Showing papers by "Moscow Institute of Physics and Technology published in 2008"

Nanosecond-Pulsed Discharges for Plasma-Assisted Combustion and Aerodynamics

Abstract: The efficiency of nanosecond discharges as an active-particle generator for plasma-assisted combustion and ignition has been shown. The kinetics of alkane oxidation have been investigated from methane to decane in stoichiometric and lean mixtures with oxygen and air at room temperature under the action of high-voltage nanosecond unform discharge. The study of nanosecond barrier discharge influence on a flame propagation and flame blowoff velocity has been carried out. A significant increase of the flame blowoff velocity has been demonstrated. A decrease of 2-3 orders of magnitude of the plasma-assisted ignition delay time in comparison with the autoignition has been registered. Detonation initiating by high-voltage gas discharge has been demonstrated. The energy deposition in the discharge ranging from 70 mJ to 12 J for propane-oxygen-nitrogen mixtures leads to the transition to detonation at a distance of less than one diameter of the detonation tube. The influence of pulsed surface dielectric discharge on the flow separation for airfoils at a high angle of attack has been investigated within the velocity range from 20 to 110 m/s for the power consumption less than 1 W/cm of the wing span. The conclusion has been made that the main mechanism of plasma impact is the boundary-layer turbulization rather than acceleration.

On existence of self-tuning solutions in static braneworlds without singularities

Abstract: A static self-tuning SO(3) × 2 symmetric and translation invariant braneworld setup with flat brane is considered. We discuss the null energy conditions (NEC) for matter on the brane and in the bulk and prove that for the static regular background with broken Lorentz invariance the NEC and positiveness of the total energy density on the brane and NEC in the bulk cannot be satisfied simultaneously. Then we give some examples and elaborate some special cases.

Receptivity of a hypersonic boundary layer over a flat plate with a porous coating

Abstract: Two-dimensional direct numerical simulation (DNS) of receptivity to acoustic disturbances radiating onto a flat plate with a sharp leading edge in the Mach 6 free stream is carried out. Numerical data obtained for fast and slow acoustic waves of zero angle of incidence are consistent with the asymptotic theory. Numerical experiments with acoustic waves of non-zero angles of incidence reveal new features of the disturbance field near the plate leading edge. The shock wave, which is formed near the leading edge owing to viscous–inviscid interaction, produces a profound effect on the acoustic near field and excitation of boundary-layer modes. DNS of the porous coating effect on stability and receptivity of the hypersonic boundary layer is carried out. A porous coating of regular porosity (equally spaced cylindrical blind micro-holes) effectively diminishes the second-mode growth rate in accordance with the predictions of linear stability theory, while weakly affecting acoustic waves. The coating end effects, associated with junctures between solid and porous surfaces, are investigated.

Error and erasure correcting algorithms for rank codes

Abstract: In this paper, transmitted signals are considered as square matrices of the Maximum rank distance (MRD) (n, k, d)-codes. A new composed decoding algorithm is proposed to correct simultaneously rank errors and rank erasures. If the rank of errors and erasures is not greater than the Singleton bound, then the algorithm gives always the correct decision. If it is not a case, then the algorithm gives still the correct solution in many cases but some times the unique solution may not exist.

Generation of fast charged particles and superstrong magnetic fields in the interaction of ultrashort high-intensity laser pulses with solid targets

Abstract: Recent experimental and theoretical investigations are reviewed concerning the generation of fast charged particles and superstrong magnetic fields in the interaction of ultrashort laser pulses with solid targets. The mechanisms of generating fast charged particles in superstrong light fields of laser radiation with intensities ranging from 1017 to 1021 W cm–2 are considered. Electron acceleration due to vacuum heating, the ponderomotive potential, resonance absorption, the laser-driven wake field in the underdense part of plasma, cyclotron mechanism and some other mechanisms are thoroughly analyzed. Experimental data on the acceleration of protons and atomic ions by spatial charge fields on thin and thick solid targets are presented and theoretically interpreted. Particular attention is paid to the generation of superstrong quasistatic magnetic fields in laser plasmas and methods for measuring them under the action of various laser pulses of both femto- and picosecond durations. The possible formation of magnetic plasma configurations and magnetic plasma confinement are discussed.

