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  • 1. Dumanov, E. V.
    et al.
    Podlesny, I. V.
    Moskalenko, S. A.
    Liberman, Michael A.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Interaction of two-dimensional magnetoexcitons2017In: Physica. E, Low-Dimensional systems and nanostructures, ISSN 1386-9477, E-ISSN 1873-1759, Vol. 88, p. 77-86Article in journal (Refereed)
    Abstract [en]

    We study interaction of the two-dimensional magnetoexcitons with in-plane wave vector (k) over right arrow (parallel to) = 0, taking into account the influence of the excited Landau levels (ELLS) and of the external electric field perpendicular to the surface of the quantum well and parallel to the external magnetic field., It is shown that the account of the ELLS gives rise to the repulsion between the spinless magnetoexcitons with (k) over right arrow (parallel to) = 0 in the Fock approximation, with the interaction constant g decreasing inverse proportional to the magnetic field strength B (g(0) similar to 1/B). In the presence of the perpendicular electric field the Rashba spin-orbit coupling (RSOC), Zeeman splitting (ZS) and nonparabolicity of the heavy-hole dispersion law affect the Landau quantization of the electrons and holes. They move along the new cyclotron orbits, change their Coulomb interactions and cause the interaction between 21) magnetoexcitons with (k) over right arrow (parallel to) = 0. The changes of the Coulomb interactions caused by the electrons and by the holes moving with new cyclotron orbits are characterized by some coefficients, which in the absence of the electric field turn to be unity. The differences between these coefficients of the electron-hole pairs forming the magnetoexcitons determine their affinities to the interactions. The interactions between the homogeneous, semihomogeneous and heterogeneous magnetoexcitons forming the symmetric states with the same signs of their affinities are attractive whereas in the case of different sign affinities are repulsive. In the heterogeneous asymmetric states the interactions have opposite signs in comparison with the symmetric states. In all these cases the interaction constant g have the dependence g(0) - 1/root B.

  • 2. Elperin, T.
    et al.
    Kleeorin, N.
    Krasovitov, B.
    Kulmala, M.
    Liberman, Michael
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Moscow Institute of Physics and Technology, Russia.
    Rogachevskii, I.
    Zilitinkevich, S.
    Acceleration of raindrop formation due to the tangling-clustering instability in a turbulent stratified atmosphere2015In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 92, no 1, article id 013012Article in journal (Refereed)
    Abstract [en]

    Condensation of water vapor on active cloud condensation nuclei produces micron-size water droplets. To form rain, they must grow rapidly into at least 50-to 100-mu m droplets. Observations show that this process takes only 15-20 min. The unexplained physical mechanism of such fast growth is crucial for understanding and modeling of rain and known as condensation-coalescence bottleneck in rain formation. We show that the recently discovered phenomenon of the tangling clustering instability of small droplets in temperature-stratified turbulence [Phys. Fluids 25, 085104 (2013)] results in the formation of droplet clusters with drastically increased droplet number densities. The mechanism of the tangling clustering instability is much more effective than the previously considered by us the inertial clustering instability caused by the centrifugal effect of turbulent vortices. This is the reason of strong enhancement of the collision-coalescence rate inside the clusters. The mean-field theory of the droplet growth developed in this study can be useful for explanation of the observed fast growth of cloud droplets in warm clouds from the initial 1-mu m-size droplets to 40- to 50-mu m-size dropletswithin 15-20 min.

  • 3. Elperin, T.
    et al.
    Kleeorin, N.
    Liberman, Michael
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Lipatnikov, A. N.
    Rogachevskii, I.
    Yu, R.
    Turbulent diffusion of chemically reacting flows: Theory and numerical simulations2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 96, no 5, article id 053111Article in journal (Refereed)
    Abstract [en]

    The theory of turbulent diffusion of chemically reacting gaseous admixtures developed previously [T. Elperin et al., Phys. Rev. E 90, 053001 (2014)] is generalized for large yet finite Reynolds numbers and the dependence of turbulent diffusion coefficient on two parameters, the Reynolds number and Damkohler number (which characterizes a ratio of turbulent and reaction time scales), is obtained. Three-dimensional direct numerical simulations (DNSs) of a finite-thickness reaction wave for the first-order chemical reactions propagating in forced, homogeneous, isotropic, and incompressible turbulence are performed to validate the theoretically predicted effect of chemical reactions on turbulent diffusion. It is shown that the obtained DNS results are in good agreement with the developed theory.

