Deeply undercritical microwave discharge excited by the field of a quasi-optical electromagnetic beam in a supersonic air jet

Electric discharge in a supersonic air jet is studied. It is ignited in a linearly polarized quasi-optical microwave electromagnetic beam the initial field intensity of which is much lower than the breakdown level. Electric breakdown is initiated by a tubular electromagnetic vibrator, one end of which has spikes and is covered by a quartz tube. Atmospheric air enters into a low-pressure working chamber through the inner channel of the vibrator. As a result, an immersed supersonic air jet forms in the chamber at the outlet from the quartz tube. A microwave discharge ignited in this jet is “attached” to aft spikes of the vibrator. The energy deposit into the discharge plasma and the effective area of energy interaction between the discharge and excited microwave field are estimated from the temperature and stagnation pressure distributions in the wake of the discharge.     

Supersonic flow of a nonequilibrium gas-discharge plasma around a body

The flow of a nonequilibrium gas-discharge plasma around a semicylindrical body is studied. The aim of the study is to see how a change in the degree of nonequilibrium of the incoming plasma changes the separation distance between a shock wave and the body. Experiments are carried out with a supersonic nozzle into which a semicylindrical body is placed. The inlet of the nozzle is connected to a shock tube. In the course of the experiment, electrodes built into the wall of the nozzle initiate a gas discharge in front of the body to produce an additional nonequilibrium ionization in the stationary incoming supersonic flow. The discharge parameters are selected such that the discharge raises the electron temperature and still minimizes heating of the gas. The degree of nonequilibrium of the flow varies with gas-discharge current. Diagnostics of the flow is carried out with a schlieren system based on a semiconductor laser. The system can record flow patterns at definite time instants after discharge initiation.     

Analysis of Plasmadynamic Nanosecond-Range Processes in the Formation of Shock Waves from Pulsed Discharges

We have studied the dynamics of the plasma glow of pulsed discharges (sliding surface discharge and combined volume discharge with plasma electrodes) in the nanosecond range (100–12 000 ns) in stationary air and in the flow behind the front of a plane shock wave with Mach numbers 1.7–5.0 in the shock tube channel. The temporal characteristics of the flow, the radiation spectra, and the discharge currents in air are compared in the pressure range 5–150 Torr, a pulsed voltage of 20–30 kV, and a current of about 1 kA. It is shown that the time of current under various conditions does not exceed 400 ns, and the duration of the glow can reach a few microseconds. It is shown that as a result of energy supply near the planar shock wave front, the decay of discontinuities occurs with the formation of shock waves and contact surfaces. The positions of the plasma glow regions are compared with the positions of discontinuity surfaces of numerically calculated gasdynamic parameters in the flow.     

Model of pulsed electric discharge expansion in dense gas taking into account electron and radiative thermal conductivities. II. Energy balance and equation

Using an ionization sensor, it was found that weakly ionized plasma with an ionization degree larger than 10⁻⁶ is formed under exposure to UV radiation of a high-current pulsed electric discharge in gas (air, nitrogen, xenon, and krypton) at atmospheric pressure at a distance of ∼1.2–2.5 cm from the discharge boundary. It was shown that the structure of such discharge includes, in addition to the discharge channel, a dense shell and a shock wave, also a region of weakly ionized and excited gas before the shock wave front. The mechanism of discharge expansion in dense gas is ionization and heating of gas involved in the discharge due to absorption of the UV energy flux from the discharge channel and the flux of the thermal energy transferred from the discharge channel to the discharge shell due to electron thermal conductivity.     

On the possibility of controlling transonic profile flow with energy deposition by means of a plasma-sheet nanosecond discharge

A way of effectively affecting the gasdynamic structures of a transonic flow over a surface by means of instantaneous local directed energy deposition into a near-surface layer is proposed. Experimental investigations into the influence of a pulsed high-current nanosecond surface discharge of the “plasma sheet” type on gas fast flow with a shock wave near the surface are carried out. The self-localization of energy deposition into a low-pressure region in front of the shock wave is described. Based on this effect, a facility for automated energy deposition into a dynamic region bounded by the moving shock front can be designed. The limiting value of the specific energy deposition on the surface in front of the shock wave is found. With the help of the direct-shadow method, an unsteady quasi-two-dimensional discontinuous flow arising when a plasma sheet is initiated on the wall in a flow with a plane shock wave is studied. By numerically solving the two-dimensional nonstationary equations of gas dynamics, the influence of the energy of a pulsed nanosecond discharge, which is applied in the frequency regime, on the aerodynamic characteristics of a high-lift profile is investigated. It is ascertained that the energy delivered to the gas before the closing shock wave in a local supersonic region that is located in the neighborhood of the profile contour in zones extended along the profile considerably decreases the wave drag of the profile.     

