Flow behind magnetogasdynamic exponential shock wave in self-gravitating gas

The propagation of a cylindrical (or spherical) shock wave driven out by a piston moving with time according to an exponential law, in a self-gravitating ideal gas with azimuthal magnetic field is investigated. The initial magnetic field is assumed to be varying according to an exponential law. Solutions are obtained for both the cases of isothermal and adiabatic flows. The effects of variation of ambient magnetic field, gravitational parameter and adiabatic exponent are worked out in detail. It is manifested that the increase in strength of ambient magnetic field has decaying effect on the shock wave however increase in the value of gravitational parameter has reverse effect on the shock strength. The compressibility of the medium is increased in the presence of gravitational field. Also, a comparison between the solutions obtained in the case of isothermal and adiabatic flows is done. Density, pressure, velocity and magnetic field increases, however mass decreases as we move inward from the shock front towards the piston.     

Propagation of a shock wave across a current sheet without reflection

A class of exact solutions is obtained for the magnetohydrodynamic equations with plane waves which describe the solid-body motion of an ideally conducting gas in a given uniform gravitational field. The gas motion is initiated by the impact of a piston which generates a shock wave propagating through the initial equilibrium state with a decreasing density. Propagation of the shock wave across a current sheet is considered within the framework of the results obtained.     

Integral inequalities in the dynamics of a gravitating gas

In the framework of Newtonian mechanics, a study is made of the spherically symmetric problem of the adiabatic motion of a gravitating perfect gas in the presence of a shock wave produced by inhomogeneous gravitational collapse or a point explosion. The method of Golubyatnikov [1, 2] is used to construct a system of integro—differential inequalities that determine, in particular, the law of motion of the shock wave if the initial state of the gas is known. The investigated examples include some self-similar and nearly self-similar solutions to the problem of the gravitational contraction of dust with the formation of a strong shock wave, possibly with the release of energy; the self-similar problem of a point explosion in a gas at rest; and also the problem of the equilibrium of a gas sphere for =4/3 and arbitrary distribution of the entropy. In these cases, the inequalities reduce to algebraic relations and can be solved numerically.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 169–173, September–October, 1980.     

Thermo-acceleration waves and shock formation in Extended Thermodynamics of gravitational gases

The possibility of shock formation as degeneration of acceleration waves in a thermoviscous gravitational ideal gas is studied by exploiting the hyperbolic system of Extended Thermodynamics. The mathematical aspects of this problem are discussed by considering the different contributions of gravity and dissipative effects. In particular, we evaluate the critical time (i.e. the instant in which a shock wave starts) proving that it exists, in the usual physical situations, only for a sufficiently large critical initial amplitude of the acceleration jump. We show that an acceleration wave can never degenerate into a shock wave except in some limiting cases and so, since gravity force is overcome by dissipative effects, our results do not differ, qualitatively, from the case without gravity: this result implies the asymptotic stability (in the sense of [1]) of the static isothermal solutions.     

一种弹体入水冲击波阴影成像可视化系统及其试验研究

扩大成像视野对于开展充水容器中弹体入水冲击波传播及弥散方面的可视化研究具有重要的实际意义。阴影成像技术适用于大视野实验,且对流场冲击波和扰动的可视化研究具有简单性和通用性,其中直接阴影成像最为简单,但可靠点光源的缺乏是阻碍其发展应用的瓶颈。因此基于国产短弧氙灯管,自制了短弧氙灯点光源,根据阴影成像原理,设计出一种弹体入水冲击波阴影成像可视化系统,详细介绍了其组成和运行原理。利用该系统对高速弹体入水进行了试验研究,获得了弹体入水冲击波的阴影成像和冲击波信号的压力时程曲线,通过阴影成像和冲击波信号相结合分析了弹体入水冲击波的传播特性,并进行了理论验证。结果表明:该弹体入水冲击波阴影成像可视化系统具有可靠性和设计的合理性。弹体高速入水后,初始冲击波的强度最大,随着冲击波的传播,冲击波强度逐渐降低,水中冲击波的传播速度不断降低,球形冲击波的半径逐渐增大。     

Radiation hydrodynamics of supernova shock breakouts

The first powerful burst of photon radiation in a supernova appears when the shock front is a few photon mean-free paths below the star photosphere. This is called “shock breakout” and it is the first observable event after the neutrino and gravitational wave bursts in core-collapsing supernovae. Any early information about collapse is vitally important for understanding the physics of explosion, constraining speed of neutrino propagation etc. Direct observations of shock breakouts have been carried out in a few supernovae. We discuss some puzzles related to those objects. Finally, we describe our current understanding of the most luminous (hyper-)supernovae. Their long living radiative shocks pause many interesting problems in numerical and laboratory astrophysics and may have important applications in cosmology.     