  • 4. Elperin, T.
    et al.
    Kleeorin, Nathan
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel; N. I. Lobachevsky State University of Nizhny Novgorod, Russia.
    Liberman, Michael
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Moscow Institute of Physics and Technology, Russia.
    Rogachevskii, Igor
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel; N. I. Lobachevsky State University of Nizhny Novgorod, Russia.
    Tangling clustering instability for small particles in temperature stratified turbulence2013In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, no 8, article id 085104Article in journal (Refereed)
    Abstract [en]

    We study tangling clustering instability of inertial particles in a temperature stratified turbulence with small finite correlation time. It is shown that the tangling mechanism in the temperature stratified turbulence strongly increases the degree of compressibility of particle velocity field. This results in the strong decrease of the threshold for the excitation of the tangling clustering instability even for small particles. The tangling clustering instability in the temperature stratified turbulence is essentially different from the inertial clustering instability that occurs in non-stratified isotropic and homogeneous turbulence. While the inertial clustering instability is caused by the centrifugal effect of the turbulent eddies, the mechanism of the tangling clustering instability is related to the temperature fluctuations generated by the tangling of the mean temperature gradient by the velocity fluctuations. Temperature fluctuations produce pressure fluctuations and cause particle accumulations in regions with increased instantaneous pressure. It is shown that the growth rate of the tangling clustering instability is root Re (l(0)/L-T)(2)/(3Ma)(4) times larger than that of the inertial clustering instability, where Re is the Reynolds number, Ma is the Mach number, l(0) is the integral turbulence scale, and L-T is the characteristic scale of the mean temperature variations. It is found that depending on the parameters of the turbulence and the mean temperature gradient there is a preferential particle size at which the particle clustering due to the tangling clustering instability is more effective. The particle number density inside the cluster after the saturation of this instability can be by several orders of magnitude larger than the mean particle number density. It is also demonstrated that the evaporation of droplets drastically changes the tangling clustering instability, e. g., it increases the instability threshold in the droplet radius. The tangling clustering instability is of a great importance, e. g., in atmospheric turbulence with temperature inversions.

  • 5. Ivanov, Mikhail F.
    et al.
    Kiverin, Alexey D.
    Liberman, Mikhail A.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Ignition of deflagration and detonation ahead of the flame due to radiative preheating of suspended micro particles2015In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 162, no 10, p. 3612-3621Article in journal (Refereed)
    Abstract [en]

    We study a flame propagating in the gaseous combustible mixture with suspended inert solid micro particles. The gaseous mixture is assumed to be transparent for thermal radiation emitted by the hot combustion products, while particles absorb and reemit the radiation. Thermal radiation heats the particles, which in turn transfer the heat to the surrounding unburned gaseous mixture by means of thermal heat transfer, so that the gas phase temperature lags that of the particles. We consider different scenarios depending on the spatial distribution of the particles, their size and the number density. In the case of uniform spatial distribution of the particles the radiation causes a modest increase of the temperature ahead of the flame and corresponding modest increase of the combustion velocity. In the case of non-uniform distribution of the particles (layered dust cloud), such that the particles number density is relatively small in the region just ahead of the flame front and increases in the distant regions ahead of the flame, the preheating caused by the thermal radiation may trigger additional independent source of ignition. Far ahead of the flame, where number density of particles increases forming a dense cloud of particles, the radiative preheating results in the formation of a temperature gradient with the maximum temperature sufficient for ignition. Depending on the steepness of the temperature gradient formed in the unburned mixture, either deflagration or detonation can be initiated via the Zel'dovich's gradient mechanism. The ignition and the resulting combustion regimes depend on the number density profile and, correspondingly, on the temperature profile (temperature gradient), which is formed in effect of radiation absorption and gas-dynamic expansion. The effect of radiation preheating as stronger as smaller is the normal flame velocity. The effect of radiation heat transfer in the case of coal dust flames propagating in layered particle-gas deposits cloud can result in the spread of combustion wave with velocity up to 1000 m/s and it is a plausible explanation of the origin of dust explosion in coal mines.

