不同边界条件下炸药水中爆炸的能量输出结构

针对不同边界条件下,炸药水下爆炸的能量输出结构,设置自由场、沉底、近水底和近水面等不同 边界条件水下爆炸实验。结果表明,炸药在距水底、水面一定距离时爆炸,边界条件对冲击波影响较小,对气 泡脉动影响较大;当炸药与水底距离小于1倍最大气泡半径处爆炸时,由于水底边界影响,不能形成完整的气 泡脉动;当炸药在距水面小于1倍最大气泡半径处爆炸时,气泡直接溢出水面,不能形成气泡脉动压力;当炸 药在距水面1~1.5倍最大气泡半径处爆炸时,部分气泡溢出水面,气泡脉动压力较自由场爆炸时小;气泡在 膨胀和收缩过程中,由于气泡不断上升,水面对气泡脉动的影响范围要大于水底。     

水中高压脉动气泡水射流形成机理及载荷特性研究

具有脉动特性的气泡(如水下爆炸气泡、螺旋桨空泡和气枪气泡)动力学行为很大程度上取决于其边界条件. 实验已证实,近自由液面气泡在坍塌过程中常常产生背离自由液面的水射流现象,而近刚性边界气泡在坍塌阶段产生朝向壁面的高速水射流,严重威胁水中结构的局部强度. 前人基于 Rayleigh-Plesset 气泡理论和 “Bjerknes” 力来预测气泡射流方向,然而理论方法难以透彻的揭示气泡射流的初生、发展和砰击过程中丰富的力学机理. 本文首先采用水下高压放电技术产生气泡,并通过高速摄影对不同边界条件下气泡的运动特性进行实验研究. 然后,采用边界积分法模拟气泡非球状坍塌过程. 研究表明,边界条件改变了气泡周围的流场压力梯度方向,进而影响气泡射流初生位置;射流在发展阶段,气泡附近流场的局部高压区和射流之间存在“正反馈效应”,从而揭示了气泡射流速度在短时间内即可增加到百米每秒的力学机理. 射流砰击会在流场中造成局部高压区,随着气泡回弹,射流速度和砰击压力逐渐减小. 本文还探讨了无量纲距离参数对气泡运动及射流砰击载荷的影响,旨为近场水下爆炸等相关领域提供参考.      

水中高压脉动气泡水射流形成机理及载荷特性研究

具有脉动特性的气泡(如水下爆炸气泡、螺旋桨空泡和气枪气泡)动力学行为很大程度上取决于其边界条件.实验已证实,近自由液面气泡在坍塌过程中常常产生背离自由液面的水射流现象,而近刚性边界气泡在坍塌阶段产生朝向壁面的高速水射流,严重威胁水中结构的局部强度.前人基于Rayleigh-Plesset气泡理论和"Bjerknes"力来预测气泡射流方向,然而理论方法难以透彻的揭示气泡射流的初生、发展和砰击过程中丰富的力学机理.本文首先采用水下高压放电技术产生气泡,并通过高速摄影对不同边界条件下气泡的运动特性进行实验研究.然后,采用边界积分法模拟气泡非球状坍塌过程.研究表明,边界条件改变了气泡周围的流场压力梯度方向,进而影响气泡射流初生位置;射流在发展阶段,气泡附近流场的局部高压区和射流之间存在"正反馈效应",从而揭示了气泡射流速度在短时间内即可增加到百米每秒的力学机理.射流砰击会在流场中造成局部高压区,随着气泡回弹,射流速度和砰击压力逐渐减小.本文还探讨了无量纲距离参数对气泡运动及射流砰击载荷的影响,旨为近场水下爆炸等相关领域提供参考.     

近固壁微米尺度空泡溃灭的数值研究

考虑空泡表面张力、液体黏性和气体可压缩性,采用VOF多相流模型对近固壁微米尺度空泡在静止流场溃灭过程进行了数值研究.获得了近固壁空泡溃灭过程的流场细节,分析了空泡与固壁的无量纲距离γ对空泡溃灭过程动力特性的影响,并揭示了不同γ条件下的固壁空蚀破坏机理.计算结果表明:随着γ的减小,泡心向固壁移动的趋势明显,射流形成前空泡上部高压区内压力减小,空泡溃灭时间延长,最大射流速度减小.模拟结果验证了空泡溃灭将产生冲击波和高速微射流,二者均会在固壁面产生脉冲压力,其是造成壁面损伤的两种主要原因.参数γ对固壁的空蚀破坏机理有重要影响.与微射流机制相比,以冲击波机制为主的空蚀破坏更显著.微射流冲击固壁的作用半径为10μm左右,将引起固壁"点"蚀坑的出现.当γ=2.0时,冲击波扫掠壁面的范围相对较广,有效作用半径约为1 mm,其导致固壁产生较大圆形蚀坑,且中心空蚀严重.     