Modeling Toroidal Networks with the Gaussian Integers

Abstract: In this paper we consider a broad family of toroidal networks, denoted as Gaussian networks, which include many previously proposed and used topologies. We will define such networks by means of the Gaussian integers, the subset of the complex numbers with integer real and imaginary parts. Nodes in Gaussian networks are labeled by Gaussian integers, which confer these topologies an algebraic structure based on quotient rings of the Gaussian integers. In this sense, Gaussian integers reveal themselves as the appropriate tool for analyzing and exploiting any type of toroidal network. Using this algebraic approach, we can characterize the main distance-related properties of Gaussian networks, providing closed expressions for their diameter and average distance. In addition, we solve some important applications, like unicast and broadcast packet routing or the perfect placement of resources over these networks.

Molecular-dynamics simulation of edge-dislocation dynamics in aluminum

Abstract: The ability of a crystal to exhibit plastic deformation is related to the presence of dislocations in the crystal lattice. The motion of dislocations provides for the formation of a real atomic structure in crystalline solids and the kinetics of deformation of crystals under load; it underlies the control of many important physical properties of solids [1‐3]. It is known that the dependence of the yield stress on deformation rate in many metals sharply increases when the deformation rate exceeds ~10 3 –10 4 s ‐1 . This phenomenon can be interpreted as the consequence of a change in the mechanism of dislocation motion. Moving at low velocities, dislocations overcome obstacles as a result of the joint action of the applied stress and thermal fluctuations. Due to this, an increase in the temperature is accompanied by a decrease in the yield stress of the material. For a high-rate deformation, it is necessary to apply higher stresses. At a deformation rate exceeding a certain threshold ( ~10 4 s ‐1 for pure metals), the acting stresses prove to be sufficient for the dynamic overcoming of obstacles without an additional contribution from thermal fluctuations. In this case, the pumping of the dislocation energy to the crystal lattice vibrations or, depending on temperature, to the electron subsystem becomes the dominating mechanism of the retardation of dislocations. In contrast to the region of thermofluctuational mobility, the dislocation velocity in the dynamic region decreases with temperature in accordance with an increase in the density of the gas of elementary excitations. For this reason, an anomalous increase in the yield stress with increasing temperature is observed for some materials at very high rates of deformation [1]. In this study, we have compared the results of simulation with the values of the dynamic yield stress for single-crystalline aluminum in the shock-wave experiments, which offer a powerful method of studying the properties of materials dynamically loaded under well controllable conditions. The behavior of materials under high-rate deformation in shock-wave experiments is very diverse, which is manifested both in the temperature dependence of the yield stress and in the character of deformation in the samples upon storage [1, 4].

The structure of gasdynamic flow in a supersonic separator of natural gas

Abstract: Results are given of calculations of flow within the two-dimensional Euler model of supersonic swirling flow of gas in a supersonic separator of natural gas. The formulation of the problem is given, numerical experiment is performed, and the basic parameters of gas flow (velocity components, pressure, and so on) are obtained as functions of radius. The process of relaxation of flow to steady state with the formation of shock wave is considered, and the shock wave structure is determined. The behavior of gasdynamic parameters is analyzed under conditions of separation in the region of shock wave and behind it.

Corona initiated from grounded objects under thunderstorm conditions and its influence on lightning attachment

Abstract: Lightning attachment to grounded structures due to the initiation of an upward connecting leader from them is considered taking into account the effect of corona space charge near the structures. It is shown that the corona space charge strongly affects the initiation and development of the connecting leader. Specific features of a non-stationary corona are analysed analytically and numerically for one-dimensional electrode geometries and for a grounded rod coronating in a slowly varying thundercloud electric field that can be enhanced by the charge of an approaching downward lightning leader. Initiation and development of an upward connecting leader or upward lightning from high ground objects are investigated. Prospects of using the effect of coronae to control downward lightning discharges are discussed.