超声速层流/湍流压缩拐角流动结构的实验研究

在Ma=3.0的超声速风洞中, 分别对上游边界层为超声速层流和湍流, 压缩角度为25°和28°的压缩拐角流动进行了实验研究. 采用纳米粒子示踪平面激光散射(NPLS)技术获得了流场整体和局部区域的精细结构, 边界层、剪切层、分离激波、回流区和再附激波等典型结构清晰可见, 测量了超声速层流压缩拐角壁面的压力系数. 从时间平均的流场结构中测量出分离激波、再附激波的角度和再附后重新发展的边界层的增长情况, 通过分析时间相关的流场NPLS图像, 可以发现流场结构随时间的演化特性. 实验结果表明: 在25°的压缩角度下, 超声速层流压缩拐角流动发生了典型的分离, 边界层迅速增长失稳转捩, 并引起一道诱导激波, 流场中出现了K-H涡、剪切层和微弱压缩波结构, 而超声速湍流压缩拐角流动没有出现分离, 湍流边界层始终表现为附着状态; 在28° 的压缩角度下, 超声速层流压缩拐角流动进一步分离, 回流区范围明显扩大, 诱导激波、分离激波向上游移动, 再附激波向下游移动, 分离区流动结构复杂, 相比之下, 超声速湍流压缩拐角流动的回流区范围明显较小, 边界层增长缓慢, 流场中没有出现诱导激波、K-H涡和压缩波, 流动分离区域的结构也相对简单, 但分离激波的强度则明显更强. 关键词： 压缩拐角 层流 湍流 流动结构     

Asymmetry of spontaneous helium radiation in retarded excitation in an electric discharge

The dynamics of formation of optical radiation in spectral lines in a longitudinal nanosecond helium discharge under conditions of wave breakdown is investigated. A discharge tube with a diameter of 0.4 cm and a length of 50 cm was placed in a metallic shield 1 cm in diameter. The gas pressure was varied within 1–60 torr for an amplitude of the voltage pulses of up to 50 kV. Asymmetry of spontaneous radiation recorded from different ends of the discharge tube is detected. This is shown to be a consequence of nonuniform distribution of the functions of the primary brightness of the flux and the absorption coefficient along the tube length. Dagestan State University, 43a, M. Gadzhiev Str., Makhachkala, 367025, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 415–420, May–June, 1999.     

Characteristics of a magnetogasdynamic diffuser under different modes of electric current switching

This paper elaborates upon a previous investigation into the influence of external electric and magnetic fields on a flow through a supersonic diffuser. The aim of the present study is to correlate a change in the configuration of a shock wave emerging near the diffuser inlet at magnetohydrodynamic interaction with the amount of force and energy actions and with total pressure losses. For this purpose, the main parameters of the shock wave structure and the total pressure are measured at the diffuser outlet when the flow is subjected to magnetic and electric fields of various strengths at different routes of current passage. In the experiments, a shock tube with a supersonic nozzle is employed. The shock tube forms a flow behind the shock wave reflecting from the end of the tube, which terminates in the nozzle. The diffuser is located directly downstream of the nozzle. The investigation is carried out in xenon. The flow is subjected to external fields at the inlet of the diffuser. The shock wave structure is visualized by frame sweeping of Schlieren patterns of the flow. The total pressure is measured with a piezoelectric transducer located at the end of the channel. The results obtained make it possible to optimize the action on the flow in terms of power consumption and total pressure losses for a given design of the diffuser.     

Application of PIV to over-expanded supersonic flows: Possibilities and limits

Two-dimensional velocity distributions outside a Mach 2.0 supersonic nozzle have been investigated using a digital particle im age velocimetry (PIV). Mean velocities , vor ticity field and volume dilatation field were obtained from PIV images using 0 .33 μm titanium dioxide (TiO₂) particle. The seeding particle of larger size , 1.4 μrn Ti0₂, was also used for the experimental comparison of velocity lag downstream of shock waves. The results have been compared and analyzed with schlieren photographs for the locations of shock waves and over-expanded shock structure to inspect possibilities and limits of a PIV technique to over-expanded supersonic flows. It is found that although the quantitative velocity measurement using PIV on over-expanded supersonic flows with large velocity and pressure gradients is limited, the locations of normal shock and oblique shock waves can be resolved by the axial/radial velocity fields, and over-expanded shock structure can be predicted by vorticity field and volume dilatation field which are acquired from the spatial differential of the velocity field.     