  • 6.
    Liberman, Michael A.
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kleeorin, Nathan
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel.
    Rogachevskii, Igor
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel.
    Haugen, Nils Erland L.
    Multipoint radiation induced ignition of dust explosions: turbulent clustering of particles and increased transparency2018In: Combustion theory and modelling, ISSN 1364-7830, E-ISSN 1741-3559, Vol. 22, no 6, p. 1084-1102Article in journal (Refereed)
    Abstract [en]

    Understanding the causes and mechanisms of large explosions, especially dust explosions, is essential for minimising devastating hazards in many industrial processes. It is known that unconfined dust explosions begin as primary (turbulent) deflagrations followed by a devastating secondary explosion. The secondary explosion may propagate with a speed of up to 1000 m/s producing overpressures of over 8-10 atm, which is comparable with overpressures produced in detonation. Since detonation is the only established theory that allows rapid burning producing a high pressure that can be sustained in open areas, the generally accepted view was that the mechanism explaining the high rate of combustion in dust explosions is deflagration-to-detonation transition. In the present work we propose a theoretical substantiation of an alternative mechanism explaining the origin of the secondary explosion producing high speeds of combustion and high overpressures in unconfined dust explosions. We show that the clustering of dust particles in a turbulent flow ahead of the advancing flame front gives rise to a significant increase of the thermal radiation absorption length. This effect ensures that clusters of dust particles are exposed to and heated by radiation from hot combustion products of dust explosions for a sufficiently long time to become multi-point ignition kernels in a large volume ahead of the advancing flame. The ignition times of a fuel-air mixture caused by radiatively heated clusters of particles is considerably reduced compared with the ignition time caused by an isolated particle. Radiation-induced multipoint ignitions of a large volume of fuel-air ahead of the primary flame efficiently increase the total flame area, giving rise to the secondary explosion, which results in the high rates of combustion and overpressures required to account for the observed level of overpressures and damage in unconfined dust explosions, such as for example the 2005 Buncefield explosion and several vapour cloud explosions of severity similar to that of the Buncefield incident.

  • 7.
    Liberman, Michael
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Kleeorin, Nathan
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel.
    Rogachevskii, Igor
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel.
    Haugen, Nils Erland L.
    Mechanism of unconfined dust explosions: Turbulent clustering and radiation-induced ignition2017In: Physical review. E, ISSN 2470-0045, E-ISSN 2470-0053, Vol. 95, no 5, article id 051101Article in journal (Refereed)
    Abstract [en]

    It is known that unconfined dust explosions typically start off with a relatively weak primary flame followed by a severe secondary explosion. We show that clustering of dust particles in a temperature stratified turbulent flow ahead of the primary flame may give rise to a significant increase in the radiation penetration length. These particle clusters, even far ahead of the flame, are sufficiently exposed and heated by the radiation from the flame to become ignition kernels capable to ignite a large volume of fuel-air mixtures. This efficiently increases the total flame surface area and the effective combustion speed, defined as the rate of reactant consumption of a given volume. We show that this mechanism explains the high rate of combustion and overpressures required to account for the observed level of damage in unconfined dust explosions, e.g., at the 2005 Buncefield vapor-cloud explosion. The effect of the strong increase of radiation transparency due to turbulent clustering of particles goes beyond the state of the art of the application to dust explosions and has many implications in atmospheric physics and astrophysics.

  • 8.
    Liberman, Michael
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Wang, Cheng
    Qian, Chengeng
    Liu, JianNan
    Influence of chemical kinetics on spontaneous waves and detonation initiation in highly reactive and low reactive mixtures2019In: Combustion theory and modelling, ISSN 1364-7830, E-ISSN 1741-3559, Vol. 23, no 3, p. 467-495Article in journal (Refereed)
    Abstract [en]

    Understanding the mechanisms of explosions is important for minimising devastating hazards. Due to the complexity of real chemistry, a single-step reaction mechanism is usually used for theoretical and numerical studies. The purpose of this study is to look more deeply into the influence of chemistry on detonation initiated by a spontaneous wave. The results of high-resolution simulations performed for one-step models are compared with simulations for detailed chemical models for highly reactive and low reactive mixtures. The calculated induction times for H-2/air and for CH4/air are validated against experimental measurements for a wide range of temperatures and pressures. It is found that the requirements in terms of temperature and size of the hot spots, which can produce a spontaneous wave capable to initiate detonation, are quantitatively and qualitatively different for one-step models compared to detailed chemical models. The time and locations when the exothermic reaction affects the coupling between the pressure wave and spontaneous wave are considerably different for a one-step and detailed models. The temperature gradients capable to produce detonation and the corresponding size of hot spots are much shallower and, correspondingly, larger than those predicted using one-step models. The impact of the detailed chemical model is particularly pronounced for the methane-air mixture. In this case, not only the hot spot size is much greater than that predicted by a one-step model, but even at the elevated pressure, the initiation of detonation by a temperature gradient is possible only if the temperature outside the gradient is rather high, so that can ignite a thermal explosion. The obtained results suggest that the one-step models do not reproduce correctly the transient and ignition processes, so that interpretation of the simulations performed using a one-step model for understanding mechanisms of flame acceleration, DDT and the origin of explosions must be considered with great caution.

