Showing papers by "Moscow Institute of Physics and Technology published in 2001"

Stabilization of Hypersonic Boundary Layers by Porous Coatings

Abstract: A second-mode stability analysis has been performed for a hypersonic boundary layer on a wall covered by a porous coating with equally spaced cylindrical blind microholes. Massive reduction of the second mode amplification is found to be due to the disturbance energy absorption by the porous layer. This stabilization effect was demonstrated by experiments recently conducted on a sharp cone in the T-5 high-enthalpy wind tunnel of the Graduate Aeronautical Laboratories of the California Institute of Technology. Their experimental confirmation of the theoretical predictions underscores the possibility that ultrasonically absorptive porous coatings may be exploited for passive laminar flow control on hypersonic vehicle surfaces.

Prehistory of Instability in a Hypersonic Boundary Layer

Abstract: The initial phase of hypersonic boundary-layer transition comprising excitation of boundary-layer modes and their downstream evolution from receptivity regions to the unstable region (instability prehistory problem) is considered. The disturbance spectrum reveals the following features: (1) the first and second modes are synchronized with acoustic waves near the leading edge; (2) further downstream, the first mode is synchronized with entropy and vorticity waves; (3) near the lower neutral branch of the Mack second mode, the first mode is synchronized with the second mode. Disturbance behavior in Regions (2) and (3) is studied using the multiple-mode method accounting for interaction between modes due to mean-flow nonparallel effects. Analysis of the disturbance behavior in Region 3) provides the intermodal exchange rule coupling input and output amplitudes of the first and second modes. It is shown that Region (3) includes branch points at which disturbance group velocity and amplitude are singular. These singularities can cause difficulties in stability analyses. In Region (2), vorticity/entropy waves are partially swallowed by the boundary layer. They may effectively generate the Mack second mode near its lower neutral branch.

Nonstationary Casimir effect and analytical solutions for quantum fields in cavities with moving boundaries

Abstract: This is a review of publications on classical and quantum electrodynamics in cavities with moving boundaries (in the quantum case this subject is labeled frequently as "nonstationary Casimir effect" or "dynamical Casimir effect"), from 1921 to October of 2000, with an emphasis on analytical results related to cavities with resonantly oscillating boundaries.

Nonstationary Casimir Effect and analytical solutions for quantum fields in cavities with moving boundaries

Abstract: This is a review of publications on classical and quantum electrodynamics in cavities with moving boundaries (in the quantum case this subject is labeled frequently as "nonstationary Casimir effect" or "dynamical Casimir effect"), from 1921 to October of 2000, with an emphasis on analytical results related to cavities with resonantly oscillating boundaries.

The effect of coronae on leader initiation and development under thunderstorm conditions and in long air gaps

Abstract: The initiation and development of a leader is theoretically studied by considering an electrode which is embedded in a cloud of space charge injected by a corona discharge The focus is on the initiation of upward lightning from a stationary grounded object in a thundercloud electric field The main results are also applicable to the leader process in long laboratory air gaps at direct voltage Simple physical models of non-stationary coronae developing in free space near a solitary stressed sphere and of a leader propagating in the space charge cloud of coronae are suggested It is shown that the electric field redistribution due to the space charge released by the long corona discharge near the top of a high object hinders the initiation and development of an upward leader from the object in a thundercloud electric field The conditions for the formation of corona streamers that are required to initiate a leader are derived The criteria are obtained for a leader to be initiated and propagate in the space charge cloud A hypothesis is proposed that the streamers are never initiated near the top of a high object under thunderstorm conditions if at ground level there is only a slowly-varying electric field of the thundercloud The streamers may be induced by the fast-rising electric field of distant downward leaders or intracloud discharges

High energy ions generated by laser driven Coulomb explosion of cluster

Abstract: We present an analytical model and three dimensional particle simulations of intense laser interaction with a cluster of overdense plasma. When laser intensity is above a critical value, it blows off all of electrons from the cluster and forms a non-neutral ion cloud. During the Coulomb explosion of the ion cloud, ions acquire their energy. Ion energy spectra are discussed in detail for different densities and sizes of clusters with various laser intensities. It is shown that ultra-fast ions are produced for relatively large clusters, and that the ion energy becomes three times greater than the maximum electrostatic potential energy of the ion cloud. The laser driven Coulomb explosion of a cluster may provide a new high energy ion source.

Efficiency of ion acceleration by a relativistically strong laser pulse in an underdense plasma

Abstract: A particle-in-cell simulation is used to investigate ion acceleration by a femtosecond laser pulse propagating in an underdense plasma slab. In plasma slabs with different thicknesses, the ions are found to be accelerated by different mechanisms. It is shown that, for laser pulse intensities in the range (5–10)×1019 W/cm2, the ions are accelerated near the plasma-vacuum interface.