Model for the shock actuation of a wedge-shaped nozzle with allowance for detachment of the flow

The motion of the starting gasdynamic discontinuities and of the detached flow arising upon the shock actuation of a wedge-shaped nozzle is examined in the framework of the problem of the decay of an arbitrary discontinuity at a jump in the cross-sectional area of the channel. The results of the calculation are compared with experimental measurements of the spatial and time dependence of the starting discontinuities upon the shock actuation of submerged jets over a wide range of variation of the parameters governing the flow: at pressure ratios and sound velocities of the issuing and immersing gases, for issuing gas species with 1.16<γ<1.67, and for various combinations of these parameters. The possibility of using this model to determine the relaxation time to an undetached supersonic flow in a wedge-shaped nozzle upon the shock actuation of slightly underexpanded and overexpanded jet flows is analyzed. Zh. Tekh. Fiz. 67, 1–4 (June 1997)     

超声速预混可燃气流的点火与燃烧

在激波风洞一激波管组合设备上开展了碳氢燃料超声速预混可燃气流的点火与燃烧实验研究。实验结果表明:利用激波对燃料进行预热,并以高温燃气作为引导火焰,可以有效缩短汽油空气超声速可燃混气的点火延迟时间,使之缩短到 0.2 ms以下。利用纹影照片对超声速燃烧流场结构作出了分析;研究了超声速预混可燃气流的温度以及当量比对超声速燃烧流场结构、点火与火焰传播特性的影响。     

Numerical simulation of gasdynamic phenomena

The work in computational hydrodynamics related to investigations of the supersonic streamlining of bodies, the interaction of shock waves, multiphase flows, and hyposonic flows in devices for semiconductor technology is reviewed. Zh. Tekh. Fiz. 69, 46–49 (September 1999)     

光腔与扩压器化学反应流场优化数值模拟

氧碘化学激光器(COIL)在化学反应条件下,由于光腔及扩压器的气流通道内存在残余化学反应放热,从而导致"热堵"现象发生,影响了扩压器的正常启动及光腔内超声速流场的流动品质。采用数值模拟方法对COIL光腔与超声速扩压器流道内的化学反应流场进行研究,对超声速扩压器插入段的长度、楔形体的数量级扩散段长度对化学反应流场的影响进行研究。数值模拟结果表明:通过优化插入段及楔形体长度、取消扩压器侧壁的半楔形体,改善了因化学反应放热对光腔及扩压器流场造成的不利影响。优化后,光腔内的流动不再受气流分离产生的斜激波的影响,扩压器二喉道内的分离现象消失,扩压器壁面的分离区减小,出口流动更加均匀,"热堵塞"现象消失。化学反应条件下的气流总压损失比冷流时提高约15%,光腔与扩压器的总压恢复系数为0.426,进出口的静压比为3.75,比优化前提高了约25%。     

Investigation of the pseudo-shock wave in a two-dimensional supersonic inlet

Abstract  

This paper describes experimental and numerical investigations into the multiple shock waves/turbulent boundary layer interaction in a supersonic inlet. The test model has a rectangular shape with an asymmetric subsonic diffuser of 5°. Experiments were conducted to obtain the visualization images and static pressure data by using supersonic wind tunnel. Numerical simulation was performed by solving the RANS equations with the Menter’s SST turbulent model. The inflow condition was a free-stream Mach number of 2.5 and a unit Reynolds number of 7.6 × 10⁷/m. Numerical results showed good agreement with the experimental results. Based on this agreement, the flow characteristics which are often very difficult to obtain experimentally alone were analyzed with the aid of numerical simulation. The structures, pressure and velocity distributions, and total pressure loss of the pseudo-shock wave in the supersonic inlet were presented in detail from flow visualization images and static pressures.     

Development of gas-dynamic perturbations propagating from a distributed sliding surface discharge

Results are presented from experimental studies of the plasma layer structure of a distributed sliding surface discharge excited in quiescent air and in a uniform gas flow behind a plane shock wave at gas densities of 0.03–0.30 kg/m³. The dynamics of weak shock waves generated after discharge initiation was studied. According to the experimental and simulation results, 40% of the discharge energy transforms into heat within a surface gas layer in the energy input stage, which lasts up to 200 ns.     

