High-Temperature Ignition of Hydrogen and Air at High Pressures Downstream of the Reflected Shock Wave

The delay and features of the ignition of a stoichiometric hydrogen–air mixture at temperatures of 830–1450 K and pressures of 2–21 atm downstream of the reflected shock wave have been investigated. The experimental data obtained were compared with the results of calculations done in accordance with the known kinetic schemes of hydrogen combustion. The influence of the interaction of the reflected shock wave with the dynamic boundary layer on the ignition and the time of induction in a shock tube has been examined.     

Shock Tube Measurements of the Equation of State of Argon

We have investigated the equation of state of argon at elevated temperatures and pressures using a new shock tube method. Temperatures in the range of 1280 to 1830 K, and pressures from 6 to 50 MPa were generated behind reflected shock waves in test gas mixtures of argon with trace amounts of CO and H₂ added. Density was determined from reflected shock pressure and incident shock speed measurements using the shock-jump relations. Temperature was determined from the modeling of the 4.7 sum infrared emission of the fundamental vibrational band of thermally-equilibrated CO. The experimentally determined argon P-p-T data points are in good agreement with the static-cell data of LeCocq and an extrapolation of the equation of state of Stewart and Jacobsen.     

Multiple shock wave reflections during an explosion in a closed volume with shielded walls

The impact of a shield made of granulated material on attenuation of a shock wave multiply reflected from walls is investigated during an explosion inside a closed volume. We performed experiments in a shock tube with a short high-pressure chamber. A low-pressure chamber was filled with air at atmospheric pressure. The flat blast wave with the reducing pressure profile was created. We discovered the dependence of the pressure reduction at a reflected wave front on a distance between the protecting shield and a closed end of the shock tube.     

Numerical simulation of the interaction between reflected shock wave and near-wall boundary layer

The interaction of a shock wave, reflected from the flat end of a cylindrical shock tube, with a cocurrent gas flow at the side wall, is considered in the axisymmetric case. The Navier-Stokes system of equations has been numerically integrated in the thin layer approximation using a predictor-corrector scheme. The appearance of a recirculation gas flow structure behind the reflected shock front and the evolution of this structure in the reflected wave going away from the tube end are analyzed. The corresponding patterns of constant density lines and the velocity vector field are presented.     

An axisymmetric electric discharge as a means for remote heating of gas and for ignition of combustible gas mixture

Experimental and theoretical investigations are performed of gasdynamic phenomena accompanying a ring electric discharge excited in air at atmospheric pressure. It is experimentally demonstrated that the discharge generates a toroidal (three-dimensional) shock wave which cumulatively converges toward the axis. A mathematical model is constructed, which describes the process of focusing of the toroidal shock wave. The results of measurements of shock-wave processes accompanying the discharge are in good agreement with the calculation results; this enables one to estimate the gas temperature which may be attained in the region of cumulative convergence at some distance from the ring center. Results are given of first experiments in the excitation of a ring shock wave in a combustible gas mixture (stoichiometric mixture of CH₄:O₂).     

Numerical simulation of a novel blast wave mitigation device

This paper proposes a novel blast wave mitigation device, consisting of a piston–cylinder assembly. A shock wave is induced inside the device when it is subject to a blast wave. The shock wave propagates inside the device and is reflected repeatedly. The physical processes within the blast wave mitigation device are simulated numerically. Numerical predictions are in excellent agreement with analytical solutions for special cases of the investigated problem that are available in the literature. The peak pressure on the base of the device caused by the blast wave is studied using a number of design parameters. The numerical simulation shows that, although the transmitted impulse remains practically unchanged, the peak pressure of the blast wave can be reduced by as much as 98%, or even higher, depending on the design parameters chosen.     

