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.     

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.      

高超声速层流绕三维压缩拐角的数值模拟

通过数值模拟研究了高超声速来流绕过压缩拐角的层流分离三维流动特性.数值方法采用三维N-S方程,结合2阶精度Roe格式以及分区结构网格有限体积法进行离散.数值模拟的空间激波结构与实验纹影结果符合较好;激波/边界层干扰区内3条纵向线上的计算压力分布与实验结果进行了对比分析,计算获得在三维楔侧面存在低压力区,与实验结果反映的规律一致,计算结果表明低压力区是由楔体侧缘尖端发起的二次涡的抽吸作用造成的.此外,在楔体后端尾流区的低压沿边界层内的亚声速区往上游传递了一定距离.      

Control of the dynamics of a boiling vapour bubble using pressure-modulated high intensity focused ultrasound without the shock scattering effect: A first proof-of-concept study

Boiling histotripsy is a promising High-Intensity Focused Ultrasound (HIFU) technique that can be used to induce mechanical tissue fractionation at the HIFU focus via cavitation. Two different types of cavitation produced during boiling histotripsy exposure can contribute towards mechanical tissue destruction: (1) a boiling vapour bubble at the HIFU focus and (2) cavitation clouds in between the boiling bubble and the HIFU source. Control of the extent and degree of mechanical damage produced by boiling histotripsy is necessary when treating a solid tumour adjacent to normal tissue or major blood vessels. This is, however, difficult to achieve with boiling histotripsy due to the stochastic formation of the shock scattering-induced inertial cavitation clouds. In the present study, a new histotripsy method termed pressure-modulated shockwave histotripsy is proposed as an alternative to or in addition to boiling histotripsy without inducing the shock scattering effect. The proposed concept is (a) to generate a boiling vapour bubble via localised shockwave heating and (b) subsequently control its extent and lifetime through manipulating peak pressure magnitudes and a HIFU pulse length. To demonstrate the feasibility of the proposed method, bubble dynamics induced at the HIFU focus in an optically transparent liver tissue phantom were investigated using a high speed camera and a passive cavitation detection systems under a single 10, 50 or 100 ms-long 2, 3.5 or 5 MHz pressure-modulated HIFU pulse with varying peak positive and negative pressure amplitudes from 5 to 89 MPa and −3.7 to −14.6 MPa at the focus. Furthermore, a numerical simulation of 2D nonlinear wave propagation with the presence of a boiling bubble at the focus of a HIFU field was conducted by numerically solving the generalised Westervelt equation. The high speed camera experimental results showed that, with the proposed pressure-modulated shockwave histotripsy, boiling bubbles generated by shockwave heating merged together, forming a larger bubble (of the order of a few hundred micron) at the HIFU focus. This coalesced boiling bubble then persisted and maintained within the HIFU focal zone until the end of the exposure (10, 50, or 100 ms). Furthermore, and most importantly, no violent cavitation clouds which typically appear in boiling histotripsy occurred during the proposed histotripsy excitation (i.e. no shock scattering effect). This was likely because that the peak negative pressure magnitude of the backscattered acoustic field by the boiling bubble was below the cavitation cloud intrinsic threshold. The size of the coalesced boiling bubble gradually increased with the peak pressure magnitudes. In addition, with the proposed method, an oval shaped lesion with a length of 0.6 mm and a width of 0.1 mm appeared at the HIFU focus in the tissue phantom, whereas a larger lesion in the form of a tadpole (length: 2.7 mm, width: 0.3 mm) was produced by boiling histotripsy. Taken together, these results suggest that the proposed pressure-modulated shockwave histotripsy could potentially be used to induce a more spatially localised tissue destruction with a desired degree of mechanical damage through controlling the size and lifetime of a boiling bubble without the shock scattering effect.     

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.     

激波与SF6球形气泡相互作用的数值研究

本文基于大涡模拟方法, 采用高阶精度格式对平面入射激波以及不同反射距离条件下的反射激波与SF₆重气泡相互作用过程进行了三维数值模拟. 数值结果清晰地显示了SF₆重气泡在激波作用下诱导Richtmyer-Meshkov不稳定性过程, 揭示了入射激波以及反射激波在气泡界面聚焦诱导射流的过程, 详细分析了不同反射距离条件下反射激波与SF₆重气泡作用过程及流场结构.     

Numerical flow visualization of a Single Expansion Ramp Nozzle with hypersonic external flow

Numerical simulation of scramjet asymmetric nozzle flow is carried out to visualize and investigate the effects of interaction between engine exhaust and hypersonic external flow. The Single Expansion Ramp Nozzle (SERN) configuration studied here consists of flat ramp and a cowl with different combinations of ramp angle and cowl geometry. UsingPARAS 3D, simulations are performed for a free stream Mach number of 6.5 that constitutes the external flow around the vehicle. Appropriate specific heats ratio has been simulated for the jet and free stream flow. External shock wave due to jet plume interaction with free stream flow, the internal barrel shock wave and the shear layer emanating from the cowl trailing edge and sidewalls are well captured. Wall static pressure distribution on the nozzle ramp for different nozzle expansion angles has been computed for both with and without side fence. Axial thrust and normal force have been evaluated by integrating the wall static pressure. Effect of cowl length variation and side fence on the SERN performance has also been studied and found to be quite significant. Based on this study, an optimum ramp angle at which the SERN generates maximum axial thrust is obtained. SERN angle of 20° was found to be optimum when the flight axis coincides with nozzle axis.     

