Experimental investigation of primary and secondary features in high-mach-number shock-bubble interaction

Experiments to study the compression and unstable evolution of an isolated soap-film bubble containing helium, subjected to a strong planar shock wave (M=2.95) in ambient nitrogen, have been performed in a vertical shock tube of square internal cross section using planar laser diagnostics. The early phase of the interaction process is dominated by the formation of a primary vortex ring due to the baroclinic source of vorticity deposited during the shock-bubble interaction, and the mass transfer from the body of the bubble to the vortex ring. The late time (long after shock interaction) study reveals the presence of a secondary baroclinic source of vorticity at high Mach number which is responsible for the formation of counterrotating secondary and tertiary vortex rings and the subsequent larger rate of elongation of the bubble.     

超声速混合层MHz级超高频流动可视化实验

研究发展了超高频基于纳米示踪的平面激光散射（nano-tracer planar laser scattering,NPLS）技术。基于多腔并联脉冲激光器技术、棱锥分光与短曝光相机集成技术以及高精度同步控制技术,实现了MHz级流场可视化和精细测量。采用超高频NPLS技术研究了对流Mach数Ma_c=0.17,0.26混合层流场,获得了时间序列的混合层高分辨率NPLS图像。采用阵列型涡流发生器开展流动控制研究,分析涡流发生器对混合层发展的影响特性。通过选取典型涡结构,分析了超声速混合层不同发展阶段的涡运动和发展演化规律。发现混合层中段的不稳定性发展阶段,涡结构以平移和旋转为主,伴随一定的拉伸;混合层后段以变形和破碎为主,有大量小尺度结构产生。并且小尺度结构会受到剪切、大尺度结构以及小激波的影响,发生明显的非定常运动。      

激波作用下SF6气泡界面演化和射流发展的数值模拟

 数值研究了平面激波冲击氮气环境中SF₆气泡界面的Richtmyer-Meshkov不稳定性,重点关注其中的激波聚焦及射流的产生和发展过程。在入射激波马赫数为1.23的情况下,给出了压力、密度、数值纹影和涡量等物理量的演化图像,定量分析了流场中压力最大值、密度最大值、射流速度、环量和斜压力矩随时间的变化关系。计算结果表明,平面激波冲击SF₆气泡过程有很强的聚能效应,在气泡内部靠近下游极点处发生激波近似理想聚焦和点爆炸现象,直接导致出现二次波系以及向下游运动的细长射流结构。相比入射激波,二次波系产生斜压力矩和涡量的能力要弱得多。     

Laser-induced bubble formation and collapse in a nonlinear Rayleigh-Taylor unstable interface in a thin layer of molten metal

Bubble formation and bubble collapse zones in a laser-induced nonlinear Rayleigh-Taylor (R-T) instability were shown to follow the contours of a series of R-T shock-fronts generated on an irregular planar (terrace-like) target surface. Bubble formation occurs in the regime of target planar vaporization. Spatial variation of the bubble density distribution, the bubble size, and the bubble-bubble distance, as a function of distance from the shock front envelope, were determined. Bubble collapse occurs in the regime of planar-to-volume boiling transition and proceeds by the so-called “chain reaction” collapse mechanism inside a 2D bubble array. The contours of bubble generation and of bubble collapse were simulated by using the analytical model of Ott with variable phase and variable amplitude of R-T modes.     

入射和反射激波诱导轻气泡变形和失稳的计算

采用NS方程对激波诱导球形轻气泡的变形失稳过程进行三维数值模拟,验证本文模型的可靠性,讨论气泡变形、涡环形成及其三维失稳过程.结果发现:球形轻气泡在入射和反射激波先后作用下,可以变形并沿流向形成多个旋转方向不同的涡环,而斜压效应是产生这些涡环的主要原因,涡环不仅在自诱导效应和流场作用下运动,而且自身会受到扰动而发生方位角不稳定.当涡环形成以小尺度流向涡为主的复杂结构时,即有可能诱发流场湍流化.     

