INVESTIGATION OF BUBBLE-BUBBLE INTERACTION EFFECT DURING THE COLLAPSE OF MULTI-BUBBLE SYSTEM

通过直接数值模拟方法对多泡在压力驱动下的溃灭过程进行了研究。气相满足理想气体正压模型,液相为不可压 流体,采用基于压力的方法求解多泡的两相流场。数值研究表明,在多泡流场中,中心气泡的溃灭过程明显不同于单泡,存在总体溃灭延迟现象和后期加速现象。随着周围气泡数的增多或气泡间距的减小,中心气泡的溃灭时间增长,溃灭时的压力峰值增大。结合理论定性分析发现,气泡运动不仅受远场压力的驱动,还受周围气泡诱导压力场的影响。周围气泡的诱导压力经历先减小后增大的过程,从而使受其影响的中心气泡产生先延迟后加速的特征。     

A study of nucleate bubble growth on microstructured surface through high speed and infrared visualization

We studied bubble growth on a microstructured surface during nucleate boiling using optical high-speed and infrared (IR) cameras. The effects of structured surfaces on bubble growth and dynamics were examined and their role analyzed with the use of simple models. A smooth, bare surface was prepared, and four microstructured test sample surfaces were fabricated with microscale gaps ranging from 5 to 80 µm. The optical high-speed camera was used to observe the bubble growth profile with high temporal resolution; the IR camera was focused on the underside of the sample for direct visualization of the boiling process. Overall, the microstructured surfaces produced more bubbles, a lower frequency and nucleation site density than the bare surface for the low heat flux range (100–300 kW/m²), corresponding to the isolated bubble growth regime. The liberated bubble size was dependent on the size of the microstructure gap. Analysis of the high-speed images revealed that the liquid between the microstructures did not evaporate during bubble growth; however, during the initial growth stage, there was a brief period in which the liquid at the nucleation site evaporated. The large surface area and relatively high number of nucleation points contributed to enhanced bubble growth on the structured surfaces.     

柱形汇聚激波冲击球形重气体界面的实验研究

采用高速纹影法实验研究了柱形汇聚激波与球形重气体界面相互作用的 Richtmyer-Meshkov不稳定性问题. 激波管实验段基于激波动力学理论设计, 将马赫数为1.2 的平面激波转化为柱形汇聚激波, 气体界面由肥皂膜分隔六氟化硫(内)和空气(外)得到. 采用高速摄影机在单次实验中拍摄激波运动的全过程, 对柱形激波的形成进行了实验验证, 并进一步观测了汇聚激波与球形气体界面相互作用过程中的波系发展和气体界面变形以及反射激波同已变形界面二次作用的流场演化. 结果表明: 当柱形汇聚激波穿过气泡界面以后, 气泡左侧界面极点沿激波传播方向保持匀速运动, 气泡右侧界面发展成为射流结构, 气泡主体发展成为涡环结构; 在反射激波的二次作用下, 流场中无序运动显著增强并很快进入湍流混合阶段.     

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。     

ADVANCE OF EXPERIMENTAL TECHNOLOGIES FOR STRUCTURAL MODAL TEST IN HIGH TEMPERATURE ENVIRONMENTS

由于高超声速飞行器长时间在大气层内工作会面临恶劣的气动热环境,飞行器结构地面模态试验必须考虑高温影响,但高温环境下结构模态试验远比常规试验复杂,目前仍存在很多技术难点亟待解决.本文首先对高温环境下的结构模态试验技术国内外发展现状进行了综述,详细分析了现有的高温环境下结构模态试验中的激励力与振动响应测量技术的特点及适用性,最后展望了高温环境下结构热模态试验技术的发展方向和研究重点.     

Particle tracking velocimetry of the flow field around a collapsing cavitation bubble

The velocity field in the vicinity of a laser-generated cavitation bubble in water is investigated by means of particle tracking velocimetry (PTV). Two situations are explored: a bubble collapsing spherically and a bubble collapsing aspherically near a rigid wall. In the first case, the accuracy of the PTV method is assessed by comparing the experimental data with the flow field around the bubble as obtained from numerical simulations of the radial bubble dynamics. The numerical results are matched to the experimental radius–time curve extracted from high-speed photographs by tuning the model parameters. Trajectories of tracer particles are calculated and used to model the experimental process of the PTV measurement. For the second case of a bubble collapsing near a rigid wall, both the bubble shape and the velocity distribution in the fluid around the bubble are measured for different standoff parameters γ at several instants in time. The results for γ > 1 are compared with the corresponding results of a boundary-integral simulation. For both cases, good agreement between simulation and experiment is found.     