高超声速火星进入环境中颗粒运动特性研究

火星大气中会发生不同规模的沙尘暴,大气中蕴含的尘埃颗粒会对高速进入的火星探测器表面造成侵蚀并导致壁面热流增加,给探测器的热防护系统设计带来巨大挑战.文章针对高超声速火星进入环境两相流动问题,基于Euler-Lagrange框架建立非平衡流场与颗粒的单向耦合计算方法,采用模态半径为0.35μm的火星大气颗粒分布模型,研究不同尺寸颗粒在流场中的运动轨迹,获得高温相变模型对颗粒运动的影响以及不同粒径颗粒的撞击能量分布.结果表明,颗粒在高温流场中运动会吸热融化甚至蒸发,高温相变模型导致的颗粒直径减小对小尺寸颗粒运动轨迹有较大影响;当前计算状态下,直径3μm以上的颗粒具有较大的Stokes数且颗粒半径在运动过程中基本保持不变,其运动轨迹受流场影响较小,该尺寸颗粒的撞击分数均达95%以上,是造成壁面撞击的主要颗粒尺寸;撞击能量分数结果表明,直径3～10μm之间的颗粒是撞击能量的主要来源,约占总撞击能量的80%.     

基于动力学模态分解法的绕水翼非定常空化流场演化分析

空化是船舶和水下航行体推进器中经常发生的一种特殊流动现象,它具有强烈的非定常性,空化的发生往往会影响推进器的水动力性能和效率. 为探究绕水翼非定常空化流场结构,本文基于 Schnerr-Sauer 空化模型和 SST $k$-$\omega $ 湍流模型,开展绕二维水翼非定常空化流动数值预报与流场结构分析. 通过将数值预报的空泡形态演变和压力数据与试验结果对比,验证了建立的数值方法的有效性. 并基于动力学模态分解方法对空化流场的速度场进行模态分解,分析了各个模态的流场特征. 结果表明,第一阶模态对应频率为 0,代表平均流场;第二阶模态对应频率约为空泡脱落频率,揭示了空泡在水翼前缘周期性地生长与脱落,第三阶模态对应频率约为第二阶模态的 2 倍,揭示了两个大尺度旋涡在水翼后方存在融合行为. 第四阶模态对应频率约为第二阶模态的 3 倍,具有更高的频率,表征流场中存在一些小尺度旋涡的融合行为. 最后对不同空化数下的空化流场进行了模态分解分析,发现脱落空泡的旋涡结构随着空化数的减小而增大,第二阶模态频率随着空化数的减小而减小.      

ANALYSIS OF UNSTEADY CAVITATION FLOW OVER HYDROFOIL BASED ON DYNAMIC MODE DECOMPOSITION$^{\bf 1)}$

空化是船舶和水下航行体推进器中经常发生的一种特殊流动现象,它具有强烈的非定常性,空化的发生往往会影响推进器的水动力性能和效率. 为探究绕水翼非定常空化流场结构,本文基于 Schnerr-Sauer 空化模型和 SST $k$-$\omega $ 湍流模型,开展绕二维水翼非定常空化流动数值预报与流场结构分析. 通过将数值预报的空泡形态演变和压力数据与试验结果对比,验证了建立的数值方法的有效性. 并基于动力学模态分解方法对空化流场的速度场进行模态分解,分析了各个模态的流场特征. 结果表明,第一阶模态对应频率为 0,代表平均流场;第二阶模态对应频率约为空泡脱落频率,揭示了空泡在水翼前缘周期性地生长与脱落,第三阶模态对应频率约为第二阶模态的 2 倍,揭示了两个大尺度旋涡在水翼后方存在融合行为. 第四阶模态对应频率约为第二阶模态的 3 倍,具有更高的频率,表征流场中存在一些小尺度旋涡的融合行为. 最后对不同空化数下的空化流场进行了模态分解分析,发现脱落空泡的旋涡结构随着空化数的减小而增大,第二阶模态频率随着空化数的减小而减小.     