Simultaneous generation of a proton beam and terahertz radiation in high-intensity laser and thin-foil interaction

Abstract: We have observed simultaneously both the fast proton generation and terahertz (THz) radiation in the laser pulse interaction with a 5-μm thick titanium target. In order to control the proton acceleration and THz radiation, we have changed the duration of the amplified spontaneous emission (ASE) preceding the main pulse generated by the high-intensity Ti:sapphire laser. A fast proton beam with the maximal energy of ∼ 490 keV has been realized by reducing the duration of the ASE. Simultaneously, an intense emission of THz radiation is observed for various ASE durations. We propose the antenna mechanism for the THz radiation, according to which the fast electrons moving along the target surface emit the low-frequency electromagnetic wave.

Numerical simulation of a surface barrier discharge in air

Abstract: The development of a surface barrier discharge in air at atmospheric pressure under the action of a constant voltage of different polarity is simulated numerically. When the polarity of the high-voltage electrode is negative, the discharge develops as an ionization wave that moves along the dielectric surface. When the polarity is positive, the discharge develops as a streamer that first moves above the dielectric surface and then comes into contact with and continues to develop along it. In the case of a high-voltage electrode of positive polarity, the discharge zone above the dielectric surface is approximately five times thicker than that in the case of negative polarity. The characteristic aspects of numerical simulation of the streamer phase of a surface barrier discharge are discussed. The numerical results on the density of the charge stored at the dielectric surface and on the length of the discharge zone agree with the experimental data.

Attacks and counter-attacks on the GPT public key cryptosystem

Abstract: The public key cryptosystem based on rank error correcting codes (the GPT cryptosystem) was proposed in 1991. Several attacks against this system were published, including Gibson's attacks and recent Overbeck's attacks. In this paper, we improve the GPT system by more careful choice of parameters to withstand these attacks.

A continuous-wave Fe2+:ZnSe laser

Abstract: A continuous-wave oscillation is obtained for the first time in a Fe2+:ZnSe laser. The laser wavelength was in the range from 4.04 to 4.08 μm. A liquid-nitrogen-cooled active element was pumped by a Cr2+:CdSe laser at 2.97 μm. The maximum output power of the laser was 160 mW with the 56% slope efficiency. The minimum absorbed pump power threshold was 18 mW. The intrinsic losses in the Fe2+:ZnSe crystal did not exceed 0.024 cm-1 during lasing.

Interaction of electromagnetic waves with caustics in plasma flows.

Abstract: An electromagnetic wave (EMW) interacting with the moving singularity of the charged particle flux undergoes the reflection and absorption as well as frequency change due to Doppler effect and nonlinearity. The singularity corresponding to a caustic in plasma flow with inhomogeneous velocity can arise during the breaking of the finite amplitude Langmuir waves due to nonlinear effects. A systematic analysis of the wave-breaking regimes and caustics formation is presented and the EMW reflection coefficients are calculated.

Efficient pulsed Cr2+ : CdSe laser continuously tunable in the spectral range from 2.26 to 3.61 μm

Abstract: The efficient lasing of a Cr2+:CdSe single crystal pumped by 1.94-μm, 300-μs pulses from a Tm:YAP laser was obtained. The Cr2+:CdSe laser with a nonselective resonator emitted up to 17 mJ at a wavelength of ~2.65 μm with the quantum slope efficiency of 63% with respect to the absorbed pump energy. The absorption coefficient of the Cr2+:CdSe crystal at the laser wavelength did not exceed 0.045 cm-1. By using a resonator with a dispersion prism, the laser wavelength was continuously tuned in the spectral range from 2.26 to 3.61 μm.