Study of density field measurement based on NPLS technique in supersonic flow

Due to the influence of shock wave and turbulence, supersonic density field exhibits strongly inhomogeneous and unsteady characteristics. Applying traditional density field measurement techniques to supersonic flows yields three problems: low spatiotemporal resolution, limitation of measuring 3D density field, and low signal to noise ratio (SNR). A new method based on Nano-based Planar Laser Scattering (NPLS) technique is proposed in this paper to measure supersonic density field. This method measures planar transient density field in 3D supersonic flow by calibrating the relationship between density and concentration of tracer particles, which would display the density fluctuation due to the influence of shock waves and vortexes. The application of this new method to density field measurement of supersonic optical bow cap is introduced in this paper, and the results reveal shock wave, turbulent boundary layer in the flow with the spatial resolution of 93.2 μm/pixel. By analyzing the results at interval of 5 μs, temporal evolution of density field can be observed. Supported by the National Natural Science Foundation of China (Grant No. 10672178)     

Autoignition of ethylene in shock waves

The delay time of ignition of various C₂H₄-O₂-Ar mixtures behind reflected shock waves were measured at temperatures of 1090–1520 K and a pressure of 0.65 ± 0.05 MPa. A kinetic scheme of the ignition of ethylene based on the known rate constants of the key elementary reactions was developed. The scheme satisfactorily describes our own and published data on the ignition of ethylene in shock waves over wide ranges of temperature (1100–2400 K), pressure (0.006–0.64 MPa) and ethylene (0.1–17.4 vol %) and oxygen (0.6–20.7 vol %) concentrations.     

On the nature of turbulence

A concept of turbulence is presented that is based on the results of an investigation of the structure of a gas flow in a tube with a square cross section in front of a nonsteady-state moving flame front. It is shown that a region of elevated pressure, consisting of alternating condensations, is formed in the gas flow near the tube walls. These condensations are the sources of waves which form a distribution of velocity fluctuations in the gas flow over a wide range of amplitudes, frequencies and directions. The dynamics of the perturbations at the walls and the configuration of the wave in the gap make it possible to consider the fluctuations in the flow as pseudochaotic and to use statistical methods to describe them. Zh. éksp. Teor. Fiz. 113, 191–203 (January 1998)     

Double-diaphragm shock tube at the Ioffe Physicotechnical Institute

Results of numerical calculations of the flow in a double-diaphragm shock tube with a tailored contact surface are reported. The calculations were carried out using a model of an ideal shock tube allowing for the real properties of the driver gas at high pressures and equilibrium thermodynamics of the processes behind the shock waves at Mach numbers M _s1 of the shock wave in the working gas varying in the range 5–25. Flow regimes with a tailored contact surface were obtained for Mach numbers M _s1=6.3, 11, and 15 using the double-diaphragm shock tube at the Ioffe Physicotechnical Institute. Under these conditions, the parameters of the working gas were kept constant for more than 1 ms. The calculated data were compared with the experimental results and it was shown that the calculated data may be used to determine the section lengths in a double-diaphragm shock tube and to estimate its operating time. The calculated values of the initial pressure in the sections of the tube were substantially lower than those achieved experimentally. Measurements were made of the static pressure along the axis of a conical nozzle during the expansion of hydrogen (initial temperature T ₀=293 K) and shock-heated nitrogen (T ₀=4000 K). It is found that the expansion of hydrogen is accompanied by deactivation of the rotational degrees of freedom, and that partial freezing of the vibrational degrees of freedom takes place in the nitrogen stream. Zh. Tekh. Fiz. 67, 88–95 (November 1997)     

High-velocity mass transfer in FCC-metals containing chains of vacancies and interstitial atoms

Using the method of molecular dynamics simulations, the dynamics of structure rearrangement in the course of relaxation in a three-dimensional aluminum is investigated via introduction of one-dimensional chains containing equal number of vacancies and interstitial atoms and located in close-packed positions along the <101 > directions. This model represents a starting material structure possessing regions with differing mass densities: m⁺ and m^–. The process of relaxation is shown to proceed via a number of phases: generation of shock waves, nucleation of vortex displacements of atoms, transformation of shock waves into acoustic waves, and correlated high-velocity collective displacements of atoms from interstitial into vacancy positions. The latter displacements are developed at velocities much higher than acoustic velocity.