Strong Shock Wave Front Research Articles

Technique for Recording the Intensity of the Emission of Gases behind the Front of Strong Shock Waves in the Region of Vacuum Ultraviolet Radiation

Technique for Recording the Intensity of the Emission of Gases behind the Front of Strong Shock Waves in the Region of Vacuum Ultraviolet Radiation

Development of a Technique for Recording the Intensity of the Emission of Gases behind the Front of Strong Shock Waves

Development of a Technique for Recording the Intensity of the Emission of Gases behind the Front of Strong Shock Waves

Numerical investigation of shock wave characteristics at microsecond underwater electrical explosion of Cu wires

Strong shock wave (SW) can be generated during underwater electrical wire explosion as the exploding wire rapidly expands and pushes the surrounding water. In this paper, a coupled model that describes the behaviours of the circuit, the exploding wire, and the evolution of SW was established. Wide range equation of state data and conductivity data were used, making the calculation from solid state possible. The calculated discharge current, voltage, and trajectories of the wire radius and SW front were generally in good agreement with the experimental results, manifesting the validity of the model. Evolution of SW peak pressure, the process of energy conversion, and the effect of the discharge period were studied using the model. Simulation results showed that the radial attenuation of SW peak pressure is largely dependent on wire diameter and energy deposition process, while the efficiency of converting the deposited electrical energy to the mechanical energy of SW is not as sensitive, varying from 40% to 50% tens of microsecond after the current starts. Fast discharge tends to generate SW with higher initial SW peak pressure, while slow discharge enables higher initial energy storage within the limit of insulation and generates SW with slower radial attenuation. The presented model and numerical results could serve as a reference for parameter selection in related applications.

Shock waves dynamics and evolution of vortex formation during the interaction of spherical air pulse with aqueous foam layer

The numerical study of the powerful air spherical shock-wave pulse interaction with the protective aqueous foam barrier with the initial liquid volume fraction of 0.2 is carried out. The foam layer thickness is selected to satisfy the condition of the non-reflection of compression wave from the foam external boundary at the considered time intervals. In studying the wave flows dynamics, we used the assumption of the foam structure destruction into the microdrops suspension behind the strong shock wave front. The two-phase medium is described on the basis of the gas-droplet mixture model, which includes the laws of conservation of mass, momentum and energy for each phase in accordance with the single-pressure, two-speed, two-temperature approximations in a two-dimensional axisymmetric formulation. The Schiller–Naumann model is used for taking into account the interfacial drag forces. The contact heat transfer influence at the interface between the phases is taken into account by the Ranz–Marshall model. To describe the properties of air and water, the Peng–Robinson and perfect fluid equations of state are used. The numerical implementation of the model is carried out using the OpenFOAM open-source software with the two-step PIMPLE algorithm. The numerical study results are presented as spatial distributions of pressure fields, velocities and streamlines. The significant attenuation of the spherical shock wave intensity during its interaction with the aqueous foam layer has been established. The causes and dynamics of the toroidal vortices series formation in the gas region behind the shock front are investigated. The results reliability is confirmed by comparison with the solutions of the similar problem, found by another numerical method.

Calculation of Nonequilibrium Radiation of Shock Waves in the Air Using Two Models

The results of the numerical modeling of the spectral radiating capacity of shock-wave-heated air are presented. Two models of nonequilibrium radiation emitted from the relaxation zone behind the strong shock wave front are used. The first model is based on Euler equations and formulated for the one-dimensional problem of kinetic processes behind the shock front. The second model is based on the two-dimensional Navier-Stokes equations for a viscous shock layer on the nose surface of a blunt body in a hypersonic flow. The problem of the interpretation of the Fire-II flight experiment are discussed using the above-mentioned models.

