An experimental study of gasdynamic windows for gas lasers

The working pressure in the cavities of most gas lasers varies from 30 to 100 torr. Thus, in transmitting the laser light into the atmosphere, the problem arises of isolating the cavity from atmospheric pressure. In low-power lasers (N l kW/cm², where N is the laser output power) optical windows of monocrystalline NaC1 (CO₂ laser), CaF₂ (CO laser), and so on, are used for this purpose [1], The absorption coefficient for radiant energy of such optical windows averages 0.02–0.2%; thus, at cw laser powers of order 3 kW/cm² or more it is difficult to send the beam out through monocrystalline windows because they are heated excessively due to absorption of radiation and, consequently, broken. In this casa so-called gasdynamic windows (shutters) with transverse gas pump-through [2] or ejector-type gasdynamic windows, which ensure the required pressure difference and tolerable return flows of gas between the laser cavity and the surrounding medium, are used in place of monocrystalline windows. Here we shall describe the design of gasdynamic windows, their characteristics, and operating regimes, and we shall present data from experimental studies of the operation of gasdynamic windows with transverse gas pumpthrough over a wide range of variation of the window size and gas-flow parameters.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 192–196, March–April, 1977.In conclusion, we thank Kh. M. Faizulin, V. M. Abramov, S. A. Danilushkin, B. Ya. Lyubimov, and L. V. Bol'shov for help in doing the experiments and for useful discussions.     

Laser diagnostics of molecular states in nonequilibrium gas flows

Laser diagnostics techniques are developed, and some typical results are presented on the vibrational CO₂ molecule level population measurements under highly nonequilibrium conditions for two types of flows: (i) in a conventional gasdynamic lasers and (ii) in a flow mixing gasdynamic laser. The measuring procedure is based on recording spectral gain coefficient distributions at several resolved rotational transitions of different vibrational bands. The laser optical system with spatial selection of single lines is described. The system allows fast (about 10^–5–10^–4 s) line tuning, thus providing a variable and arbitrary choice of the selected line sequences. The optimum choice of the laser generation spectrum is discussed, and the sources of measuring uncertainties are analyzed. Typical illustrations and results are given and discussed.     

Subsonic Radiation Gas Dynamics in a Laser Plasma Generator Channel

The radiation gasdynamic processes in the channel of an air laser plasma generator operating at atmospheric pressure are analyzed. In the multigroup approximation a numerical radiation gasdynamic model is formulated on the basis of the equations of motion of a viscous heat-conducting gas and the selective thermal radiation transport equation. Laminar and turbulent subsonic generator operation regimes are considered.For the purpose of approximately describing the turbulent gas and plasma mixing the Navier-Stokes equations averaged after Reynolds and the k-ε turbulence model are used. The problem is solved in the time-dependent two-dimensional axisymmetric formulation.Strong radiation-gasdynamic interaction regimes are investigated. In these regimes the energy losses due to radiation from the high-temperature region of the laser plasma and the absorption of its thermal self-radiation by the surrounding plasma and gas layers (radiation reabsorption) appreciably affect the gasdynamic flow structure. Two methods of integrating the selective thermal radiation transport equation in the generator channel are discussed. In one of these the thermal radiation transport is calculated inside the heated volume and in the other the radiation heat fluxes are calculated on the surfaces bounding the volume. The results of calculating the spectral and integral radiation heat fluxes on the inner surface of the generator are given.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 3, 2005, pp. 126–143.Original Russian Text Copyright © 2005 by Surzhikov.     

Optimization of the parameters of a CO2 gasdynamic laser

In order to maximize the gain coefficient of a CO₂-N₂-He gasdynamic laser, a total optimization of the stagnation parameters, the concentration coefficient, and the parameters which determine the shape of the nozzle (assuming a quasi-one-dimensional flow model) was carried out. The dependence of the optimized parameters on the limitations imposed by the stagnation temperature of the flow and the critical cross section have been determined.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 3–8, September–October, 1974.     

Flow around a sphere by a two-phase supersonic jet

Results are presented of a calculation of the flow around a sphere of a two-phase supersonic jet, discharging into a vacuum. Calculations were performed by the determination method with use of a difference grid constructed on the basis of characteristic ratios [1], The parameters of the unperturbed jet were determined with the two-velocity and two-temperature model of mutually penetrating flows of continuous media (gas and particles) [2, 3] by the network method [4]. In calculating the flow around the sphere, as in [5–7], it was assumed that the particles do not affect the gas flow in the shock layer. An analysis of the effect of particles on gasdynamic parameters in a shock layer was performed in [8].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 171–176, November–December, 1978.The authors are grateful to A. N. Nikulin for providing the program for calculation of flow about a blunt body by a uniform supersonic flow.     