大断面隧道钻爆冲击波的衰减规律

隧道开挖爆破产生的空气冲击波的破坏效应,将会对人员、机具设备与周围环境造成危害。隧道钻孔爆破冲击波的影响因素比裸露药包爆炸更多、更复杂,研究其衰减规律对采取合适的防护措施意义重大。本文中开展了时速350 km双线铁路大断面隧道钻孔爆破空气冲击波的现场测试,分析了不同工况下冲击波传播规律及影响因素。结果表明:钻爆冲击波超压时程曲线存在多个不同幅值的超压波峰,波峰之间具有明显微差延时的短间隔性,传播至远场未形成稳定的单一平面波,与单一药包爆炸冲击波的传播规律存在差异;钻爆冲击波超压信号由多段与微差延时相对应的子信号叠加而成,子信号数量与毫秒延期雷管段数相同,呈现出典型的时域特征;相同爆破条件下,大断面隧道钻爆时的乳化炸药冲击波转化因数小于小断面巷道工况下的;相较于总药量及最大段药量,按掏槽药量计算的超压峰值与实测超压峰值之间的相关性最强,钻爆冲击波最大超压峰值宜按掏槽段炸药TNT当量确定;隧道内大型机械设备等障碍物改变了钻爆冲击波流场的传播规律,呈现较明显的叠加放大效应。     

炸药水中爆炸冲击因子的新型计算方法

针对水中爆炸冲击因子在近场范围内的一些不足,利用水中爆炸冲击波的最大峰值压力与正相冲量的乘积推导了冲击因子的表达式。通过水中爆炸实验,得到了几种典型炸药的冲击波参数及其相似方程。利用该公式计算了水中爆炸冲击因子及其装药指数,并与基于平面波的水下爆炸冲击因子进行了对比。结果表明:冲击因子中的装药指数n＝0.5不仅适合所有以TNT为基本组分的炸药,也适合于RDX、HMX基的炸药。在修正冲击波形状的基础上,由峰值压力与冲量的乘积推导的冲击因子计算公式,从冲击波的毁伤作用的角度表述了水中爆炸冲击因子的物理意义,在计算近场冲击因子时具有更高的准确性。     

Numerical simulation of expansion/compression effect on shock induced turbulent flow

The interaction of homogeneous and isotropic turbulence with a shock wave is observed by solving the Reynolds-averaged Navier–Stokes equations with the k–? turbulence model. All turbulent fluctuations are measured at the period of expansion in the turbulent field and during compression by the reflected shock on turbulent field, and it is observed that the longitudinal turbulent velocity fluctuation is enhanced more at the period of expansion due to incident shock wave movement far from the turbulent field. The amplification of the turbulent kinetic energy (TKE) level in the shock/turbulence interaction depends on the shock wave strength and the longitudinal velocity difference across the shock wave. On decreasing the longitudinal velocity difference across the shock, the turbulent kinetic energy (TKE) level is less amplified. The TKE level is amplified by the factor of 1.5–1.8 in the shock/turbulence interaction where the dissipation rate of TKE decreases in all cases of shock/turbulence interaction. After the shock/turbulence interaction, the turbulent dissipative-length scale is amplified slightly and the amplification of the length scales decreases when increasing the shock strength. To cite this article: M.A. Jinnah, K. Takayama, C. R. Mecanique 333 (2005).     