  • 9. Moskalenko, S. A.
    et al.
    Khadzhi, P. I.
    Podlesny, I. V.
    Dumanov, E. V.
    Liberman, Michael A.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Zubac, I. A.
    Metastable Bound States of the Two-Dimensional Bimagnetoexcitons in the Lowest Landau Levels Approximation2018In: Semiconductors (Woodbury, N.Y.), ISSN 1063-7826, E-ISSN 1090-6479, Vol. 52, no 14, p. 1801-1805Article in journal (Refereed)
    Abstract [en]

    The possible existence of the bound states of the interacting two-dimensional (2D) magnetoexcitons in the lowest Landau levels (LLLs) approximation was investigated using the Landau gauge description. The magnetoexcitons taking part in the formation of the bound state with resultant wave vector have opposite in-plane wave vectors and and look as two electric dipoles with the arms oriented in-plane perpendicularly to the corresponding wave vectors. The bound state of two antiparallel dipoles moving with equal probability in any direction of the plane with equal but antiparallel wave vectors is characterized by the variational wave function of the relative motion depending on the modulus . The spins of two electrons and the effective spins of two holes forming the bound states were combined separately in the symmetric or in the antisymmetric forms with the same parameter for electrons and holes. In the case of the variational wave function the maximum density of the magnetoexcitons in the momentum space representation is concentrated on the in-plane ring with the radius The stable bound states of the bimagnetoexciton molecule do not exist for both spin orientations. Instead of them, a deep metastable bound state with the activation barrier comparable with the ionization potential of the magnetoexciton with was revealed in the case and . In the case and only a shallow metastable bound state can appear.

  • 10. Moskalenko, S. A.
    et al.
    Khadzhi, P.
    Podlesny, I.
    Dumanov, E.
    Liberman, Michael A.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Zubac, I. A.
    Metastable bound states of the interacting two-dimensional magnetoexcitons2018In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 283, p. 14-21Article in journal (Refereed)
    Abstract [en]

    The possible formation of two-dimensional (2D) magnetic biexcitons composed of two 2D magnetoexcitons with electrons and holes on the lowest Landau levels (LLLs) with opposite center-of-mass wave vectors (k)over-right-arrow and -(k)over-right-arrow and with antiparallel electric dipole moments perpendicular to the corresponding wave vectors was investigated. Two spinor structures of two electrons and of two holes were considered. In the singlet-singlet state the spins of two electrons as well as the effective spins of two holes create the combinations with the total spin S = 0 and its projection on the magnetic field S-z = 0. The triplet-triplet state corresponds to S = 1 and S-z = 0. Two orbital Gaussian variational wave functions depending on vertical bar(k)over-right-arrow vertical bar and describing the relative motion of two magnetoexcitons inside the molecule were used. Analytical calculations show that in the LLLs approximation the stable bound states of bimagnetoexcitons do not exist, but there is a metastable bound state with the orbital wave function, having the maximum on the in-plane ring for the triplet-triplet spin configuration. The metastable bound state has an energy activation barrier comparable with the magnetoexciton ionization potential and gives rise to the new luminescence band due to the metastable biexciton-para exciton conversion with the frequencies higher than those of the para magnetoexciton luminescence line.

  • 11. Moskalenko, S. A.
    et al.
    Podlesny, I. V.
    Dumanov, E. V.
    Liberman, Michael A.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Two-dimensional cavity polaritons under the influence of the perpendicular strong magnetic and electric fields. The gyrotropy effects2015In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 222, p. 58-64Article in journal (Refereed)
    Abstract [en]

    The properties of the two-dimensional cavity polaritons subjected to the action of a strong perpendicular magnetic and electric fields, giving rise to the Landau quantization (LQ) of the 2D electrons and holes accompanied by the Rashba spin-orbit coupling, by the Zeeman splitting and by the nonparabolicity of the heavy-hole dispersion law are investigated. We use the method proposed by Rashba (1960) [1] and the obtained results are based on the exact solutions for the eigenfunctions and for the eigenvalues of the Pauli-type Hamilonians with third order chirality terms and nonparabolic dispersion law for heavy-holes and with the first order chirality terms for electrons. The selection rules of the band-to-band optical quantum transitions as well as of the quantum transitions from the ground state of the crystal to the magnetoexciton states depend essentially on the numbers n(e) and n(h) of the LQ levels of the (e-h) pair forming the magnetoexciton. It is shown that the Rabi frequency Omega(R) of the polariton branches and the magnetoexciton oscillator strength f(osc), increase with the magnetic field strength B as Omega(R) similar to root B and f(osc) similar to B. The optical gyrotropy effects may be revealed changing the sign of the photon circular polarization at a given sign of the wave vector longitudinal projection k(z) or equivalently changing the sign of k(z) at the same selected circular polarization.