Primary cause and mechanisms of structural instability of strong shock waves in gases

This paper is aimed at finding the primary cause and mechanisms of flow structure rearrangement (stable/unstable) behind the front of strong shock waves (SW) in gases. Results of experimental and calculated investigations of strong shock waves in atomic gases obtained by the authors of this work and other researchers for the last several decades are analyzed. A unique analytical approach, referred to by the authors as the source of electrons method, is used, which makes it possible to determine the relationship between the kinetic processes of ionization of particles and the energy exchange in a non-equilibrium flow (avalanche ionization region) behind a shock wave in argon in a wide range of flow conditions: M = 10–30, pressure ahead of the shock wave front p1 = 1–25 torr. In addition to the basic processes, the following additional mechanisms are considered: (a) associative ionization in interatomic collisions with participation of highly excited atoms, (b) integral radiation losses, (c) convective energy transfer, and (d) electrons heating in the polarization electric field. It is found that the primary cause of flow structure instability in shock waves is the presence of flow boundaries responsible for (i) weak gas-dynamic perturbations and (ii) local (near-wall) inhomogeneity of the medium ahead of the shock wave front and behind it. The mechanism of structural instability of the shock wave in atomic gases at low Mach numbers is caused by a cardinal change in the relationship of the basic and additional kinetic and energy exchange processes between the ionized flow components behind the front, even in the case of a weak perturbation of the atomic–ionic component temperature. In the range of high Mach numbers, the instability mechanism is due to the thermal effect induced by wall and near-wall gas layers heating ahead of the SW front by precursor radiation and also due to possible energy release initiated by polarization of the electric field near the wall behind the shock wave front.

Radiation Characteristics of Air in the Ultraviolet and Vacuum Ultraviolet Regions of the Spectrum behind the Front of Strong Shock Waves

Experiments on the measurement of air emission intensity behind the front of incident shock wave were carried out in a shock tube at an initial pressure of 0.25 Torr and shock wave velocities of 6.3–8.4 km/s. The emission intensity was measured in absolute units both in the form of an integral spectral distribution in a wavelength range of 120−400 nm (panoramic spectra) and as the time evolution of emission at the individual atomic lines of nitrogen and oxygen atoms. The results of the measurements demonstrated that the emission in air behind a shock wave in the vacuum ultraviolet region of 120–200 nm had a much higher radiation flux level than the emission in a range of 200–900 nm.

Molecular dynamics simulations of microscopic structure of ultra strong shock waves in dense helium

Hydrodynamic properties and structure of strong shock waves in classical dense helium are simulated using non-equilibrium molecular dynamics methods. The shock speed in the simulation reaches 100 km/s and the Mach number is over 250, which are close to the parameters of shock waves in the implosion process of inertial confinement fusion. The simulations show that the high-Mach-number shock waves in dense media have notable differences from weak shock waves or those in dilute gases. These results will provide useful information on the implosion process, especially the structure of strong shock wave front, which remains an open question in hydrodynamic simulations.

Calculation of elastic modules behind strong shock wave

As part of a unified system of the few-parameter equation of state, an approach is implemented to calculate the mechanical properties of materials behind the front of strong shock waves. For aluminum and copper, the results of calculations are compared with the experimental data available for high energy densities; a close match is observed.

Modeling of vibration-dissociation oxygen kinetics at temperatures of 4,000-11,000 K

The time profiles of vibrational molecular oxygen temperature Tv measured earlier in experiments behind a strong shock wave were used for testing the theoretical and empirical models of thermal nonequilibrium dissociation of molecules. To do this, dissociating gas flows behind the strong shock wave front were calculated with account for these models. If the initial gas temperature behind the wave front T0 7×103 K neither of the models tested describes the measured temperature profiles satisfactorily. Using the empirical model proposed in the present study made it possible to satisfactorily describe the vibrational temperature evolution observed in experiments at temperatures up to 11×103 K.

