Strong Shock Waves In Air Research Articles

Shock Wave Generated Megahertz Radiation of Atmosphere

Based on intuitive reasoning and a simple mathematical model, it is shown that the radio frequency electromagnetic radiation in 1-2-MHz range is generated during the propagation of a directed strong shock wave in air. It is due to the radiation from free electrons (which are produced by the shock wave) in the magnetic field of the earth. The possibility of detection of this radiation is discussed.

Absorption Spectroscopy for Temperature Measurement Behind Shock Waves at Super-Orbital Velocity

The distribution of the translational temperature of atomic oxygen behind the strong shock wave in air produced in a shock tube was measured by means of an unique absorption spectroscopic technique. The translational temperature was determined by fitting a Voigt profile to the measured absorption spectral line shape

Nonequilibrium Phenomena Behind Strong Shock Waves Generated in Superorbital Reentry Flight

The aerothermodynamic behavior behind the strong shock wave in air that is associated with a superorbital reentry flight was investigated experimentally. For the experiment, a free-piston-driven shock tube was used, and radiation from behind the shock front was investigated spectroscopically. Based on the spectroscopic measurement, the spatial distributions of the rotational/vibrational temperatures of molecules (N 2 and N + 2), the electron excitation temperature of atomic nitrogen N, and the electron density were determined in the region behind the shock front. These results were reviewed by comparison with those previously determined for a nitrogen gas

Nanoparticle removal from trenches and pinholes with pulsed-laser induced plasma and shock waves

Removing nanometer scale particles from patterned substrates is a critical, yet challenging, technical task with numerous applications. Due to environmental concerns, there is a drive to reduce chemical usage and waste in semiconductor and other nano-technology industries. The laser plasma method is a novel removal method for nanoparticles. For the work reported in this study, the method is applied to substrates with features, i.e. trenches and pinholes. The technique, which is dry and non-contact, takes advantage of shock wavefronts initiated by plasma formation under a focused laser beam pulse and their interaction with the substrate. In the reported experiments, a Q-switched Nd : YAG pulsed laser is employed as a plasma and shockwave generation source. Various mechanisms are responsible for the removal effect. The strong shock wave in air generates complex pressure wavefields resulting in both drag and lift on the particle and acceleration of the substrate. However, shock waves are not transmitted into the solid substrate as discontinuous shock fronts due to a large difference between the relevant wave phase speeds in the two media. Also, damage concerns due to cavitation, which is a common effect in megasonic cleaning, are avoided. However, damage due to the high temperatures associated with the plasma formation is found to be an issue. Patterned silicon wafers and micrometer level pinholes were used for the particle removal experiments. The effects of the distance between the surface and the plasma boundary on the removal efficiency are reported. With this method, we were able to remove particles from the wall of a micrometer level pinhole and patterned silicon wafers.

Experimental and numerical studies of rotational relaxation behind a strong shock wave in air

This paper describes the experimental and numerical investigations of unknown characteristics of the rotational nonequilibrium phenomena behind a strong shock wave in air. Experiments were carried out using a piston-driven shock tube with helium as driving gas and air as driven (test) gas, operated as a two-stage shock tube. In the experiments, emission spectra of NO were measured to evaluate the rotational temperature behind a strong shock wave. The numerical calculations use the computational code for the thermal and chemical nonequilibrium flow behind a strong shock wave developed by the present author's group, where 11 chemical species (N $_2$ , O $_2$ , NO, N, O, N $_2^+$ , O $_2^+$ , NO $^+$ , N $^+$ , O $^+$ , e $^-$ ) and 48 chemical reactions of high-temperature air are considered. The thermal nonequilibrium is expressed by introducing an 8 temperature model composed of translational temperature, rotational and vibrational temperatures for N $_2$ , O $_2$ , NO, and electron temperature. The coupling of a rotation, vibration and dissociation (CRVD) model was incorporated to take sufficiently into account the rotational nonequilibrium. The calculations were conducted for the same conditions as the experimental ones. From the calculated flow properties, emission spectra were re-constructed using the code for computing spectra of high temperature air “SPRADIAN”. Furthermore, rotational and vibrational temperatures of NO $\gamma$ (0,1) were determined from a curve fitting method and compared with the computed results.