基于三角波瓣混合器的超声速流场精细结构和掺混特性

在超声速吸气式混合层风洞中,采用基于纳米粒子的平面激光散射(NPLS)技术对平板混合层和三角波瓣混合器诱导的混合层流场精细结构进行了对比实验研究.上下两层来流的实测马赫数分别为1.98和2.84,对流马赫数为0.2.NPLS图像清晰地展示了Kelvin-Helmholtz涡、流向涡、波系结构以及大尺度涡结构的配对合并过程.通过对比分析时间相关的NPLS流场图像,发现了大尺度拟序结构随时间发展演化的非定常特性.基于流动显示结果,采用分形维数和间歇因子指标对流场结构和混合特性进行了定量分析.实验研究表明,三角波瓣混合器诱导的流向涡结构显著提高了上下两层来流的掺混效率,其流动远场的分形维数突破了平板混合层中完全湍流区的分形维数值,达到了1.88,流场结构表现出明显的破碎性,有利于流动在标量层面的扩散和掺混.流动间歇性分析表明,流向涡与展向涡的相互剪切作用主导着混合层的掺混特性,同时由于流向涡的卷吸作用,三角波瓣混合器诱导的混合层混合区域更大,更多的流质被卷入混合区完成混合.     

波后滑移线引起的界面变形和R-M不稳定性

实验研究复杂波形结构引起平面界面变形和反射激波冲击下的R-M不稳定性的问题.在竖直激波管中生成稳定的N₂/SF₆平面界面,激波在圆柱绕射后,冲击平面界面,由此研究复杂激波引起的界面变形.平面激波在圆柱绕射后的流场,演化成具有初始入射波、三波点、弯曲反射波、Mach波和Mach反射产生的滑移线等复杂结构.研究复杂结构激波对界面的作用,对认识界面扰动的生成具有较大帮助.绕柱激波冲击后,平面界面仅在两对滑移线内部发生变形.绕柱激波冲击界面后,两对滑移线将界面分成"内界面"和"外界面",界面变形形态同滑移线和界面相交位置相关.反射激波二次冲击下,界面扰动的增长与Jacobs-Sheeley涡量模型较吻合.      

Numerical Study of Air Flow Induced by Shock Impact on an Array of Perforated Plates

This study is focused on the propagation behavior and attenuation characteristics of a planar incident shock wave when propagating through an array of perforated plates. Based on a density-based coupled explicit algorithm, combined with a third-order MUSCL scheme and the Roe averaged flux difference splitting method, the Navier–Stokes equations and the realizable k-ε turbulence model equations describing the air flow are numerically solved. The evolution of the dynamic wave and ring vortex systems is effectively captured and analyzed. The influence of incident shock Mach number, perforated-plate porosity, and plate number on the propagation and attenuation of the shock wave was studied by using pressure- and entropy-based attenuation rates. The results indicate that the reflection, diffraction, transmission, and interference behaviors of the leading shock wave and the superimposed effects due to the trailing secondary shock wave are the main reasons that cause the intensity of the leading shock wave to experience a complex process consisting of attenuation, local enhancement, attenuation, enhancement, and attenuation. The reflected shock interactions with transmitted shock induced ring vortices and jets lead to the deformation and local intensification of the shock wave. The formation of nearly steady jets following the array of perforated plates is attributed to the generation of an oscillation chamber for the inside dynamic wave system between two perforated plates. The vorticity diffusion, merging and splitting of vortex cores dissipate the wave energy. Furthermore, the leading transmitted shock wave attenuates more significantly whereas the reflected shock wave from the first plate of the array attenuates less significantly as the shock Mach number increases. The increase in the porosity weakens the suppression effects on the leading shock wave while increases the attenuation rate of the reflected shock wave. The first perforated plate in the array plays a major role in the attenuation of the shock wave.     

Visualization of shock-vortex interaction radiating acoustic waves

Unsteady compressible flow fields past a wedge and a cone, evolved by propagation and interaction of shock waves, slip lines, and vortices, are studied by shadowgraphs and holographic interferograms taken during the shock tube experiment. The supplementary numerical calculation also presented time-accurate solution of the shock wave physics which was essential to recognize the similarity and dissimilarity between the wedge and the conical flows. The decelerated shock detained by the vortex interacts with the small vortexlets along the slip layer, producing diverging acoustics: this phenomenon is more distinct in the case of wedge flow for a given shock Mach number. The decelerated shock penetrated through the vortex core constitutes a transmitted shock, which eventually merges with the diaphragm shock that bridges the vortex pair/vortex ring. This phenomenon became remarkably salient in the case of conical flow.     

