因子作用下 舱段结构的动态响应,并结合水中压力测量结果,探讨了气泡射流的成因。在此基础之上,分析了气泡射流载 荷的影响范围以及外板在不同载荷作用下的毁伤模式。实验结果表明,相比冲击波载荷,在某些情况下射流 冲击载荷引起的结构响应更剧烈,毁伤效果更严重,因此在中近场水下爆炸作用下,气泡射流冲击载荷对船体 结构的影响不能忽视。     

Fusion dynamics of underwater explosion bubbles

Fusion of bubbles is a common and significant phenomenon in the nature. In this work, the potential flow theory and the boundary integral method are employed to simulate the fusion of underwater explosion bubbles. Based on the numerical and experimental results in the literature, a three-dimensional model of bubble fusion is established. A good agreement is shown between the numerical results and experimental data. Influences of characteristic parameters such as distance and depth are specifically investigated by using the developed three-dimensional program, resulting in favorable curves and conclusions. This work provides references for the relevant research on fusion mechanics and dynamic characteristics of bubbles.     

CL-20基炸药水中爆炸气泡脉动实验研究

为研究CL-20基炸药、CL-20基含铝炸药水下爆炸气泡脉动情况,在2 m×2 m×2 m的实验水箱中开展小当量实验,采用高速摄影技术,得到炸药水中爆炸冲击波传播曲线,同时清晰地观测到气泡的产生、膨胀和收缩过程。拟合得到气泡脉动过程中气泡半径、速度、加速度对时间的变化曲线,对比分析了CL-20含铝与非含铝炸药水下爆炸气泡脉动规律。在实验条件下,首次直观地拍摄到CL-20含铝炸药水下爆炸的二次反应放热现象。实验表明:CL-20基含铝炸药的气泡半径、脉动周期都明显升高,半径增大13.7%,周期增大6.9%;冲击波峰值压力略有下降;水下爆炸测试技术以及高速摄影技术是研究观测含铝炸药二次反应的有效手段。     

On the breakup of bubbles at high Reynolds numbers and subcritical Weber numbers

In this paper we propose a simplified two-dimensional model to describe some aspects of the turbulent breakage of bubbles at subcritical Weber numbers. In particular we focus on the breakup of bubbles owing to their interaction with an array of successive eddies, modeled by a train of straining flows. Our simulations show that, under certain conditions, a bubble accumulates energy due to its interaction with a sequence of turbulent structures until it eventually breaks, even if none of the eddies is sufficiently energetic to split the particle by itself. It is also shown that the different strain directions of the eddies acting on the surface of the bubble, and the resonance effect between their characteristic frequency and the natural oscillation frequency of the bubble immersed into the straining flow are the two key factors in the bubble deformation, and subsequent breakup mechanism. Moreover, the breakup patterns obtained from our simulations seem to agree qualitatively well with the experimental observations.     

On the role of the lateral lift force in poly-dispersed bubbly flows

An extensive experimental database comprising air–water as well as steam-water upwards vertical pipe flows for a pressure up to 6.5 MPa was used to investigate the effect of the lateral lift force on turbulent poly-dispersed flows with medium or high gas volume fraction. It was clearly shown that the lift force plays an important role also in such flows. Several effects such as bubble coalescence and breakup as well as fast rising large bubbles which push small bubbles towards the pipe wall superpose the effect of the lift force but can be separated from this effect. The critical bubble diameter, at which the lift force changes its sign, predicted by using Tomiyama’s correlation agrees well with experimental data obtained for turbulent air–water and steam-water flows with medium and high void fraction and a broad spectrum of bubbles sizes. The values for this critical bubble diameter are confirmed by the experimental data within the frame of the uncertainty of the data. Consequences of the action of the lateral lift force on flow structures in different flow situations are discussed. From the investigations it can be concluded that the lift force including the bubble size dependent change of its sign should be considered in a proper numerical 2D or 3D-simulation on flows in which bubbles in the range of several millimeters are present.     