Measurement of the radiation intensity beyond the front of strong shock waves for a methane-nitrogen gas mixture

Measurement of the radiation intensity beyond the front of strong shock waves for a methane-nitrogen gas mixture

Comprehensive analysis of the effect of atomic and molecular metastable state excitation on air plasma composition behind strong shock waves

A detailed analysis of the processes of charged and neutral species formation in N2–O2–Ar system behind strong shock waves is conducted on the basis of an extended thermally non-equilibrium kinetic model with careful allowance of reactions with electronically excited atoms and molecules and vibration–electron–chemistry coupling. The model is validated against experimental data such as measurements of electron number density behind the strong shock wave front, temporal profile of O2 vibrational temperature, temporal evolution of NO(γ) radiation intensity in the post-shock region and values of time instants at which maximum quantities of radiation intensity of N2(1+) and bands are achieved. It is demonstrated that the model without the allowance of processes involving electronically excited atoms and molecules overestimates the concentrations of both charged and neutral species. For electron and NO concentrations the overestimation can come up to a factor of 2. The computations also indicate the existence of a strong vibration–electron–chemistry coupling in the air plasma produced by the strong shock wave. Only the model, which takes into account such an interaction, makes it possible to properly predict the variation of gas dynamic parameters and species concentrations in the post-shock region.

Efficiency studies of particle removal with pulsed-laser induced plasma

The detachment and removal of micro- and nanoparticles from surfaces is of importance in many industries. The cleaning of silicon wafers is of great interest in the semiconductor, microelectronics, and optics industries. The development and adoption of dry, rapid, non-contact, and non-damaging particle removal technologies is a critical process. The proposed laser-induced plasma (LIP) removal technique is a novel method for detaching and removing fine particles from substrates. The current technique is a dry, non-contact method that takes advantage of the strong shock wavefront from expanding plasma, created by focusing a laser pulse in air above the substrate. The transient pressure field acts on the target particles to produce a rotational moment and a rolling mode of detachment from the substrate. In the current study, the LIP removal technique is employed repeatedly to remove particles over an area of a silicon wafer, and a systematic efficiency study for the removal effectiveness is performed. A Q-switched Nd:YAG pulsed laser operating at 1064 nm with a 370 mJ pulse energy and 5 ns pulse length is used in experiments. 0.99 μm diameter silica spheres and 3.063 μm diameter polystyrene spheres were successfully detached and removed with no substrate damage. The removal efficiency at various gap distances between plasma core and substrate is determined and reported. This work is the first laboratory demonstration of the LIP technique over an extended area. The reported results substantiate the LIP removal technique as a serious option for particle detachment and removal from extended areas.

Investigation of the fractal characteristics of the radiation behind a strong shock-wave front in an instability regime

Investigation of the fractal characteristics of the radiation behind a strong shock-wave front in an instability regime

Plastic deformation behind strong shock waves

The dislocation produced plastic deformation that must occur behind the front of strong shock waves is analysed in this paper. (A strong shock has a driving stress that is large compared with the bulk elastic modulus). Because the front of strong shock waves must be exceedingly narrow, there is a problem of how the effective shear stress, which is probably of the order of the theoretical shear strength at the immediate front itself, is relaxed in the region behind the shock front. Our analysis (Weertman and Follansbee, 1983)) of moderate strength shock waves is used for the relaxing region behind a strong shock wave front. It is concluded that a strong shock wave front has the following dislocation structure: The shock front, of atomic dimensions, consists of a Smith dislocation interface of dislocations that keep up with the front by moving at transonic or supersonic velocities. Immediately behind the Smith interface is a region of moving dislocations that do not keep up with the shock front. In this region normal dislocation motion and multiplication takes place. Within this region of normal plastic deformation the shock pressure rises by an amount that is quite small compared with the shock pressure itself.

The initiation process of heterogeneous detonation waves (Kerosene spray and film)

Experiments were performed on the initiation of a heterogeneous detonation wave propagating through mixtures of liquid fuel and a gaseous oxydizer. The liquid fuel tested was kerosene in the forms of a spray and a film attached on tube walls. The ignition was performed by oxyhydrogen detonation. The effects of the content of oxygen in the oxydizer were investigated. It was found that for pure oxygen as an oxydizer, detonation waves were established by the combustion of kerosene, but for an oxydizer with a small amount of oxygen, only low Mach number shock waves with combustion waves following after were formed. For the former case, the ignition was initiated by the strong shock wave front, but for the latter case, the liquid phase seemed to be ignited by the combustion products of the initiator gas.

Estimation of the width of the front of strong shock waves in gases

Estimation of the width of the front of strong shock waves in gases