DYNAMICAL BEHAVIORS OF DOUBLE CAVITATION BUBBLES UNDER ULTRASONIC HONING

为了探讨超声珩磨作用下磨削区的空化机理,基于速度势叠加原理,考虑超声珩磨速度和珩磨压力,建立了磨削区两空化泡的动力学模型. 数值模拟了磨削区空化泡初始半径、两空化泡间距、超声声压幅值、珩磨压力、珩磨头转速对磨削区两空化泡动力学特性的影响. 研究表明,考虑两空化泡之间的相互作用时,要想获得良好的空化效果,可将两空化泡初始半径之比控制在3 倍以内;选择较高的超声波声压幅值与较低的珩磨压力,并且使超声波声压幅值与珩磨压力和液体静压力之差介于0.66~1.89MPa 之间;增大珩磨头转速空化泡溃灭也略有加速;通过试验测量材料表面粗糙度的方法间接验证了理论分析的合理性.     

高速列车轴承可靠性评估关键力学参量研究进展

轴承是高速列车牵引传动和轮轴系统的关键零部件. 受列车运行过程中电机转矩、齿轮啮合以及轮轨随机激励的影响,轴承可能发生疲劳破坏, 严重影响高速列车的行车安全.我国特有的复杂运用条件对轴承部件的疲劳性能提出了更高的要求,而轴承疲劳可靠性的基础理论和关键技术是我国轴承正向设计研发中的薄弱环节.可靠性评估方面的相关研究在解决轴承可靠性研究的瓶颈问题中起到了承上启下的关键作用.高速列车轴承可靠性评估手段与技术旨在获得使用环境中轴承可靠性评估的关键力学参量,并以此推动复杂激励下轴承疲劳可靠性理论研究. 因此,需要哪些关键力学参量并且在复杂的实际使用环境下如何去获取这些力学参量是进行高速列车轴承可靠性评估的关键所在.本文首先概述了高速列车轴承所处的复杂使用环境及运用中的主要失效模式,并据此分析了高速列车轴承可靠性评估所需的关键力学参量,强调了轴承内部滚滑行为和载荷分布在可靠性评估和轴承状态监测中的重要作用,之后从计算模型和测试技术等方面系统阐述了针对这两个关键力学参量的研究进展.最后提出了在高速列车轴承可靠性评估关键力学参量特征及测试技术研究中值得关注的若干问题.      

EXPERIMENTALLY STUDY OF THE RICHTMYER-MESHKOV INSTABILITY AT N2/SF6 FLAT INTERFACES BY DIFFRACTED INCIDENT SHOCK WAVES AND RESHOCK

利用高速纹影测试实验研究低马赫数入射激波绕圆柱体后冲击N₂/SF₆平面界面,以及来自固壁的反射激波再冲击过程的（Richmyer-Meshkov,R-M）不稳定性特征.与平面激波作用不同的是,绕射后的激波会在界面处生成局部扰动.实验结果显示,入射激波作用下界面宽度增长缓慢,而反射激波再冲击后,局部扰动会产生大的“尖钉”和“气泡”结构;以及反射激波与边界层相互作用产生壁面涡,它们会加剧湍流混合区的增长;实验中反射激波过后混合区增长率不十分依赖于波前状态,增长规律同Mikaelian模型较吻合;来自尾部固壁的反射稀疏波会再次加剧湍流混合区的增长.     

Medical image based hemodynamic analysis

随着计算机成像技术的发展,计算机断层扫描(CT)和核磁共振(MRI)已经广泛应用于临床,特别是血管类疾病的诊断,比如动脉狭窄、血管瘤、血管畸形等.除了可以提供高分辨率的静态图像,先进的MRI技术还可以通过时间序列直接反映血流动力学的变化.而基于计算机成像和三维重建技术,血流动力学的参数又可以通过计算流体力学的方法进行详细的分析.如何将血流动力学参数和临床诊断相结合是近年来在转化医学领域研究的热点.结合文献调研和作者自己的研究工作对基于医学图像的血流动力学分析进行综述,并探讨未来的研究方向.     