Wave processes at outflow of water coolant with initial supercritical parameters

Simulation is presented for the case of water coolant outflow with initial supercritical parameters after high pressure pipeline breaking. The nonequilibrium relaxation model of phase transition was developed and validated. The model describes both boiling and condensation processes.     

对运动斜激波绕射波纹壁面的研究

使用计算流体动力学的方法,对经典的运动激波绕射现象做数值模拟,研究了一类复杂激波反射问题一入射的运动斜激波绕射现象.给出一组运动斜激波绕射波纹壁面的非定常过程的模拟结果。计算结果显示出由运动斜激波绕射诱导的多波干扰产生的复杂流场结构。     

Determination of Slip Length in Couette Flow Based on an Analytical Simulation Incorporating Surface Interaction

An analytical simulation based on a new model incorporating surface interaction is conducted to study the slip phenomenon in the Couette flow at different scales.The velocity profile is calculated by taking account of the micro-force between molecules and macro-force from the viscous shearing effect,as they contribute to the achievement of the slip length.The calculated results are compared with those obtained from the molecular dynamics simulation,showing an excellent agreement.Further,the effect of the shear rate on the slip is investigated.The results can well predict the fluid flow behaviors on a solid substrate,but has to be proved by experiment.     

Controlled tissue emulsification produced by high intensity focused ultrasound shock waves and millisecond boiling

In high intensity focused ultrasound (HIFU) applications, tissue may be thermally necrosed by heating, emulsified by cavitation, or, as was recently discovered, emulsified using repetitive millisecond boiling caused by shock wave heating. Here, this last approach was further investigated. Experiments were performed in transparent gels and ex vivo bovine heart tissue using 1, 2, and 3 MHz focused transducers and different pulsing schemes in which the pressure, duty factor, and pulse duration were varied. A previously developed derating procedure to determine in situ shock amplitudes and the time-to-boil was refined. Treatments were monitored using B-mode ultrasound. Both inertial cavitation and boiling were observed during exposures, but emulsification occurred only when shocks and boiling were present. Emulsified lesions without thermal denaturation were produced with shock amplitudes sufficient to induce boiling in less than 20 ms, duty factors of less than 0.02, and pulse lengths shorter than 30 ms. Higher duty factors or longer pulses produced varying degrees of thermal denaturation combined with mechanical emulsification. Larger lesions were obtained using lower ultrasound frequencies. The results show that shock wave heating and millisecond boiling is an effective and reliable way to emulsify tissue while monitoring the treatment with ultrasound.     

激波与层流/湍流边界层相互作用实验研究

在超声速风洞中,分别对层流和湍流来流条件下的边界层和斜激波(激波强度足以引起流动分离)相互干扰进行了实验研究,利用纳米粒子示踪平面激光散射(NPLS)技术获得了两种条件下流场的精细结构图像;利用粒子图像测速(PIV)技术获得了两种条件下流场的速度场和涡量场;综合运用NPLS结果和PIV结果对比分析了两种流动的瞬时流动结构和时间相关性,实验结果表明:层流边界层内的分离区呈现出狭长的条状,而湍流边界层内分离区呈现出较规则的椭圆;在入射激波上游距入射点较远的位置,层流边界层外围拟序结构会诱导出一系列压缩波系,进而汇聚成空间位置不稳定的诱导激波,而湍流边界层则是在入射激波上游较近的地方直接形成较强且稳定的诱导激波;在入射激波下游,层流边界层内的膨胀区域较小且急促,膨胀后产生的再附激波很弱,而湍流边界层内的膨胀区域较大,膨胀后产生的激波较强。     

冲击波在铝靶中传播的数值模拟研究

利用一维辐射流体力学数值模拟程序对激光驱动的冲击波在平面铝靶中传播的实验结果进行了模拟研究.分析了空间单元层的厚度对模拟结果的影响,给出了最佳单元层厚度.通过将数值模拟与实验结果相比较给出了实验中两种激光光强的实际的吸收系数.结果显示,波长为1.053μm,强度分别为0.81×10¹⁴和1.65×10¹⁴W/cm²的激光驱动的冲击波在铝靶中传播速度分别为16.52,18.56μm/ns,冲击波的峰值压力分别为0.386和0.537TPa,这些模拟结果与实验结果是一致的 关键词： 冲击波 辐射流体力学 激光等离子体     

Molecular Dynamic Simulation of High Speed Deformation Effect on Microstructure Change in Thermal Heated Metals