Nanoparticle removal from substrates with pulsed-laser induced plasma and shock waves

Removing particles with nanometer scale diameters from substrates is a challenging task with numerous critical applications. A novel removal method for nanoparticles is developed and tested. The technique, which is dry and non-contact, takes advantages of shock wavefronts initiated by plasma formation under a focused laser beam pulse and its interaction with the substrate. Experimental results indicate that silica particles down to 500 nm on silicon wafers can be removed without substrate damage. In the reported experiments, a Q-switched Nd : YAG pulsed laser with a 5-ns pulse width and 360-mJ pulse energy at 1064 nm wavelength is employed as a plasma generation source. It is reported that the traditional dry laser cleaning method based on the rapid thermal expansion under direct laser irradiation often results in surface damage in the nanometer scale due to light diffraction around nanoparticles and/or stress localization in the thermal skin. This occurs when the characteristic dimensions of the particles are comparable to the wavelength of incident beam. In the laser plasma method, various mechanisms are responsible for the removal effect. The strong shock wave in air generates complex pressure wavefields resulting in both drag/lift on the particle and acceleration of the substrate. However, shock waves are not transmitted to the solid substrate due to a large difference between the relevant wave phase speeds in the two media. The effects of the number of shots and the distance between the surface and the plasma boundary on the removal efficiency are reported.

Radiation of strong shock waves in air: Part II

The problem on the structure of a radiating shock wave propagating in air at an altitudeH ≤50 km at a velocityD = 10–50 km/s is studied. The treatment is based on the real thermodynamic and optical characteristics of air. The distributions of the gasdynamic parameters and radiant fluxes are obtained in the heated region before the shock wave front and in the relaxation zone behind the viscous shock along the average optical variable; these distributions serve as a basis for the calculation of the space profiles and radiation flows generated by the shock wave and going to “infinity”.

Initial formation process of laser-induced plasma shock wave in air

The initial formation process of laser-induced plasma shock wave in air during laser damage process has been studied experimentally with a well-designed optical fiber sensor based on detecting beam deflection. The experimental results indicate that the formation of the laser-induced plasma shock wave is the result of the overlap of vapor- and explosion-shock waves. The plasma explosion shock wave, which generates behind the vapor shock wave but has a higher speed, catches up with the vapor shock wave and overlaps it at a certain distance, giving the result of a strong shock wave in air. The distance is about 3 mm when a 170 mJ laser at 1.06 μm with 30 ns duration irradiates an aluminum target in air.

Radiation of strong shock waves in air: Part I

Radiation of strong shock waves in air: Part I

Features of nonequilibrium processes of titrogen oxide formation behind strong shock waves in air

The kinetics of NO and NO2 of behind shock fronts propagating in air are analyzed. It is shown that in certain cases it is necessary to use fairly detailed chemical reaction schemes involving not only N2, O2, NO, N, and O, but also NO2, N2O, H2, OH, and H and to take into account the mutual effects of vibrational relaxation and chemical transformations. It is established that neglecting the chemical processes involving NO2 only can lead to significant errors in the length of the relaxation zone (up to 25 times), the gasdynamic parameters (up to 20%), and the NO concentration (up to 3 times).

Density reconstruction from laser schlieren signal in shock tube experiments

Using a diffraction approach the convolution-type integral equation for the laser schlieren signal created by an arbitrary disturbance at low pressure, where refractive index of disturbance is close to unity, in a shock tube (thin optical layer) has been deduced. In the equation electric circuit relaxation processes were taken into account by a response function. The equation was solved with the aid of the regularization method worked out for ill-posed problems. The density structures of the strong shock waves in air have numerically been reconstructed from experimental data ranging shock wave Mach number of $M_s=20$ –30, and $P_0=10$ –30 Pa.

空気中の強い衝撃波背後の発光特性（第１報）全放射発光形態

This paper describes high-speed digital photography and numerical calculation for radiation emanating from strong shock waves in air. By using a high-speed gated image intensified CCD camera, the time-frozen images of total radiation emission are obtained. The radiation profiles have 1 peak for the velocities up to 11km/s, while the profiles have 2 peaks when the velocities exceed 11km/s. In addition, there is a transition region between 10.7km/s and 11.4km/s where both radiation profiles can exist. The experimental results are compared with the numerical ones. The numerical calculation is based on the Park model which takes into account 11 species and 47 chemical reactions. The experimental profiles are found to be narrower than the numerical ones.