Wave curves for the riemann problem of plane waves in isotropic elastic solids

A modified Riemann problem in which the initial and boundary conditions are constants is considered for plane waves in a half space occupied by an elastic solid. The governing quasilinear differential equations form a system of hyperbolic conservation laws which possesses three wave speeds c₁ ≥ c₂ ≥ c₃. The system is genuinely nonlinear with respect to c₁ and c₃ and linearly degenerate with respect to c₂. Thus it is sufficient to study a two-wave-speed system with c₁ and c₃. Wave curves for simple waves and shock waves are used to construct the solution. Second-order hyperelastic materials which contain four material constants are considered and the solution in the form of wave curves is obtained for all possible combinations of initial and boundary conditions. With a proper nondimensionalization, the wave curves depend only on one material parameter k. The solutions are thermodynamically correct because entropy effects do not come into the picture until the third-order terms in stresses are included in the constitutive laws. The two-wave-speed system has one umbilic point at which c₁ = c₃ and hence the system is not totally hyperbolic (or not strictly hyperbolic). Several interesting and unexpected results are obtained due to the existence of the umbilic point. In one example, we find that a shock wave satisfies the Lax stability condition for a V₁ shock as well as a V₃, shock. In another, a shock wave which involves only one stress component does not satisfy the Lax stability condition for either a V₁ shock or a V₃ shock. However, it satisfies the Lax stability condition if we consider it under the context of a one-wave-speed system. Finally we consider the effects on the solution when the third-order terms are included. We show that although the entropy affects the shock wave solution, it does not appear in the simple wave solution until the fourth-order terms are included. With the third-order terms, there may be as many as three umbilic points, one of which may be an umbilic line.     

Analytical and experimental evaluation of energies during shock wave loading

Analytical expressions were developed using gas dynamic equations to evaluate the energy associated with the incident shock wave (incident energy) and the reflected shock wave (remaining energy) for a shock tube experiment. The real time deformation profile of the specimen being loaded by a shock tube was also used to calculate the deformation energy. A shock wave loading on a homogeneous aluminum panel in a simple shock tube experiment was used to illustrate the implementation of these methods.     

The effect of MHD interactions on the input shock waves in a supersonic diffuser

The interaction with an external magnetic field modifies the variation of the shock wave configuration in a pure inert gas plasma at the entrance of a supersonic diffuser. The phenomenon was studied using an experimental setup based on a shock tube with a flat nozzle and the model supersonic diffuser. The experiments were conducted in krypton, for the shock wave Mach number in the shock tube M=7.8 and the Mach number at the nozzle exit M=4.2. The gasodynamic discontinuities and their structural variations induced by the magnetic induction changes were by visualized by the schlieren method and by photography of the intrinsic emission accompanying the process. Three regions of the MHD interaction affecting the shock wave configuration in the gas flow were revealed.     

Correlations study in shock wave/turbulent boundary layer interaction

The shock waves/turbulent boundary layer interaction is a problem of critical importance that is frequently encountered in designing flying vehicles. Presently, the most topical issue is the investigation of nonstationary phenomena (in particular, low-frequency effects) involved in this interaction. We have experimentally studied separated flows in the zone of interaction between an obliquely incident shock wave and a turbulent boundary layer at a Mach number of M = 2. Correlation data in the separation zone and the upstream flow were obtained. It is established that low-frequency oscillations in the reflected shock are related to the pulsation in the inflow boundary layer.     

Experimental study of λ-phase transition induced by shock compression

The λ-phase transition induced by shock compression was experimentally investigated by using the superfluid shock tube facility. A compression shock wave is generated in He II by the impingement of a gas dynamic shock wave onto a He II free surface. The shock-compressed He II can be converted to He I crossing the λ-line in the case of initial He II temperature close to T_λ. The resultant temperature variation has been measured with in-house-made superconductive temperature sensors. Furthermore, the visualization photographs were taken by a digital still camera with the Schlieren optical method. It is found that the magnitude of the entropy production Δs induced by shock compression process highly depends on the temperature difference between the temperature in the shock compressed state and the lambda temperature as well as on the shock strength. In particular, Δs becomes extremely large in the case where the λ-phase transition is induced.     

Traveling wave solution of heat transfer for small sample moving in the sub-cooled liquid nitrogen

The temperature field of a traveling wave in a system is analyzed. The system consists of a rapidly moving small specimen tube and sub-cooled liquid nitrogen (SLN₂). The temporal and spatial temperature distribution of the subsonic wave is investigated. The relationship between a dimensionless cooling rate and the thermal Mach number (M) is derived. When the quenching speed of sample tube in the sub-cooled liquid nitrogen is equal to or faster than that of the thermal wave propagated, a shock wave is expected.     