Analogue of the identity Log Det = Trace Log for resultants

Abstract: Resultant $R_{r_1, ..., r_n}$ defines a condition of solvability for a system of $n$ homogeneous polynomials of degrees $r_1, ..., r_n$ in $n$ variables, just in the same way as determinant does for a system of linear equations. Because of this, resultants are important special functions of upcoming non-linear physics and begin to play a role in various topics related to string theory. Unfortunately, there is a lack of convenient formulas for resultants when the number of variables is large. To cure this problem, we generalize the well-known identity Log Det = Trace Log from determinants to resultants. The generalized identity allows to obtain explicit polynomial formulas for multidimensional resultants: for any number of variables, resultant is given by a Schur polynomial. We also give several integral representations for resultants, as well as a sum-over-paths representation.

Cavitation in liquid metals under negative pressures. Molecular dynamics modeling and simulation

Abstract: An approach to study cavitation in stretched liquids via molecular dynamics (MD) simulation is presented. It is based on the stochastic properties of MD and allows one to study cavitation as a stochastic phenomenon. The approach is used to study equation of state and stability limits of the metastable liquid phase, cavitation kinetics and dynamics properties for different temperatures. Particular examples of metals under consideration include Pb, Li and Pb83Li17. Quantitative and qualitative disagreements between the classic nucleation theory estimates and the MD results are found. The Kolmogorov‐Johnson‐Mehl‐Avrami equation is used as an alternative way to estimate cavitation rate. The two methods show good mutual agreement. Decay at a constant stretching rate is also considered.

Modeling hexagonal constellations with Eisenstein-Jacobi graphs

Abstract: A set of signal points is called a hexagonal constellation if it is possible to define a metric so that each point has exactly six neighbors at distance 1 from it. As sets of signal points, quotient rings of the ring of Eisenstein-Jacobi integers are considered. For each quotient ring, the corresponding graph is defined. In turn, the distance between two points of a quotient ring is defined as the corresponding graph distance. Under certain restrictions, a quotient ring is a hexagonal constellation with respect to this metric. For the considered hexagonal constellations, some classes of perfect codes are known. Using graphs leads to a new way of constructing these codes based on solving a standard graph-theoretic problem of finding a perfect dominating set. Also, a relation between the proposed metric and the well-known Lee metric is considered.

Metal-insulator transition in two dimensions: experimental test of the two-parameter scaling.

Abstract: We report a detailed scaling analysis of resistivity rho(T,n) measured for several high-mobility 2D electron systems in the vicinity of the 2D metal-insulator transition. We analyzed the data using the two-parameter scaling approach and general scaling ideas. This enables us to determine the critical electron density, two critical indices, and temperature dependence for the separatrix in the self-consistent manner. In addition, we reconstruct the empirical scaling function describing a two-parameter surface which fits well the rho(T,n) data.

Stability of Temporally Evolving Supersonic Boundary Layers over Micro-Cavities for Ultrasonic Absorptive Coatings

Abstract: Ultrasonic absorptive coatings, consisting of regularly-spaced arrays of micro-cavities, have previously been shown to effectively damp second-mode instability for the purpose of delaying transition in hypersonic boundary layers. However, previous simulations and stability analysis have used approximate porous-wall boundary conditions. Here we investigate the feasibility of using direct numerical simulation to directly compute the hypersonic boundary layer including the micro-cavities. In order to keep the problem computationally tractable, we restrict our attention to the two-dimensional case (which is relevant since the second-mode is initially two dimensional), and we show that temporally evolving layers display qualitatively similar behavior to spatially developing boundary layer and instabilities. We validate the numerical method by comparing the simulation results to temporal linear stability analysis of the (frozen) velocity and temperature profiles from the direct numerical simulation. Two-dimensional linear simulations of the boundary layer on a flat plate and over a porous coating are performed, and it is shown that the presence of the cavities attenuates the instability waves, as expected from theory.

Proximity-induced superconductivity in graphene

Abstract: We propose a way of making graphene superconductive by putting on it small superconductive islands which cover a tiny fraction of graphene area. We show that the critical temperature, T c , can reach several Kelvins at the experimentally accessible range of parameters. At low temperatures, T T c , and zero magnetic field, the density of states is characterized by a small gap E g ≤ T c resulting from the collective proximity effect. Transverse magnetic field H g (T) ∝ E g is expected to destroy the spectral gap driving graphene layer to a kind of a superconductive glass state. Melting of the glass state into a metal occurs at a higher field H g2(T).