爆轰波与激波对撞的实验研究

对乙炔氧气混合气体中爆轰波与激波的正面对撞现象的实验研究是以高速摄影获取两波对撞的x-t纹影图,以烟迹板记录对撞中的爆轰胞格图案,并基于激波理论和经典CJ爆轰理论求解了两波对撞的稳态解并探寻其规律. 研究发现透射波系包括一道激波和爆轰波,以及紧随爆轰波后的稀疏波区,这一结果对应于一维理论分析中的CJ解. 透射波系基本不受初始压强影响;初始温度也只成比例地改变流场整体速度,温度越高,速度越快;对波系起实质影响作用的是入射激波强度,激波越强,则整个透射流场呈现偏向激波的趋势;理论分析还指出,稀疏波区的出现不可避免,当激波强度趋于声波稀疏波区趋于消失,激波越强则疏波区趋于扩大. 两波对撞存在一个有限的转变阶段,透射爆轰首先减缓,接着迅速迸发为过驱爆轰,然后再逐渐平衡为CJ爆轰. 对于强不稳定的燃气,对撞后爆轰波在空间上的发展极不均衡,一些区域发生火焰面与诱导激波的严重脱离,随后的火焰面失稳发展为诱导激波区内的爆轰波,实验观察到了这种爆轰在烟迹板上留下的极为精细的迹线.      

装药动爆冲击波特性研究

导弹、炮弹等战斗部爆炸时具有一定的速度,较大的运动速度会使爆炸冲击波场分布发生变化,进而对弹药的毁伤威力产生影响。本文中采用AUTODYN软件对速度分别为0、272、340、680、1 020和1 700 m/s的TNT球形裸装药在空气中爆炸的冲击波场进行了仿真计算,定量研究装药在动爆条件下的峰值超压、比冲量和正压作用时间等威力参数特性。结果表明,方位角小于90°时装药速度与冲击波超压、比冲量成正相关,与正压作用时间成负相关;方位角大于90°时装药速度与冲击波超压、比冲量成负相关,与正压作用时间成正相关。超压峰值大小沿方位角成正弦变化。最后,分析了冲击波峰值超压数据,建立了动爆冲击波超压的计算模型,该模型计算结果与仿真和实验结果吻合较好。     

The plane piston problem in a weak gravitational field

We analyze a gasdynamical process in the stellar atmosphere that is driven by a piston moving with constant velocity in a weak gravitational field. Ahead of the piston, the gas is compressed, and this compressed gas uses part of its internal energy and somewhere its kinetic energy to overcome the applied gravity.If we expand the quantities as a series of a small parameter, which is the ratio of a typical escape velocity to the plasma velocity, the basic state gives a uniform flow, as shown by the case of gasdynamical theory without gravity. The first-order relationships show the influence of the applied gravity on the flow fields, that is, the strength of the shock wave changes slightly, the internal energy of the gas exhausts. For the cases of strong shock wave and near the piston, an analytical solution may be approximately obtained and has the similar features.Because of the importance of the applied gravity in the astrophysical and atmospheric physical processes, these results may shed light on the mechanics of transient process in the stellar and planetary atmosphere.     

激波喷出过程和在林带中的传播

运用激波管技术研究了激波从管口喷出和在林带中传播这两个基本问题。给出了三方面的结果 :揭示了激波从管口喷出时的复杂流场 ,显示了主激波弯曲和衰减 ,二次激波的形成和合拢 ,涡环的长大和发展 ,以及涡环前缘出现正激波等基本物理现象 ;显示了激波在林带中的传播过程 ,取得了激波遇林地的反射 ,马赫杆形成 ,激波与林冠的相遇 ,林冠对涡环的阻滞效应等纹影照片 ;沿激波在林地的传播方向测量了地面压力 ,证实林带有明显的消波效应。     

The influence of magnetic field upon the collapse of a cylindrical shock wave

In the present paper, the problem of propagation of collapsing cylindrical shock wave in an ideal gas permeated by a transverse magnetic field with infinite electrical conductivity is investigated. Here it is assumed that the medium ahead of the shock front is uniform and at rest. Also, its counter pressure concerning the motion of the wave front is neglected. This problem admits a self similar solution of second kind. The similarity exponent has been computed by solving a nonlinear eigenvalue problem and integrating numerically the self-similar equations for various values of adiabatic heat exponent and Cowling number. Numerical computations have been performed to determine the flow field behind the shock wave. The influence of magnetic field strength and adiabatic heat exponent on the flow parameters for various cases is presented.     