金属导爆索的爆炸水声特性

为了研究金属导爆索的水声特性,进行了水下爆炸压力测试和气泡脉动实验,获得了金属导爆索水下爆炸冲击波传播和衰减特性及气泡脉动特性.研究了金属导爆索水下爆炸的声压级、声持续时间、混响效应和功率谱特性,结果表明:(1)金属导爆索水下爆炸声压级较高,具有很强的声功率;(2)金属导爆索水下爆炸后产生的气泡脉动和随后产生的大量小气...     

水下爆炸气泡运动的理论研究

在适当深度的无黏、无旋的流体中对水下爆炸气泡运动特性进行理论研究。综合运用势流理论、能量方程以及拉格朗日方程建立气泡在不可压缩流体中的运动方程。并以此为基础,考虑重力、浮力以及阻力等多种因素对气泡运动特性的影响,通过引入新的边界积分方程,结合分析力学中完整非保守系统的Hamilton原理建立气泡在可压缩流体中的运动微分方程,并对微分方程进行求解。将方程的数值解与MSC.DYTRAN非线性有限元软件的计算结果以及经验公式进行对比,方程数值解与二者都具有较好的一致性。结果表明,基于非保守系统可压缩流体建立的气泡运动方程正确、可行,相关的理论研究和计算具有一定参考价值。     

NUMERICAL ANALYSIS ON THE VELOCITY AND PRESSURE FIELDS INDUCED BYMULTI-OSCILLATIONS OF AN UNDERWATER EXPLOSION BUBBLE

假设水下爆炸气泡的内部气体在膨胀收缩过程中满足绝热条件,周围流体无黏无旋不可压缩. 基于势流理论,采用边界元法研究气泡动力学行为,重点关注气泡引起的流场脉动载荷以及滞后流特性,给出了相关的理论推导和数值计算方法. 通过将数值结果与解析解、实验值进行对比,数值模型的收敛性和有效性能够得到保证. 利用编写的程序进行计算和分析,发现在气泡加速膨胀阶段,流场压力在气泡径向不一定是逐渐衰减,还有可能以先增后减的规律变化;气泡射流后,为了能够继续描述环状气泡的运动以及流场特性,将此时的流场分为无旋场和一个布置在气泡内部涡环的叠加,计算过程中采用了一些数值技巧处理气泡的拓扑结构,得以连续模拟多个周期的气泡运动. 环状气泡具有相对较高的上浮迁移速度,而且在其顶部和底部附近分别形成两个高压区,顶部的高压区峰值相对较大,底部的高压区范围相对较大. 环状气泡中心轴上的流场速度会在气泡中心有一个加速过程,在气泡顶部附近又迅速减小.     

气泡多周期运动时引起的流场压力与速度

假设水下爆炸气泡的内部气体在膨胀收缩过程中满足绝热条件,周围流体无黏无旋不可压缩. 基于势流理论,采用边界元法研究气泡动力学行为,重点关注气泡引起的流场脉动载荷以及滞后流特性,给出了相关的理论推导和数值计算方法. 通过将数值结果与解析解、实验值进行对比,数值模型的收敛性和有效性能够得到保证. 利用编写的程序进行计算和分析,发现在气泡加速膨胀阶段,流场压力在气泡径向不一定是逐渐衰减,还有可能以先增后减的规律变化;气泡射流后,为了能够继续描述环状气泡的运动以及流场特性,将此时的流场分为无旋场和一个布置在气泡内部涡环的叠加,计算过程中采用了一些数值技巧处理气泡的拓扑结构,得以连续模拟多个周期的气泡运动. 环状气泡具有相对较高的上浮迁移速度,而且在其顶部和底部附近分别形成两个高压区,顶部的高压区峰值相对较大,底部的高压区范围相对较大. 环状气泡中心轴上的流场速度会在气泡中心有一个加速过程,在气泡顶部附近又迅速减小.