Testbed for wireless vehicle communication: a simulation approach based on three-phase traffic theory

Abstract: A testbed for wireless vehicle communication based on a microscopic model in the framework of three-phase traffic theory is presented. In this testbed, vehicle motion in traffic flow and analyses of a vehicle communication channel access based on IEEE 802.11e mechanisms, radio propagation modeling, message reception characteristics as well as all other effects associated with ad-hoc networks are integrated into a three-phase traffic flow model. Thus simulations of both vehicle ad-hoc network and traffic flow are integrated onto a single testbed and perform simultaneously. This allows us to make simulations of ad-hoc network performance as well as diverse scenarios of the effect of wireless vehicle communications on traffic flow during simulation times, which can be comparable with real characteristic times in traffic flow. In addition, the testbed allows us to simulate cooperative vehicle motion together with various traffic phenomena, like traffic breakdown at bottlenecks. Based on simulations of this testbed, some statistical features of ad-hoc vehicle networks as well as the effect of C2C communication on increase in the efficiency and safety of traffic are studied.

Parametric studies of hypersonic laminar flow control using a porous coating of regular microstructure

Abstract: To aid in the design of an ultrasonically absorptive coating (UAC) to be tested on a 7degree half-angle sharp cone in the CUBRC LENS I shock tunnel, parametric studies of the coating laminar-flow-control performance are conducted for Mach=7 and Mach=10 freestream conditions. The second-mode amplification factors, N, are calculated using the reduced-order computational package that includes the compressible Blasius mean flow and the local-parallel linear stability solver. These N-factors agree well with those predicted by the STABL solver that opens up an opportunity to conduct quick turn-around computations of the UAC performance. Stability calculations are carried out for the uncoated (solid) and coated (porous) wall. A porous coating of regular microstructure, which comprises equally spaced vertical cylindrical blind micro-holes of fixed radius, spacing and depth, is analyzed. The UAC parameters, at which the coating massively suppresses the second mode and can lead to significant (more than twice) increase of the laminar run, are predicted. Estimates of the UAC roughness effect indicate that the coating can be treated as aerodynamically smooth in the unit Reynolds number range required for transition experiments. It is shown that the UAC performance strongly increases with porosity. In this connection, it is suggested to investigate a rectangular or honeycomb patterns, which allow for coatings of substantially higher porosity compared with pores of circular cross-section.

Atomistic simulation of plasticity and fracture of nanocrystalline copper under high-rate tension

Abstract: A molecular dynamics simulation of the plastic deformation and the onset of fracture of nanocrystalline metals is performed using the example of copper. Successive stages of the response of the microstructure of a metal to deformation are considered, namely, grain boundary sliding, the nucleation and gliding of dislocations, and the formation and growth of microdamage nuclei. The influence of the grain size of a nanocrystal on its plasticity and strength is studied.

Interaction of Acoustic Disturbances with Micro-Cavities for Ultrasonic Absorptive Coatings

Abstract: Numerical simulations are performed to investigate the interaction of acoustic waves with an array of equally-spaced two-dimensional micro-cavities on an otherwise flat plate without external boundary-layer flow. This acoustic scattering problem is important in the design of ultrasonic absorptive coatings (UAC) for hypersonic laminar flow control. The reflection coefficient, characterizing the ratio of the reflected wave amplitude to the incident wave amplitude, is computed as a function of the acoustic wave frequency and angle of incidence, for coatings of different porosity, at various acoustic Reynolds numbers relevant to hypersonic flight. Overall, the numerical results validate predictions from existing theoretical modeling. In general, the amplitude of the reflection coefficient has local minima at some specific frequencies. A simple model to predict these frequencies is presented. The simulations also highlight the presence of resonant acoustic modes caused by coupling of small-scale scattered waves near the UAC surface. Finally, the cavity depth and the porosity are identified as the most important parameters for UAC design. Guidelines for the choice of these parameters are suggested.