燃气射流起始冲击波场形态的实验研究

介绍利用激光摩尔偏折仪和压阻式压力传感器对发动机喷流起始冲击波的基本形态进行的研究． 研究结果验证了在燃气流团前存在着起始冲击波及发展初期具有非球面的特性,并提出了喷口起始冲击波波阵面发展的新结构,对前人有关起始冲击波发展结构的研究结果进行了修正．     

TNT药柱水中爆炸近场压力轴向衰减规律

通过TNT药柱水中爆炸实验,建立了利用高速相机测量水中爆炸近场冲击波的实验方法,得到了药柱轴向的扫描图像,并对图像进行了数字化分析;利用Rankine-Hugoniot关系从冲击波的扫描轨迹求得冲击波阵面压力,并外推至冲击波的初始压力;用LS-DYNA计算了柱形装药近场压力沿轴向的衰减曲线;用锰铜压力传感器测量了冲击波的初始压力。研究结果表明,测试结果与计算结果比较符合,TNT药柱的近场轴向压力遵循指数衰减规律。     

Stability of a radiation-absorbing ionizing shock wave in an electromagnetic field

The linear stability of a radiation-absorbing ionizing shock wave (light detonation waves) in the presence of a uniform electromagnetic field is investigated. The applied electric field is considered to be normal to the wave front and the magnetic field to be parallel to the front and perpendicular to the plane in which perturbations propagate. The medium satisfies a two-parameter equation of state. Analytic stability criteria are obtained. For a perfect gas the effect of the electromagnetic field and radiation on shock wave stability is determined.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 23–30, January–February, 1996.     

柱形装药条件下锥形水中爆炸激波管内的冲击波特性

锥形水中爆炸激波管是进行水中爆炸实验的一种装置,该装置能够通过较小装药量在相同距离处实现自由场水中较大装药量爆炸的冲击波峰值。为了获得柱形装药条件下锥形水中爆炸激波管内的冲击波特性,本文通过数值计算的方式,对不同圆锥角和不同柱形装药质量下锥形激波管内的冲击波传播过程进行了模拟,通过对不同工况下激波管内冲击波特性进行分析,发现其初始冲击波的衰减规律符合自由场水中的指数衰减形式,并拟合得到了与自由场水中爆炸相容的冲击波峰值、比冲量和能流密度经验公式;发现其二次脉动压力周期与炸药质量呈反常规的变化规律,并引入等效静水压深度解释了这一现象;发现其二次脉动压力幅值与初始冲击波幅值之比比自由场水中更大,而二次脉动压力的比冲量与初始冲击波冲量之比与自由场水中相当。     

Three-dimensional magnetohydrodynamic description of the process of collision of a solar wind shock wave and the Earth’s bow shock

The propagation of a solar wind shock wave along the surface of the Earth’s bow shock is investigated within the framework of an ideal magnetohydrodynamic model in the three-dimensional non-plane-polarized formulation. The most characteristic values of the solar wind parameters and the interplanetary magnetic field strength are considered for the plane front of a solar wind shock wave moving at various velocities along the Sun-Earth radius. The global three-dimensional pattern of the interaction is constructed as a function of the angle of inclination of the surface of the bow shock to the solar wind velocity and the azimuthal angle along the curve of intersection of the fronts of the interacting shock waves. The evolution of the flow developed in the neighborhood of the bow shock is investigated and the parameters of the medium and magnetic field are calculated.     

流场非均匀性对非平面激波诱导的Richtmyer-Meshkov不稳定性影响的数值研究

基于Navier-Stokes方程组,采用可压缩多介质黏性流动和湍流大涡模拟程序MVFT (multi-viscousflow and turbulence),模拟了均匀流场与初始密度呈现高斯函数分布的非均匀流场中马赫数为1.25的非平面激波加载初始扰动air/SF6界面的Richtmyer-Meshkov (RM)不稳定性现象。数值模拟结果表明,初始流场非均匀性将会影响非平面激波诱导的RM不稳定性演化过程。反射激波加载前,非平面激波导致的界面扰动振幅随着流场非均匀性增强而增大;反射激波加载后,非均匀流场与均匀流场条件下的界面扰动振幅差异有所减小。进一步,定量分析流场中环量分布及脉动速度统计量揭示了前述规律的原因。此外,还与平面激波诱导的RM不稳定性进行了简单对比,发现由于非平面激波波阵面区域的涡量与激波冲击界面时产生的涡量的共同作用,使得非平面激波与平面激波诱导的界面失稳过程存在差异。