入射和反射激波诱导重气泡变形和失稳的三维数值研究

 采用大涡模拟方法,对入射激波及其反射激波诱导球形重气泡的变形失稳过程进行了三维数值模拟,利用已有实验验证了计算模型的可靠性,重点考察了反射激波与已经失稳的气泡界面的再次作用,讨论了涡环的形成及其三维失稳的过程。研究结果显示：入射和反射激波与球形重气泡作用产生斜压效应,会在流场中产生旋转方向截然相反的多个涡环;反射激波诱导的涡环具有较小的强度,故更加容易失稳,甚至能完全形成具有流向涡量的复杂小尺度涡结构。     

带喷流超声速后台阶流场精细结构及其运动特性研究

采用基于纳米示踪的平面激光散射技术(NPLS)对带超声速喷流的后台阶流动精细结构进行了研究. 来流马赫数为3.4, 喷流实测马赫数为2.45, 而名义马赫数为2.5. 结果清晰地揭示了激波、剪切层、混合层、Kelvin-Helmholtz涡、羊角涡及湍流大尺度结构等大量典型流场结构. 基于大量流场精细结构图像, 对典型位置处的结构进行了空间两点相关性分析, 在喷流混合层前端涡结构小于湍流充分发展的尾端, 结构角相对也小. 喷流工作时, 模型台阶下游表面由一薄层气膜覆盖. 获得了模型流向和不同高度展向平面内的流场结构, 对照纹影试验结果, 分析了流动特点及时间演化规律. 采用微型压力扫描系统测试了模型表面的压力系数分布, 靠近喷流下游处压力系数区域0.0146. 针对NPLS图像做了流动的分形维数的分析, 发现在流动初始阶段分形维数接近于1, 越靠下游分形维数越高.     

Surface tension and instability of shock waves in gases

The pressure anisotropy within a shock layer results in an additional property of the shock wave: the surface tension. Its value amounts to 10³ dyn/cm for Mach number 10. The surface tension term in the characteristic equation causes an absolute shock wave instability in the spontaneous sound emission regime.     

平面弱激波加载下球形气泡演化的实验研究

 利用高速摄影机直接观测了激波加载下球形气泡的演化过程。采用白色的烟雾颗粒对气泡内透明的测试气体进行染色,直接观察到了气泡界面。通过对平面弱激波加载下轻气体气泡和重气体气泡进行研究,成功验证了轻气体气泡和重气体气泡演化过程中出现的典型界面结构,获得了弱激波加载轻气体气泡中背风涡环的环状几何结构。运用相关方法分析了轻气体气泡实验初期的流场分布,得到的结果与理论分析结果吻合得很好,对于此类实验的数据处理具有一定的启发意义。同时,实验中使用的烟雾颗粒法为今后在球形气泡实验中引入更为精确的平面激光诱导荧光技术（PLIF）、激光粒子图像测速技术（PIV）等实验测试系统提供了布撒示踪粒子的范例。     

Pressure effect in a shock-wave-plasma interaction induced by a focused laser pulse

The effect of ambient pressure on the interaction between a laser plasma bubble and a shock wave involving a Richtmyer-Meshkov instability was experimentally studied via framing Schlieren visualization. A sharp plasma interface could be formed without any separation material that causes undesired disturbances. The fundamental vortex structure which was produced via baroclinic effects was self-similar with respect to the laser energy-ambient pressure ratio. Yet, the higher the ambient pressure, the more high-wave-number instabilities were enhanced so as to contaminate the self-similarity.     

磁控条件下激波冲击三角形气柱过程的数值研究

本文基于磁流体动力学方程组,在保证磁场散度为零的条件下,采用CTU+CT（corner transport upwind+constrained transport）算法,对有无磁场控制下激波与重质或轻质三角形气柱相互作用过程进行数值研究.结果表明：无论有无磁场,两气柱在激波冲击下均具有完全不同的波系结构和射流现象.其中,入射激波与重气柱发生常规折射,形成介质射流,而与轻气柱作用则发生非常规折射,形成反相空气射流.无磁场时,气柱在激波冲击下,产生Richtmyer-Meshkov和Kelvin-Helmholtz不稳定性,界面出现次级涡序列,重气柱上下角卷起形成主涡对,轻气柱空气射流穿过下游界面后形成偶极子涡.施加横向磁场后,次级涡序列、主涡对以及偶极子涡均消失.进一步研究表明,在磁场作用下,洛伦兹力将不稳定性诱导产生的涡量向界面两侧的Alfvén波上输运,减少界面涡量沉积,抑制界面卷起失稳.最终,涡量沿界面两侧形成相互远离的涡层,界面不稳定性得到控制.此外,定量分析表明磁场能加快两气柱上游界面的运动,抑制下游界面的运动,且对轻气柱的控制效果更好.     