Theory of two-component gasdynamic laser

In the present paper a numerical calculation is made of the vibrational relaxation of a binary mixture of molecular nitrogen and carbon dioxide gas. The calculation is performed for the entire range of variation of the concentrations of the components and over a wide range of mixture temperatures and pressures for various geometries of the supersonic part of the nozzle (throat dimensions, degree of expansion). It is shown that population inversion of the CO₂ molecules exists within a certain range of variation of the parameters of the mixture and the nozzle. The population inversion of the vibrational levels and the gain of the gaseous mixture are calculated as functions of these parameters and of distance measured from the critical cross section of the nozzle. The energy characteristics of the two-component gasdynamic laser are optimized.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 23–30, May–June, 1974.     

Stationary supersonic conducting gas flow in a channel with nonconducting walls under weak magnetohydrodynamic interaction

A solution of the problem of flow in a channel with nonconducting walls for a small magnetohydrodynamic interaction parameter N is obtained by numerical methods. In the 0–10 range of variation of the Hall and magnetic Reynolds number parameters the distributions of the electrical parameters and the average (over the cross section) and local gasdynamic flow parameters are computed for two different geometries of the applied magnetic field. It is shown that an increase in the Hall and magnetic Reynolds number parameters is accompanied by a diminution in the Joule dissipation and the perturbation of the average (over the cross section) gasdynamic flow characteristics. It is disclosed that the distribution of the gasdynamic parameters over the channel cross section is extremely nonmonotonic in the end current zones.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 20–29, July–August, 1970.In conclusion, the author is grateful to A. B. Vatazhin for useful comments and constant attention to the research and to I. U. Tolmach for valuable comments.     

Calculation of the motion of a gas behind the front of a luminous detonation wave taking account of the lateral expansion of the plasma column

Nonsteady-state gasdynamic processes are considered in a plasma column (hot channel), formed behind the front of a shock wave moving toward a laser beam. A quasi-one-dimensional approximation is used-the parameters in the channel are assumed to be compensated with respect to cross section, but depend on the time and distance along the axis. Motion in the cold dense casing surrounding the channel is assumed to be one-dimensional and cylindrically symmetrical. The solutions of the corresponding systems of equations in partial derivatives permit the parameters to be determined approximately both in the case when the mean free path of the radiation is small in comparison with the radius of the beam (luminous detonation) and also in the case when the mean free path is comparable with the radius. Examples are given of the corresponding numerical calculations. It is shown that in the latter case, a cycle of incomplete absorption can be achieved when behind the shock wave front, moving with constant velocity up to the Jouguet plane, only a part of the radiation energy incident on the front is released.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 18–28, May–June, 1976.     

Planar gas flows with weak energy supply

Perfect gas flows in an unlimited space, which occur during rectilinear motion of a system of distributed heat sources, are investigated. The next modes in order of growth of the number M are examined: the heat conductive, convective, subsonic, transonic, supersonic, hypersonic. Examples of computations are presented. Flows with distributed heat sources attract ever-increasing attention. Such flows are important, e.g., in the problem of radiation propagation [1–5], in the analysis of a gasdynamic laser resonator and the optical characteristics of a ray [6]. Changes in the density because of absorption of the ray energy, which can result in an essential redistribution of the radiation intensity, are of great interest in these problems. Theoretical investigations of a general nature with distributed heat supply [7–10] are also important for the development of further applications. Gas flows for a given distribution of relatively weak heat sources switched on at a certain time are examined in this paper.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 95–102, September–October, 1978.     

Some data of an experimental study of a supersonic twisted jet

Experimental data are presented on the aerodynamics of a supersonic twisted jet with different values of the system parameters. Shadow photographs of the gasdynamic section and profiles of the average characteristics of the jet are obtained. The experimental data on the stagnation temperature and velocity components were analyzed in universal coordinates. The experimental results are compared with a calculation by the method of the equivalent problem of the theory of heat conduction.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 184–186, January–February, 1977.     

Applicability of a one-dimensional model to the calculation of the properties of a plasma generator

Most engineering methods for calculating the properties of plasma generators use similarity theory to derive dimensionless equations to generalize experimental results [1]. Although their accuracy is acceptable for practical calculations, the equations cannot be used for a physical analysis of the local phenomena occurring in the working channel of a plasma generator. In the present paper the experimental data are compared with the results of a calculation of the local and integrated heat and gasdynamic properties of a dc plasma generator with a longitudinally injected arc. The basis of the computational method is a quasi-one-dimensional gasdynamic model of the flow of an electrically conducting gas in the channel of the plasma generator developed and studied in [2].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 122–126, July–August, 1977.In conclusion, the authors thank G. A. Lyubimov for valuable remarks.     

Effect of parameters of the discharge circuit on the mode of energy input into the gas in a self-sustained discharge

A model is proposed for calculating the characteristics of volumetric gas discharges used in the pumping of electric-discharge lasers. The dependence of the mode of energy input into the plasma of the gas discharge on the parameters of the discharge circuit is discussed.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 2, pp. 42–46, March–April, 1976.In conclusion, the authors thank Yu. D. Korolev for useful discussions.     