通气超空泡形态控制方法的数值模拟

根据Logvinovich独立膨胀原理发展了一种用于非定常通气超空泡形态的计算方法,并运用该方法对通气超空泡形态控制进行了数值仿真.研究表明:调整流场压力、模型速度、空化器阻力系数对超空泡长度的控制范围较大,响应速度较快;调整通气量对超空泡长度控制范围较小,响应速度较慢;此外改变超空泡内部模型的尺度和结构也可以对通气超空泡形态进行控制.     

细长体水下运动空化流场及弹道特性实验

为了研究细长体水下高速运动时空泡的产生、闭合及脱落特性,及影响细长体空泡形态及弹道特性的复杂因素等,初步开展了细长体模型水下高速运动的实验研究,分析了不同初始空化数下细长体模型在水中高速运动的一系列流动现象,重点研究了空泡的发展、闭合、尾部回射流和尾部脱落特性,以及轴对称细长体模型弹道特性与空泡形态变化之间的关系和转动特性随时间的变化历程等。结果表明:细长体水下高速运动时形成超空泡,空泡头部光滑透明,尾部凝结有汽水混合物且有交替脱落的含气漩涡;初始空化数对细长体的速度衰减有所影响;受初始扰动影响,细长体水下运动伴随有绕头部的转动且初始扰动影响细长体俯仰角随时间的变化历程。     

基于相变松弛法则的水下爆炸空化模型研究

水下爆炸过程中存在着大量的空化现象,空化的产生、演化及其溃灭过程对于水下冲击波传播、爆炸气泡运动以及水下结构物冲击损伤都会产生重要影响。本文基于多相可压缩流体理论模型,考虑空化发生过程中汽-液两相流体亚平衡状态下两相之间发生的热力学-化学平衡机制,分析汽-液两相介质之间的质量和热量交换,从而实现对相变过程的自动捕捉。该系统的控制方程采用分步法处理,首先利用二阶MUSCL-Hancock格式和HLLC黎曼求解器来求解齐次双曲型方程,再采用牛顿迭代法求解相变方程。数值测试结果表明,本文的计算模型对于空化相变过程具有较好的捕捉能力。最后将该模型应用到水下近水面爆炸空化的数值模拟当中,研究发现空泡的溃灭压力峰值约为冲击波压力峰值的15%,有效作用时间是冲击波载荷有效作用时间的2倍以上。本文的空化相变模型能够为水下爆炸空化现象的机理研究提供重要支撑。     

Cavitation: a contributory factor in the transition from symmetric to asymmetric jets in cross-flow nozzles

The structure and evolution of cavitation and its influence on jet patterns from two transparent cross-flow nozzles with holes inclined at 90 degrees (nozzle A) and 80 degrees (nozzle B) to the nozzle axis have been investigated using high-speed motion pictures, flash photography and stroboscopic visualization. At the onset, cavitation inception was in the form of travelling bubbles, which were transported along the flow and clearly detached from the wall. As the flow was increased the bubbles grew and merged into a dense group of bubbles (cloud cavitation), partly unsteady and shedding. Further increasing the flow caused the cavitation at the entrance to transform mainly into a glassy appearance and at this stage the cavitation was well inside the hole and the spray appeared symmetric. When the flow was increased beyond this stage, cavitation extended to the exit of the hole, occupying a significant part of the hole on one side, resulting in a jet that atomized on the side where cavitation was most extensive and a non-atomizing jet on the side with less cavitation. The distribution of cavitation in the hole is very sensitive to the nozzle geometry and it substantially influences the spray dispersion.     

超声波作用下气泡的非线性振动

Flynn方程在引入泡内气体的压缩性修正后能够更好地描述超声波作用下液体内气泡的非线性振动。我们以此为基础通过数值分析得到了不同初始半径的气泡在同一声场中的振动位移时间图像和相轨迹,发现气泡的运动行为对其初始状态有很强的依赖关系,在频率为26.5 kHz,幅度为1.35 atm的声波作用下,初始半径小于1 μm的气泡作小幅受迫振动,大于200 μm的气泡作小幅准本征振动,不具备空化气泡特征。声场中的小气泡对声波强度变化的反应更为激烈,当增加声强度时,可将一定范围内的小幅受迫振动气泡转化为空化气泡,并且,当驱动声波压力幅值增加时,初始半径越小的气泡的最大位移增加幅度越大。驱动声波频率同样影响气泡的振动。随着声波频率的升高,空化气泡的初始半径取值区间越来越小,空化振荡也越来越弱。本文还通过高速摄影系统对换能器作用于未除气的自来水所引起的变化进行了实验研究,结果表明,超声波作用下液体内的气泡场是一个混合场,场内气泡尺寸呈一定的分布状态,不仅有空化气泡,还有毫米级的大气泡。气泡的运动行为直接影响空化效果。超声空化场是一个复杂的物理场,场内除了有气泡的振动外,还有气泡间的相互吸引、碰撞和结合。      