Hydrogen oxidation in a stoichiometric hydrogen-air mixture in the fast ionization wave

Abstract: Oxidation of molecular hydrogen in a stoichiometric hydrogen-air mixture in the fast ionization wave (FIW) was studied at total pressures p = 1–8 Torr, and the detailed kinetics of the process has been numerically investigated. The excitation of the gas in FIW and the dynamics of molecular hydrogen concentration were monitored using measurements of absolute H2 radiation intensity (transition a 3Σ+ g → b 3Σ+ u). A comparison of calculated and experiment results allows one to draw the conclusion that the gas is predominantly excited behind the FIW front in relatively low electric fields E/n ≃ 300–600 Td at electron concentration n e ≃ (1–2) × 1012 cm−3 for a time of the order of 10 ns and can be described with good accuracy using the two-term approximation of Boltzmann's equation. In the following processes the reactions including electron-excited particles play a dominant role for times up to 100 ns, ion-molecular reactions - for the microsecond time range, reactions including radicals mostly contribute fo...

Initial-Value Problem for Hypersonic Boundary-Layer Flows

Abstract: An initial-value problem is analyzed for a two-dimensional wave packet induced by a local two-dimensional disturbance in a hypersonic boundary layer. The problem is solved using Fourier transform with respect to the streamwisecoordinateandLaplacetransform with respectto time. The temporal continuous spectrum isrevisited, and the uncertainty associated with the overlapping of continuous-spectrum branches is resolved. It is shown that thediscretespectrum’ s dispersion relationship isnonanalyticbecause of thesynchronization of thee rst mode with the vorticity/entropy waves of the continuous spectrum. However, the inverse Laplace transform is regular at the synchronism point. Characteristics of the wave packet generated by an initial temperature spot are numerically calculated. It is shown that the hypersonic boundary layer is highly receptive to vorticity/entropy disturbances in the synchronism region. The feasibility of experimental verie cation of this receptivity mechanism is discussed.

Stabilization of hypersonic boundary layers by porous coatings

Abstract: A second-mode stability analysis has been performed for a hypersonic boundary layer on a wall covered by a porous coating with equally spaced cylindrical blind microholes. Massive reduction of the second mode amplification is found to be due to the disturbance energy absorption by the porous layer. This stabilization effect was demonstrated by experiments recently conducted on a sharp cone in the T-5 high-enthalpy wind tunnel of the Graduate Aeronautical Laboratories of the California Institute of Technology. Their experimental confirmation of the theoretical predictions underscores the possibility that ultrasonically absorptive porous coatings may be exploited for passive laminar flow control on hypersonic vehicle surfaces.

The effect of small O2 addition on the properties of a long positive streamer in Ar

Abstract: Experiments are described in which spark discharges in long non-uniform gaps are studied in Ar with a small addition (1-5%) of O2 under standard conditions. The results obtained show that even the addition of 1% O2 drastically reduces the conductivity of the streamer channel and increases the average electric field required to bridge the discharge gap by a streamer. Breakdown, producing short circuiting, is developed through leader propagation in Ar:O2 mixtures, in contrast to a non-thermal streamer mechanism of breakdown that was previously observed in pure Ar. A 1.5-dimensional simulation was carried out which supports the general observed properties of streamers in long gaps in Ar:O2. An analysis of the simulation results shows that the effect of small O2 additions on the streamer properties in Ar is explained by the quenching of excited Ar atoms by O2 molecules, which leads to the deceleration of two-step ionization in the streamer channel.

Spontaneous (non) Emergence of Property Rights

Abstract: It is often argued that secured property rights are established in response to grassroots demand of private owners. The paper analyzes preferences of private owners over the degree of protection of property rights. The framework for this analysis is an equilibrium model, which combines production and appropriation activities. It is shown that inequality in resource ownership and/or relative inefficiency of production technologies could make wealthier agents to favor less than full protection of property rights. If such agents decide the outcome of public choice of a property rights regime, then fully secured property rights will not emerge from the grassroots. This conclusion is consistent with the failure to establish an efficient system of property rights regime in Russia.

Comments on 'The role of photoionization in positive streamer dynamics'

Abstract: It was noted that the model of streamer pseudo-branching, proposed in `The role of photoionization in positive streamer dynamics' (Kulikovsky A A 2000 J. Phys. D: Appl. Phys. 33 1514-24) in 2D geometry, cannot be vindicated within the frame of the physical model used in the article. It was also shown that the results of numerical simulation represented in the article are not repeatable.