Formation of active medium in a supersonic recombination-nonequilibrium plasma flow

A method of calculating a plasmodynamic laser from the given characteristics of the working fluid and the supersonic nozzle and from the stagnation parameters of the plasma in the nozzle forechamber is proposed. This method gives an analytic expression for the dependence of the inversion coefficient and the laser power on the above-mentioned parameters and serves mainly for the scientific planning of physical and detailed numerical experiments.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 109–114, January–February, 1988.     

Gasdynamics of the scattering of the detonation products of a cylindrical explosive charge with inert metal particles

A mathematical model of the scattering of the detonation products of a condensed explosive with inert metal particles when the wave travels along the axis of a cylindrical charge is proposed and numerical calculations are carried out. Detonation product scattering is simulated by a two-phase nonequilibrium axisymmetric jet flow in a supersonic external airstream. A technique for calculating the product gas-suspension behind the detonation wave is developed. The optimal values of the difference scheme parameters ensuring the required calculation accuracy are found. The gasdynamic properties of the process associated with the particles and the lateral spread are investigated.Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 5, pp. 101–111, September–October, 1995.     

Determination of optimum nozzle parameters in a gasdynamic laser

In connection with the use of supersonic nozzles to create lasers, the question arises of the optimum parameters of the nozzle and the gas mixture from the aspect of obtaining the greatest population inversion of the energy levels of internal degrees of freedom of molecules of the working gas and the greatest output power of the lasers. A rather complete concept of the kinetic processes taking place during the escape of a relaxing gas mixture containing carbon dioxide through a supersonic nozzle has now been developed on the basis of calculated and experimental data. In [1–4] the problems of optimization of the parameters of a CO₂-N₂-H₂O-He mixture and of the shape of the nozzle were set up and solved in a one-dimensional steady-state formulation. The influence of the two-dimensionality of the stream in an optimum nozzle on the laser characteristics is studied in the present report. The method of through calculation suggested in [5] is used to calculate the two-dimensional flow of a relaxing gas.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 23–26, September–October, 1978.     

Interference of stationary and non-stationary shock waves

The mathematical model of a gasdynamic discontinuity is used in the area of study concerning gas flows with large gradients of gasdynamic functions. Gasdynamic functions before and after the discontinuity meet non-linear algebraic equations called the dynamic compatibility conditions on the discontinuities. Different modes of shock wave structures forming as a result of regular or irregular interference of the incoming discontinuities of different types are described. Ranges of the initial flow parameters definition such that either shock wave structures of different modes take place or interference equations have no solutions are determined. Most attention is given to arbitrary triple shock-wave configurations. Their classification is proposed. Differential characteristics of the steady flow are studied. The notion “differential characteristics” includes first derivatives of the fundamental gasdynamic parameters with respect to natural coordinates and curvatures of the discontinuities surfaces. Effect of unsteadiness on the triple-shock configuration is examined. Some problems arising at creation of complete local theory of steady and propagating gasdynamic discontinuities interference are formulated.     

Modeling of the free-surface shape in laser cutting of metals. 2. M odel of multiple reflection and absorption of radiation

A physical model of multiple reflection of focused laser radiation propagating in narrow channels and slots is proposed to be applied in laser treatment of metals. Based on the laws of geometric optics, an algorithm for calculating the shape of the surface formed by interaction of laser radiation and metal with the use of the trajectory technique is proposed. The method used allows one to describe energy transfer inward the channel in simulation of laser-treatment processes (drilling, cutting, and welding). It is shown that the efficiency of multiple reflection is most significant in cutting thick materials with a large ratio of the plate thickness to the Gaussian beam width.Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 1, pp. 16–20, January–February, 2005     

Radiative heat transfer at the leading edge of a body during intense evaporation

The problem of the flow of a radiating gas near the leading part of a body during intense vaporization of its surface is solved. Significant radiative heating occurs when a sufficiently dense gas flows past the body, thus a gasdynamic model of the flow is used according to which the flow takes place in a shock layer and a vapor layer separated by the contact surface. The radiant energy flux from the shock layer is partially absorbed by a vapor layer and falls on the surface of the body, causing intense vaporization.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 190–192, March–April, 1975.     

Calculation of two-phase flow in an axisymmetric laval nozzle with allowance for particle reflection from the walls

A method and the results of calculating the parameters of two-phase monodisperse and polydisperse flows in a Laval nozzle are presented. A general particle-wall interaction model based on the results of experimental research is used. It is shown that taking the real characteristics of the interaction between the condensate and the walls into account makes it possible to determine a number of important flow parameters with much greater accuracy.deceasedTranslated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 79–85, March–April, 1990.     

Effect of periodic action on boundary layer stability

The effect of a wave traveling over the surface and suction-blowing in the form of a traveling wave on boundary layer stability and laminarturbulent transition is investigated. The perturbation parameters are assumed not to be related to the parameters of the Tollmien-Schlichting wave. The parameters corresponding to an increase in the critical Reynolds number by a factor of 2–2.5 are determined.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 109–115, May–June, 1988.The author is grateful to V. A. Kuparev for supplying the program for calculating the stability of the boundary layer.