Cavitation structures formed during the rebound of a sphere from a wetted surface

We use high-speed imaging to observe the dynamics of cavitation, caused by the impact and subsequent rebound of a sphere from a solid surface covered with a thin layer of highly viscous liquid. We note marked qualitative differences between the cavitation structures with increase in viscosity, as well as between Newtonian and non-Newtonian liquids. The patterns observed are quite unexpected and intricate, appearing in concentric ring formations around the site of impact. In all cases, we identify a distinct radius from which the primary bubbles emanate. This radius is modelled with a modified form of Hertz contact theory. Within this radius, we show that some fine cavitation structure may exist or that it may be one large cavitation bubble. For the non-Newtonian fluids, we observe foam-like structures extending radially with diminishing bubble sizes with increase in radial position. Whereas for the Newtonian fluids, the opposite trend is observed with increasing bubble size for increasing radial position. Finally, we compare our experimental observations of cavitation to the maximum tension criterion proposed by Joseph (J Fluid Mech 366:367–378, 1998) showing that this provides the lower limit for the onset of cavitation in our experiments.     

EQUIVALENCE OF TWO METHODS FOR AIRDROP DYNAMICS

定义了空投系统的两个元素，即“载机”和“装载物”。介绍了用来研究空投问题的两种动力学建模方法，即“整体法”和“多体法”。证明了两种方法对于平动动力学是等价的，但对于转动动力学是不等价的，原因是整体法并非总是存在的。     

竖向载荷对半刚性连接钢框架抗震性能的影响

利用拟动力试验对T 型钢半刚性连接钢框架在2 种地震峰值加速度下的抗震性能进行研究, 并对比分析了钢框架在柱顶施加竖向载荷和无竖向载荷等2 种工况下的应变、位移和载荷变化, 以及载荷-位移滞回曲线. 结果表明: T 型钢半刚性连接钢框架具有较好的变形能力, 滞回性能良好, 抗震能力较强, 且随着地震作用的增大, 钢框架的动力反应增大; 竖向载荷对钢框架的层间位移反应影响较大, 而对节点区域的应变和层间载荷的影响较小.     

柱形装药自由场水中爆炸气泡的射流特性

为研究柱形装药水中爆炸气泡射流特性,采用高速摄影方法在实验水箱中对PETN药柱进行水中爆炸实验,初步获得了有关柱形装药自由场水中爆炸气泡运动的数据资料.研究结果表明:柱形装药自由场水中爆炸形成气泡的运动与球形装药的存在差别,药柱放置方式的变化也会导致水中爆炸气泡产生性质迥异的运动.当药柱水平放置时,气泡表面的运动呈非对...     

Nanofluidic energy absorption system: A review

基于纳米流控行为设计的新一代能量吸收耗散系统（nanofluidic en-ergy absorption system,NEAS）将会比传统吸能材料具有更高的能量吸收密度,而且还可以重复使用,特别是在小体积应用环境下具有显著的优势.本文从实验和计算模拟两方面综述了目前关于NEAS能量吸收耗散行为的最新研究进展,其中实验研究主要包括准静态压缩和动态压缩测试,计算模拟研究主要是采用基于经验势的分子动力学模拟方法.通过准静态压缩实验,可以测量NEAS模型的载荷–位移关系曲线,从而获得NEAS模型的临界渗透压强,了解卸载后系统是否能够恢复到加载前的状态（即是否可以重复使用）,并通过载荷–位移关系曲线下面积估算NEAS模型的吸能密度;通过动态压缩实验可以测量NEAS模型对脉冲载荷的缓冲保护作用,主要体现为降低脉冲载荷幅值和扩展脉冲宽度.计算模型研究可以明确给出NEAS对外载荷的微观响应,从而可以准确了解NEAS的能量吸收耗散机制以及吸能密度的主要影响因素.本研究可以帮助我们全面了解NEAS的研究进展,为NEAS的设计与优化提供重要指导.     