230 空気中の強い衝撃波背後における発光の時間分解画像分光計測

This paper describes the imaging spectroscopic technique applied to the experimental study of radiation emanating from strong shock waves in air. Shock velocity is about 11km/s and initial pressure is 13.3Pa in air. The spectral diagnostic system consists of an imaging spectrograph, a streak camera, a gated image-intensified CCD camera and a personal computer for data acquisition/processing. This spectral diagnostic system is capable of simultaneous wavelength-, intensity and time-resolved spectroscopic measurements in the nanosecond order. Wavelength range is chosen primarily to investigate the first negative band of N2+ and the second positive band of N2. Spectra with enhanced spectral resolution are obtained.

高エンタルピ風洞 画像分光法による空気中の強い衝撃波の観測

高エンタルピ風洞 画像分光法による空気中の強い衝撃波の観測

空気中の強い衝撃波背後の発光観測

Two-dimensional structures of radiation emission behind strong shock waves in air are observed by using an image converter camera, and the photos are processed by an image processor. Strong shock waves are generated by a free-piston, double diaphragm shock tube. The shock speed are measured to be from 9 km/s to 11 km/s for the initial pressure of 13.3 Pa. It is shown that the double-peak characteristic becomes increasingly clear as the shock speed increases over 10 km/s. This can be mainly attributed to the increase of continuum radiation from free-free and free-bound transitions of electrons.

Features of ionization and emission behind strong shock waves in air

Features of ionization and emission behind strong shock waves in air

Brightness of strong shock waves in air with a reduced density

Brightness of strong shock waves in air with a reduced density

Experimental determination of ionization time behind a strong shock wave in air

Experimental determination of ionization time behind a strong shock wave in air

Structure and luminosity of strong shock waves in air

A set of numerical computations of the thermal structure and optical luminosity of strong shock waves in air is described. The model includes radiation transport coupled with hydrodynamics under assumed conditions of steady flow. With shock velocities above 10 km/sec a thermal precursor develops ahead of the shock front, and increases in prominence with increasing shock strength. For velocities above 50 km/sec the precursor is partially opaque to visible light and the computed brightness is smaller than that of a blackbody at the shock temperature. At higher velocities the brightness decreases with increasing shock strength. Steady solutions could not be found for velocities above 80 km/sec.

The structure of shock waves from electrodeless discharges in air and argon

gas ahead of the gaseous driver piston, i . e . , the discharge plasma [1]. Separation of the discharge plasma from the thermal plasma is observed only for initial pressures above 0.05-1 mm Hg, and Mach numbers in air below 30-35, and below 10-15 in argon [2,3]. The use of an electrodeless (inductive) discharge to obtain shock waves, as will be shown below, does not remove this kind of restriction nor does it obviate the difficulty. The existence of appreciable deviations from uniformity and regularityrof the flow in electric shock tubes imposes special requirements for control of the state of the gas behind the generated shock waves, and measurements of the gas temperature distribution are of paramount importance in this regard. In this paper we present results of systematic measurements of plasma temperature distribution behind strong shock waves in air and argon, obtained by three independent methods. A justification is given for the "hydrodynamic" method of measuring temperature in a supersonic plasma stream, as described in [4, 5], based on a comparison with simultaneous spectrographic measurements. A discussion is given of the peculiarities of formation and propagation of shock waves in electrodeless discharge Dlbes~ 1. Description of Equipment and Methods of Measurement. The tests were carried out on the equipment shown schematically in Fig. 1. A 1-pF capacitor, charged to 30 kV, is discharged through an 8turn coil of length 10 era, located at the end of a glass tube of diameter 8.6 era. The period of oscillation of the discharge circuit is 22 gsee, and rupture occurs at pressures of air or argon (usually with addition of, l% hydrogen) in the range 1-10 -s mm Hg. Measurement of electron temperature distribution behind the shock wave was performed by the method in which one uses the relative intensities of two spectral lines of hydrogen (H$ and Ha), the light flux being limited by two slits of width 0.5 mm at the entrance of an ISP-81 spectrograph (chamber with ] = 270 mm at a mean dispersion of about 30 %/ram): Separate measurements of the light as a function of time are carried out by means of two photomultipliers. The reduction of the oscillograms, calibration of the channels, and corn-