Shock tube exit flow fields through thin membranes

The flow field characteristics that form for a shock wave propagating through a membrane-like termination at the exit of a shock tube are studied. The strength of the shock wave reflected back into the tube, as well as the strength of the shock wave transmitted, is examined. Six different materials are used, ranging from copper and aluminium foils to a variety of elastic and plastic sheets, in a few cases with different initial pressure differentials. High-speed shearing interferometry imaging is done of the external flow. Three principal characteristics are present in the transmitted flow: a diffracted shock wave, an expansion wave and a re-compression shock wave. It is found that the prominence of these features varies depending on the material type. For the later flow development and material rupture, there are a number of principal characteristics: small vortices, secondary shock waves, a vortex ring, oblique waves and a Mach disc.     

Direct numerical simulation of shock/turbulent boundary layer interaction in a supersonic compression ramp

A direct numerical simulation of the shock/turbulent boundary layer interaction flow in a supersonic 24-degree compression ramp is conducted with the free stream Mach number 2.9.The blow-and-suction disturbance in the upstream wall boundary is used to trigger the transition.Both the mean wall pressure and the velocity profiles agree with those of the experimental data,which validates the simulation.The turbulent kinetic energy budget in the separation region is analyzed.Results show that the turbulent production term increases fast in the separation region,while the turbulent dissipation term reaches its peak in the near-wall region.The turbulent transport term contributes to the balance of the turbulent conduction and turbulent dissipation.Based on the analysis of instantaneous pressure in the downstream region of the mean shock and that in the separation bubble,the authors suggest that the low frequency oscillation of the shock is not caused by the upstream turbulent disturbance,but rather the instability of separation bubble.     

Experimental investigation of a swirling flow in a cubic container

A flow induced by a rotating disk mounted at the top of a cubic container totally filled with a liquid is studied experimentally. The flow pattern is visualized for Reynolds numbers in the interval 1500–6000, and the velocity variation along the axis of the container, which coincides with the axis of revolution of the disk, is observed by means of Doppler laser anemometry (DLA). As Re grows, the velocity axial component starts fluctuating because of the vortex core precession. The breakdown of the vortex helical structure becomes pronounced at Re>4000 without the formation of the return flow region (vortex breakdown bubble) at the axis. With the Reynolds number and the container height-to-disk radius ratio being the same, the axial flow patterns in standard cylindrical [1] and cubic containers differ radically. In the latter vessel at low Re, the steady flow regime and axisymmetric bubble breakdown of the vortex structure near the axis are absent.     

一个用于界面不稳定性实验研究的竖式激波管

 研制了一个可用于气-气界面Rayleigh-Taylor（RT）不稳定性、气-液界面Richtmyer-Meshkov（RM）不稳定性、气-液界面RT和RM耦合不稳定性实验的竖式激波管装置,配备了相应的光学电学测量系统,给出了装置的主要技术参数,以及典型实验的压力-时间曲线。利用这套实验装置,可以精确地控制不稳定性的初始扰动界面,观测记录界面演化发展全过程。     

Flow visualization and density measurement of compressible flows by a laser speckle method

Laser speckle method is a well known technique that is useful for both visualization and quantitative measurement. This technique was applied to the density measurement of Mach reflection of shock waves in the present experiment. The Object of the measurement is the density field of simple Mach reflection in relatively low shock Mach number. The non-uniform flow field is divided into three regions by incident, Mach and reflected shock waves. A shock tube was employed in the present experiment. Wedges of 20 degrees and 45 degrees were placed in the test section. YAG laser was employed as a light source. Speckle photograph was taken by a digital still camera. Simple subtraction between the reference and flow images shows a shock pattern and a degree of the correlation of speckle pattern in the flow field. Thus, we can obtain a visualized flow image showing a configuration of Mach reflection from speckle photograph. Speckle photographs which was obtained in the experiments were processed with cross-correlation method. A reconstructed density gradient vector map of Mach reflection was obtained. Comparing the experimental result with numerical one, the measured density gradient shows a good agreement with theoretical prediction.