壳体约束对液体爆炸抛撒流场特性的影响

进行了不同壳体约束条件下的液体爆炸抛撒实验,讨论了壳体对起爆后早期抛撒流场特征的影响,分析了爆炸分散过程中液体的状态变化和破碎分散机理,并通过数值模拟进行了对比分析。结果表明,在爆炸反射稀疏波的作用下液体发生空化现象,壳体强度越高,空化发生的延迟越大,发生位置越靠近爆心,同时,爆炸产物气体与液体在其界面附近越早发生掺混。     

绕水翼空化流动多尺度数值研究

空化的多尺度效应是一种涉及连续介质尺度、微尺度空化泡以及不同尺度间相互转化的复杂水动力学现象, 跨尺度模型的构建是解析该多尺度现象的关键. 本文基于欧拉-拉格朗日联合算法, 通过界面捕捉法求解欧拉体系下大尺度空穴演化, 通过拉格朗日体系下离散空泡模型求解亚网格尺度离散空泡的运动及生长溃灭. 同时, 通过判断空泡与网格尺度间的关系判定不同尺度空化泡的求解模型. 基于建立的多尺度算法对绕NACA66水翼空化流动进行模拟, 将数值结果与实验进行对比, 验证了数值计算方法的准确性. 研究结果表明, 离散空泡数量与空化发展阶段密切相关, 在附着型片状空穴生长阶段, 离散空泡数量波动较小, 离散空泡主要分布在气液交界面位置; 在回射流发展阶段, 离散空泡逐渐增加并分布在回射流扰动区; 在云状空穴溃灭阶段, 离散空泡数量增多且主要分布在气液掺混剧烈的空化云团溃灭区. 在各空化发展阶段, 离散空泡直径概率密度函数均符合伽玛分布. 空化湍流流场特性对拉格朗日空泡空间分布具有重要影响, 离散空泡主要分布在强湍脉动区、旋涡及回射流发展区域.      

Bubble-size distributions produced by wall injection of air into flowing freshwater,saltwater and surfactant solutions

As air is injected into a flowing liquid, the resultant bubble characteristics depend on the properties of the injector, near-wall flow, and flowing liquid. Previous research has shown that near-wall bubbles can significantly reduce skin-friction drag. Air was injected into the turbulent boundary layer on a test section wall of a water tunnel containing various concentrations of salt and surfactant (Triton-X-100, Union Carbide). Photographic records show that the mean bubble diameter decreased monotonically with increasing salt and surfactant concentrations. Here, 33 ppt saltwater bubbles had one quarter, and 20 ppm Triton-X-100 bubbles had one half of the mean diameter of freshwater bubbles.     

Time-resolved PIV measurements of the interaction of polystyrene beads with near-wall-coherent structures in a turbulent channel flow

Time-resolved PIV measurements were performed in a dilute particle-laden flow tracking near-neutrally buoyant polystyrene beads and the velocity field of a near wall turbulent boundary layer. Data were taken in a vertical light sheet aligned in the streamwise direction at the center of a horizontal, closed loop, transparent square water channel facility. In addition, low speed measurements were performed characterizing the effects of the dispersed phase on mean and turbulence flow quantities. Reynolds shear stress slightly differed from clear water conditions whereas fluid mean and rms values were not affected. A case study for several beads revealed a clear relation between their movement and near-wall coherent structures. Several structures having 2D vorticity signatures of near-wall hairpin vortices and hairpin packets, directly affected bead movement. A statistical analysis showed that the mean streamwise velocity of ascending beads lagged behind the mean fluid velocity and bead rms values were higher than fluid ones. Particle Reynolds numbers based on the magnitude of the instantaneous relative velocity vector peaked near the wall; values not exceeding 100, too low for vortex shedding to occur. Quadrant analysis showed a clear preference for ascending beads to reside in ejections while for descending beads the preference for sweeps was less.     