Absorption of Electromagnetic Energy by Slow Electrons under Scattering from Coulomb Centers

Abstract: Simple analytical expressions are obtained for the rate of the inverse stimulated bremsstrahlung absorption under electron scattering from a Coulomb center with charge Z in the presence of the electromagnetic field. The initial and final values of electron energy are assumed to be small compared to the Rydberg energy Z2 (atomic units are used throughout). Single-photon processes of absorption and induced radiation of photon by electron are treated. It is assumed that the electromagnetic field frequency ω is rather low, so that the condition Zω/p3 ≪ 1, where p is the electron momentum, and the condition ħω ≪ p2 are valid. However, this frequency is assumed to be fairly high compared to the electron-Coulomb center collision frequency: ω ≫ vnei. The dependences of the rates of photon absorption and induced radiation on the angle θ between the direction of incident electron and the electromagnetic field polarization vector (assumed to be linearly polarized) are obtained. It is demonstrated that, for any angles θ, the rate of photon absorption is higher than the rate of induced radiation and, therefore, the Marcuse effect for slow electrons (electromagnetic field amplification) is absent. It is further demonstrated that a slow electron on the average absorbs double ponderomotive energy per collision with an ion (Coulomb center) in Maxwellian plasma. This agrees both with the known results calculation for fast electrons and with the known results of the calculation based on the classical Boltzmann kinetic equation for plasma.

Thermophysical Properties of Shock Compressed Argon and Xenon

Abstract: The problem of the nature of thermodynamic properties and the high level electrical conductivity of substances at high pressures and temperatures is one of the most key issues of physics of high energy densities. So called pressure ionization is one of the most impressive demonstrations of the strong coupling effects in plasma under compression. Noble gases are the simplest object of studying of these phenomena because of absence of molecules and spherical symmetry of their atoms. In the present paper we are trying to have a common look from the chemical plasma picture on the whole available massive of the experimental data on Ar and Xe in a wide range of the parameters: from gaseous densities of 0,01 g/cc and pressures of several kilobars up to extremely high densities corresponding to the insulator-metal transition and megabar pressure range. (orig.)

Dynamics of Coulomb explosion of large Xe clusters irradiated by a super-intense ultra-short laser pulse

Abstract: Dynamics of large Xe clusters irradiated by a super-intense ultra-short laser pulse is investigated within the frames of the simple Bethe model. Inner and outer classical above-barrier multiple ionization of the clusters, and also the Coulomb expansion of the cluster ion are calculated. As a typical example we consider a Xe cluster with 1000 atoms irradiated by a laser pulse with the wavelength 800 nm, pulse duration 100 fs and the peak intensity 1016 W cm-2. We found that electrons do not absorb practically the energy from the external electromagnetic field in the process of elastic reflection from the inner cluster surface or at the collisions with the atomic ions inside the cluster. The heating of electrons is determined by their electrostatic Coulomb energy which is transformed into the kinetic energy after ejection of electrons from the cluster.

Plasma Parameters in the Channel of a Long Leader in Air

Abstract: The time evolution of the electric field in the leader channel and other characteristics of the leader plasma in long air gaps are simulated. Calculations are performed in the one-dimensional time-dependent model with allowance for the time-varying energy deposition in the channel, the channel expansion, and the nonequilibrium ionization kinetics in the leader plasma. The calculations show that, at a gas temperature of 4500–6000 K, associative ionization becomes a dominant ionization mechanism in the leader channel; as a result, the electric field decreases to 100–200 V/cm in 10−4–10−3 s under the conditions typical of the leader discharge. The calculated electric field agrees well with the data from the experimental modeling of long leaders by a spark discharge in short gaps.

Streamer breakdown of long gas gaps

Abstract: Results obtained from numerical simulations of the streamer breakdown of long (longer than 10 cm) gas gaps at atmospheric pressure are reviewed. Most attention is focused on air under normal conditions and at elevated temperatures characteristic of the rebreakdown in the postspark channel that is cooled after the primary spark discharge has come to an end. The main stages of the evolution of a streamer discharge into an arc are considered, and the features of this phenomenon are analyzed as functions of the initial conditions. The main macroscopic processes that govern the composition and dynamics of the streamer plasma in different discharge stages are revealed. The experimental data and the results of computer simulations provide evidence for a nonthermal mechanism for streamer breakdown in noble gases.