气波机振荡管内削弱反射激波的研究进展

反射激波的存在是制约制冷效率的一个重要因素，因此，消除反射激波成为气波制冷技术研究的一个重要课题。本文分析了振荡管内激波的行为及反射激波对制冷效应的影响。对几种消除反射激波技术进行了评述及对比，分析了其效果及优缺点。最后，对消波技术的研究方向提出了建议。     

Pulsed 5-GW resonance relativistic BWT for a decimeter wavelength range

The results of numerical modeling and experimental investigation of a high-power, resonance relativistic backward wave tube are presented. By using the working TM₀₁ mode reflections from the ends of the electrodynamic system, optimum conditions for the electron beam interaction with both the (−1)st harmonic of the backward electromagnetic wave and the main harmonic of the concurrent wave are achieved. A single mode generation with 5 GW output power and a 30% efficiency was obtained in experiments at a frequency of 3.6 GHz. The working frequency can be controlled within 15% (at the half maximum power level) by changing the slow-wave structure period at the constant electron beam parameters.     

Interpreting experimental data on the electric explosion of thin wires in air

Experimental data on the electric explosion of thin wires in air at a current of up to 10 kA growing at a rate of up to 50 A/ns are reported. The possible mechanisms of shock wave generation are analyzed, and differences in the development of secondary breakdown during the explosion of copper and tungsten wires in air are considered. The intervals of the parameters of the medium observed during the explosion-induced shock wave propagation are presented and compared to the estimates based on a model of the shock wave front structure.     

The formation of vortex shocks in a 3D subsonic flow behind a weak shock wave emerging from a channel

The structure of a 3D subsonic flow behind a diffracted shock wave was studied by experimental and numerical methods for the incident shock wave Mach numbers M ₀ close to unity. It is established that vortex shocks appear in the flow behind the diffracted shock wave even when M ₀ decreases to 1.04, which is much lower than the threshold Mach number obtained analytically for a 2D automodel case. The time interval from the outflow start to the local supersonic zone formation, as well as the experimentally measured time of appearance of the first vortex shock, increase with decreasing M ₀.     

Calculation of heat flux from the region behind a reflected shock wave into the shock tube edge

We have numerically calculated the process of conjugated unsteady heat exchange during the formation of a boundary transient temperature layer as a result of the interaction of a shock wave with a flat edge surface of a shock tube. The results of calculations can be used in the analysis of the experimental data. The time variation of the heat flux and the temperature at the gas-solid interface is determined. The heat flux distribution profiles are obtained for two limiting variants of the boundary conditions on the outer surface of the shock tube.     

Reflection and transmission of nonlinear waves from arterial branching

In this work, treating the arteries as a prestressed thin walled elastic tube and the blood as an inviscid fluid, we have studied the reflection and transmission of nonlinear waves from arterial branching, through the use of reductive perturbation method. The reflected and the transmitted waves at the bifurcation point are calculated in terms of the incident wave. The numerical results indicate that the reflected wave is comparatively small whereas the transmitted waves in branches are comparable with the incident wave. This result is quite consistent with the experimental measurements [N. Sergiopulos, M. Spiridon, F. Pythoud, J.J. Meister, On wave transmission and reflection properties of stenosis, J. Biomech. 26 (1996) 31-38].     

激波能量的改变对制冷效率及管内波系的影响研究

建立了一套实验装置,系统地考察了气波机接受管端部加入蓄冷填料,由反射激波强度削弱所引起的制冷效率的影响程度.理论上初步建立了管内非定常流动数学模型,对接受管内激波的流动进行了数值模拟,计算结果与实验测量值趋势相同,可为气波制冷机的改造、设计及优化提供理论上的指导.     

Second harmonic generation in the strong absorption regime

Abstract

A detailed theoretical analysis of the second harmonic generation in the strong absorption regime (great absorption index K _2w) is presented. Inside the nonlinear medium, it is found that only the harmonic wave travelling in the same direction as the fundamental one survives. Outside the nonlinear medium, the transmitted and reflected intensities of the harmonic waves show a peculiar dependence on k _2w- The ratio between transmitted and reflected intensities ranges from 5 to 75. Reflected and transmitted harmonic waves appear to be generated in the outer layers within a thickness of a fraction of wavelength. Experimental support is reported with application to a proton-exchanged LiNbO₃ waveguide.