Gas-dynamic signs of explosive eruptions of volcanoes. 2. Model of homogeneous-heterogeneous nucleation. Specific features of destruction of the cavitating magma

The dynamics of state of the crystallite-containing magma is studied within the framework of the gas-dynamic model of bubble cavitation. The effect of crystallites on flow evolution is considered for two cases: where the crystallites are cavitation nuclei (homogeneous-heterogeneous nucleation model) and where large clusters of crystallites are formed in the magma in the period between eruptions. In the first case, decompression jumps are demonstrated to arise as early as in the wave precursor; the intensity of these jumps turns out to be sufficient to form a series of discrete zones of nucleation ahead of the front of the main decompression wave. Results of experimental modeling of an explosive eruption with ejection of crystallite clusters (magmatic “bombs”) suggest that a cocurrent flow of the cavitating magma with dynamically varying properties (mean density and viscosity) transforms to an independent unsteady flow whose velocity is greater than the magma flow velocity. Experimental results on modeling the flow structure during the eruption show that coalescence of bubbles in the flow leads to the formation of spatial “slugs” consisting of the gas and particles. This process is analyzed within a combined nucleation model including the two-phase Iordansky-Kogarko-van Wijngaarden model and the model of the “frozen” field of mass velocities in the cavitation zone. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 2, pp. 167–177, March–April, 2009.     

Velocity field measurements of cavitating flows

A particle Image Velocimetry (PIV) system has been developed to study the microfluid mechanics of cavitating flows. Planar PIV was used to examine the non-cavitating flow in the thin boundary layer near a hydrofoil surface for the cases of a naturally developing boundary layer and a boundary layer stimulated to turbulence by roughness near the foil leading edge. PIV was also used to examine the flow near the surface of individual cavitation bubbles and incipient attached cavitation. A system was devised to create a single nucleus in the flow upstream of a hydrofoil, and planar PIV was used to study the flow around the resulting traveling cavitation bubble. Velocity vectors were determined close to the solid surfaces and the gas/liquid interfaces of the bubbles. Seeding of the flow with particles did not result in the addition of active cavitation nuclei.     

微颗粒对超声空蚀影响的试验研究

为了探明微颗粒对超声空蚀影响的具体机制,使用超声振动空蚀试验装置对45钢在四种不同的微颗粒-纯水的悬浊液中的空蚀行为进行了研究,使用扫描电子显微镜和激光共聚焦显微镜观测空蚀后的样品表面,对空蚀坑的数量分布及微观结构进行分析,结果表明空蚀破坏程度与SiO₂悬浊液浓度有重要关联. SiO₂悬浊液存在1个临界浓度,小于临界浓度时,空蚀破坏程度随浓度的增加而增大;大于临界浓度时,空蚀破坏程度随浓度的增加而减弱. 空蚀破坏程度随粒径的增大而减弱,但空蚀坑的坑径随微颗粒粒径的增大而增大. 相比于纯水介质,添加SiC、Al₂O₃和SiO₂微颗粒均会加剧空蚀破坏程度,其中,SiC微颗粒对空蚀破坏的促进最为显著. 相比之下,Al微颗粒添加后空蚀破坏却明显减弱,起到了抑制超声空蚀的作用,这可能与不同颗粒所负荷不同电荷类型有关.      

Study of cavitation phenomena based on a technique for visualizing bubbles in a liquid pressurized chamber

In this paper, the influence of nozzle geometry on cavitation and near-nozzle spray behavior under liquid pressurized ambient is studied. For this purpose, eight steel drilled plates, with different diameters and degrees of conicity of their holes, are analyzed. A special near-nozzle field visualization technique, using a test rig pressurized with fuel, is used. Due to the difference in refractive index between liquid and vapor phase, bubbles present at the outlet of the orifice are visualized. The pressure conditions at which bubbles start appearing at the orifice outlet are compared with those at which choked flow appears. The results showed that pressure conditions for inception of cavitation obtained in the visualization tests differs from those seen for choked flow (5–8% in terms of cavitation number). In addition to this, the images taken are analyzed to get the angle of the jet formed by fuel bubbles, showing that it increases significantly for those conditions more prone to cavitate. Furthermore, comparison of bubbles generation when increasing or decreasing backpressure indicates the presence of hysteresis in cavitation inception phenomena.