Equation of State of UO2

Abstract: An international project supported by INTAS (International Association for Promotion of Cooperation with Scientists from the New Independent States of the former Soviet Union) was started in 1994 with the intent of constructing an equation of state (EOS) for liquid and gaseous UO2, which fully reproduces the comprehensive thermodynamic database for this compound. The new equation of state was devised for applications encompassing hypo- and hyper-stoichiometric compositions. A so-called “chemical model” was used for the theoretical description of liquid urania. This model is based on the thermodynamic perturbation theory (TPT) modified in order to account for the specific properties of the system investigated. It describes, in a unified formalism, a multicomponent mixture of chemically reactive, strongly interacting neutral and charged molecules and atoms. Comparisons of the predicted equilibrium vapor pressures with literature data provided an initial validation of the model up to temperatures of the order of 5500 K. A further, positive result is the fairly good agreement of the predicted heat capacity with experimental values, which extend up to 8000 K. A characteristic feature of non-congruentvaporization in UO2±x is the production of a very high maximum vapor pressure (P max∼1 GPa) as well as a substantial oxygen enrichment of the vapor phase over boiling UO2 ((O/U)max∼7). The critical point of a truly non-congruent phase transition in UO2 was also calculated. This point essentially differs from that defined for a gas–liquid phase transition in simple liquids; in particular, the equation (∂P/∂V) c ∼(P/V)≠0 applies here. The predicted critical parameters are: T c≈10120 K, P c≈965 MPa, ρ c≈2.61 g·cm−3.

Experiments on passive hypervelocity boundary layer control using a porous surface

Abstract: Recently performed linear stability analyses suggested that transition could be delayed in hypersonic boundary layers by using an ultrasonically absorptive surface that would damp the second mode (Mack mode). Boundary layer transition experiments were performed on a 5.06° half-angle cone at zero angle-of-attack in the T5 Hypervelocity Shock Tunnel in order to test this concept. The cone was constructed with a smooth surface on half the cone (to serve as a control) and an acoustically absorptive porous surface on the other half. It was instrumented with flush-mounted thermocouples to detect the transition location. Test gases investigated included nitrogen and carbon dioxide at MQQ ~ 5 with specific reservoir enthalpy ranging from 1.3 MJ/kg to 13.0 MJ/kg and reservoir pressure ranging from 9.0 MPa to 50 MPa. Detailed comparisons were performed to insure that previous results obtained in similar boundary layer transition experiments (on a regular smooth surface) were reproduced and the results were extended to examine the effects of the porous surface. These experiments indicated that the porous surface was highly effective in delaying transition provided that the pore size was significantly smaller than the viscous length scale.

An algorithm for the numerical solution of linear differential games

Abstract: A numerical algorithm for the construction of stable Krasovskii bridges, Pontryagin alternating sets, and also of piecewise program strategies solving two-person linear differential (pursuit or evasion) games on a fixed time interval is developed on the basis of a general theory. The aim of the first player (the pursuer) is to hit a prescribed target (terminal) set by the phase vector of the control system at the prescribed time. The aim of the second player (the evader) is the opposite. A description of numerical algorithms used in the solution of differential games of the type under consideration is presented and estimates of the errors resulting from the approximation of the game sets by polyhedra are presented.

Rydberg matter as a metastable state of strongly nonideal plasma

Abstract: The phase diagram of cesium is complemented by an isolated region of metastable states of a strongly nonideal plasma.

On Stochastic Mixing Caused by the Rayleigh Taylor Instability

Abstract: The mixing of contacting substances is considered The evolution of the mixing layer over a long time period from multimode initial perturbations is investigated numerically in the short-scale and wide-range cases In the case of a short-scale initiation, the flow is stochastic in the sense that the time of the considered evolution exceeds the period of correlation The effect of the amplitude of wide-range perturbations on the dynamics of mixing is analyzed The scale-invariant properties of the spectral and statistical parameters of turbulent mixing are investigated for the first time The universal spectra characterizing the turbulence mixing in the entire self-similar interval on a unified basis are obtained The simulation is based on the effective algorithms with high approximating qualities, which have been tested earlier

"Green" noise in quasistationary stochastic systems.

Abstract: We consider the behavior of stochastic systems driven by noise with a zero value of spectral density at zero frequency (“green” noise). For this purpose we propose the version of the Krylov–Bogoliubov averaging method to study the systems which are not stationary in the case of an external white noise. We use the ergodicity of a nonlinear random function in the method, and obtain equations for any approximation of the theory. In particular, it is shown in the first approximation that there is an effective potential to describe the averaged motion of the system. We consider a phase-locked loop as an example and show that metastable states are possible. The lifetime of these states essentially increases if the form of a green noise spectrum becomes sharper in the low-frequency region. The high stability of the system driven by green noise is confirmed by numerical simulation. It is important that the theoretical result obtained by the averaging method and the one obtained in the simulation coincide with sufficient accuracy. In conclusion, we discuss some of the unsolved green noise problems.