Numerical investigation of ice breaking by a high-pressure bubble based on a coupled BEM-PD model

In this paper, by combining the boundary element method (BEM) and peridynamics (PD), a bubble-ice interaction model is established, which can investigate the dynamic interactions between a high-pressure bubble and an ice plate with particular focus on the mechanical behaviors of ice breaking. The bubble dynamics are solved by BEM based on the potential flow theory. Ice cracks initiation and propagation are simulated by the bond-based peridynamics which is validated by a three-point bending test. The fluid–structure interaction (FSI) is achieved by matching the normal velocity and hydrodynamic loads at the fluid–structure interface. To validate the proposed FSI model, an experiment is carried out in which an oscillating bubble is generated under an ice plate by underwater discharge system. The whole interaction process is captured by a Phantom V711 high-speed camera. Qualitative agreements are achieved between the numerical and experimental results. The underlying mechanism of cracks initiation, propagation, branching, and coalescence of the ice plate is found to highly depend on three parameters, i.e., bubble–ice distance, ice thickness and bubble size. The present study is expected to provide further assists in the understanding of ice breaking problems.     

Numerical and experimental study on the motion characteristics of single bubble in a complex channel

This paper is an extended study from previous work. In this study, the focus is paid to the dynamics of bubble rising and deformation in a complex channel, while the previous work is in straight channel. For this purpose, a three-dimensional lattice Boltzmann method (LBM) is employed to simulate the dynamics behaviour of a bubble rising in a complex channel consisting of three half-round throats. To validate the numerical method, a visual experiment was carried out by means of a high-speed digital camera and computer image processing technology. The behaviour of the rising bubble through glycerine solution in a complex channel was recorded. Some physical parameters such as rising velocities, trajectory and shapes of the bubble were calculated and processed based on the experimental data. In the same conditions, the trajectory, shapes and rising velocities of the bubble were simulated during its rising process by the proposed LBM. The numerical results are in good agreement with the experimental results. It demonstrates that LBM used in this work is feasible for simulating two-phase flow in such a complex channel.     

Lagrangian tracking of bubbles interacting with pin-fins in a microchannel

This paper presents new image analysis algorithms to measure the trajectories of breaking and coalescing bubbles in microscale bubbly flows. Image analysis of high-speed movies provides information on bubble dynamics and bubble interaction including bubble coalescence and breakage events. Individual bubbles that overlap in the image are recognized with a presented breakline method. The breakline method discriminates the overlapping bubbles with lines based on the bubble perimeter curvature analysis. Coalescence and breakage events are automatically recognized, and the path lines of bubbles travelling through the field of view are analyzed. The functionality of the algorithms was examined in bubbly flow in a microchannel encompassing two pin-fins in tandem.     

Characteristics of impact-driven high-speed liquid jets in water

This paper describes a preliminary investigation of the characteristics of high-speed water jets injected into water from an orifice. The high-speed jets were generated by the impact of a projectile launched by a horizontal single-stage powder gun and submerged in a water test chamber. The ensuing impact-driven high-speed water jets in the water were visualized by the shadowgraph technique, and the images were recorded by a high-speed digital video camera. The processes following such jet injection into water, the jet-induced shock waves, shock wave propagation, the bubble behavior, bubble collapse-induced rebound shock waves and bubble cloud re-generation were observed. Peak over-pressures of about 24 and 35 GPa measured by a Polyvinylidence difluoride (PVDF) piezoelectric film pressure sensor were generated by the jet impingement and the bubble impingement, respectively. The peak over-pressure was found to decrease exponentially as the stand-off distance between the PVDF pressure sensor and the nozzle exit increases.     

Air transport in chute flows

The air bubble rise velocity in still water depends mainly on the bubble size and is basically influenced by buoyancy, viscosity and surface tension. In high-speed flows the number of forces acting on air bubbles increases with turbulence, non-hydrostatic pressure gradient, shear forces, bubble clouds and free-surface entrainment. Air bubbles in these flows are used for cavitation protection of hydraulic structures such as chutes, spillways and bottom outlets. Here, air is normally added by means of aerators upstream of regions where the cavitation number falls below a critical value mainly to reduce the sonic velocity of the fluid and cushion the cavitation bubble collapse process. The distance between successive aerators depends basically on the bubble rise velocity. Until today, the bubble rise velocity in high-speed flows was not thoroughly investigated because of limited laboratory instrumentation. The present project focused on the streamwise development of air concentrations in high-speed flows along a 14 m long model chute. The bubble rise velocity was indirectly derived from the air detrainment gradient of the air concentration contour lines downstream of an aeration device. It accounts for the main hydraulic parameters chute slope, Froude number and air concentration. It is demonstrated that the bubble rise velocity in high-speed flow and stagnant water differ significantly due to fracturing processes, turbulence, and the ambient air concentration.