In the present paper computer simulation of high-speed deformation (shock wave propagation) by molecular dynamic method is performed in thin copper sample, having the form of rectangular parallelepiped (10 a ‐ 10 a ‐ 20 a , where a is the lattice constant) with 8000 atoms. On the surfaces Z 0 =0 and Z max =20 a the mirror boundary conditions with rigid walls and the periodic boundary conditions along X and Y directions corresponding to short sides of deformed crystal are used, which allows to investigate the reflection of shock wave from the surfaces in Z direction. The changes of microstructure have been investigated up to 12 ps. The numerical calculations of microstructure changes have been performed here taking into account the effect of thermal heating of crystal lattice before shock wave front. The numerical results show that comparing with the propagation of shock waves under room temperature in thermal heated structure additional displaced atoms (vacancies and interstitials) are produced. The obtained results show that the production of point defects during high-speed deformation is determined by the thermal softening of microstructure and generation rate of point defects very strong increases with an increasing of high speed deformation rate. The peculiarities of microstructure changes in deformed copper are analyzed here at the different initial temperatures and various high-speed deformations (average ion velocities behind shock wave).     

Asymmetric interaction of blast waves and shock waves with a body flying at supersonic velocity

The problems of asymmetric interaction of a blunt wedge traveling at supersonic velocity with a cylindrical blast wave from a point explosion and with a plane shock wave are investigated by numerical simulation. The evolution of the interaction flow is analyzed, and data are obtained on how the structure of the shock layer changes. Zh. Tekh. Fiz. 69, 15–19 (May 1999)     

Effect of the incident shock wave interacting with transversal jet flow on the mixing and combustion

The interaction between an incident shock wave and a transverse jet flow for mixing and combustion in a supersonic airstream was investigated experimentally and numerically. NO planar laser induced fluorescence (NO-PLIF) and particle imaging velocimetry (PIV) for non-reactive flows and three-dimensional reactive/non-reactive numerical simulations were conducted to examine the effect of the incident shock wave on the three-dimensional flow structure and mixing mechanism between the airstream and the injected gas downstream of the injection slot. Results of NO-PLIF measurement and numerical simulation showed that, in the case without the incident shock wave, injected gas is seldom seen in the recirculation zone just downstream of the injection slot, while the injected gas with higher concentration is almost uniformly distributed in the recirculation zone when the incident shock wave is introduced downstream of the injection slot. Moreover, it was shown by the numerical simulations that the profiles of the local equivalence ratio is in the combustible range due to the enhanced entrainment of the airstream when the incident shock wave is introduced downstream of the injection slot. A large-scale recirculation in the direction parallel to the wall is generated by the three-dimensional flow effects, which enhances the mixing and extends the residence time in the recirculation zone in the case with incident shock wave downstream of the injection slot, the recirculation flow being confirmed successfully by PIV measurements as well. The results of three-dimensional reactive numerical simulations were in good agreement with the experimental flame-holding characteristics at a lower total temperature, which showed that flame-holding can be attained only when the incident shock wave was introduced downstream of the injection slot, confirming that the formation of three-dimensional and large-scale recirculation flow downstream of the injection slot enlarges the recirculation zone and enhances the mixing to produce the conditions for robust flame-holding.     

Shock wave structure in simple liquids

The shock wave structure in a liquid is studied by a molecular dynamics simulation method. The interaction between atoms is described by the Lennard-Jones (6–12) potential. In contrast to earlier works, the simulation is performed in a reference frame tied to the shock wave front. This approach reduces non-physical fluctuations and makes it possible to calculate the distribution functions of the kinetic and potential energy for several cross sections within the shock layer. The profiles of flow variables and their fluctuations are found. The surface tension connected with pressure anisotropy within the shock front is calculated. It is shown that the main contribution to the surface tension coefficient comes from the mean virial. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 722–727 (10 May 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Monte Carlo solution of the Boltzmann equation via a discrete velocity model

A new discrete velocity scheme for solving the Boltzmann equation is described. Directly solving the Boltzmann equation is computationally expensive because, in addition to working in physical space, the nonlinear collision integral must also be evaluated in a velocity space. Collisions between each point in velocity space with all other points in velocity space must be considered in order to compute the collision integral most accurately, but this is expensive. The computational costs in the present method are reduced by randomly sampling a set of collision partners for each point in velocity space analogous to the Direct Simulation Monte Carlo (DSMC) method. The present method has been applied to a traveling 1D shock wave. The jump conditions across the shock wave match the Rankine–Hugoniot jump conditions. The internal shock wave structure was compared to DSMC solutions, and good agreement was found for Mach numbers ranging from 1.2 to 10. Since a coarse velocity discretization is required for efficient calculation, the effects of different velocity grid resolutions are examined. Additionally, the new scheme’s performance is compared to DSMC and it was found that upstream of the shock wave the new scheme performed nearly an order of magnitude faster than DSMC for the same upstream noise. The noise levels are comparable for the same computational effort downstream of the shock wave.