  • 12. Moskalenko, Sveatoslav A.
    et al.
    Podlesny, Igor V.
    Dumanov, Evgheni V.
    Liberman, Michael A.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Effects of Rashba spin-orbit coupling, Zeeman splitting and gyrotropy in two-dimensional cavity polaritons under the influence of the Landau quantization2015In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 88, no 9, article id 218Article in journal (Refereed)
    Abstract [en]

    We consider the energy spectrum of the two-dimensional cavity polaritons under the influence of a strong magnetic and electric fields perpendicular to the surface of the GaAs-type quantum wells (QWs) with p-type valence band embedded into the resonators. As the first step in this direction the Landau quantization (LQ) of the electrons and heavy-holes (hh) was investigated taking into account the Rashba spin-orbit coupling (RSOC) with third-order chirality terms for hh and with nonparabolicity terms in their dispersion low including as well the Zeeman splitting (ZS) effects. The nonparabolicity term is proportional to the strength of the electric field and was introduced to avoid the collapse of the semiconductor energy gap under the influence of the third order chirality terms. The exact solutions for the eigenfunctions and eigenenergies were obtained using the Rashba method [E.I. Rashba, Fiz. Tverd. Tela 2, 1224 (1960) [Sov. Phys. Solid State 2, 1109 (1960)]]. On the second step we derive in the second quantization representation the IIamiltonians describing the Coulomb electron-electron and the electron-radiation interactions. This allow us to determine the magnetoexciton energy branches and to deduce the Hamiltonian of the magnetoexciton-photon interaction. On the third step the fifth order dispersion equation describing the energy spectrum of the cavity magnetoexciton-polariton is investigated. It takes into account the interaction of the cavity photons with two dipole-active and with two quadrupole-active 2D magnetoexciton energy branches. The cavity photons have the circular polarizations sigma(+/-)(k) oriented along their wave vectors k, which has the quantized longitudinal component k(z) = +/-pi/L-c, where L-c is the resonator length and another small transverse component k(parallel to) oriented in the plane of the QW. The 2D magnetoexcitons are characterized by the in-plane wave vectors k(parallel to) and by circular polarizations sigma(M) arising in the p-type valence band with magnetic momentum projection M = +/- 1 on the direction of the magnetic field. The selection rules of the exciton-photon interaction have two origins. The first one, of geometrical-type, is expressed through the scalar products of the two-types circular polarizations. They depend on the in-plane wave vectors k(parallel to) even in the case of dipole-active transitions, because the cavity photons have an oblique incidence to the surface of the QW. Another origin is related with the numbers ne and n(h) of the LQ levels of electrons and heavy-holes taking part in the magnetoexciton formation. So, the dipole-active transitions take place for the condition n(e) = n(h), whereas in the quadrupole-active transitions the relation is n(e) = n(h) +/- 1. It was shown that the Rabi frequency Omega(R) of the polariton branches and the magnetoexciton oscillator strength f(osc) increase in dependence on the magnetic field strength B as Omega(R) similar to root B, and f(osc) similar to B. The optical gyrotropy effects may be revealed if changing the sign of the photon circular polarization at a given sign of the wave vector longitudinal projection k(z) or equivalently changing the sign of the longitudinal projection k(z) at the same selected light circular polarization.

  • 13. Moskalenko, Sveatoslav A.
    et al.
    Podlesny, Igor V.
    Dumanov, Evgheni V.
    Liberman, Michael A.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Novikov, Boris V.
    Dispersion laws of the two-dimensional cavity magnetoexciton-polaritons2016In: Journal of Nanophotonics, ISSN 1934-2608, Vol. 10, no 3, article id 036006Article in journal (Refereed)
    Abstract [en]

    The energy spectrum of the two-dimensional cavity magnetoexciton-polaritons has been investigated previously, using exact solutions for the Landau quantization (LQ) of conduction electrons and heavy holes (hhs) provided by the Rashba method. Two lowest LQ levels for electrons and three lowest Landau levels for hhs lead to the construction of the six lowest magnetoexciton sates. They consist of two dipole-active, two quadrupole-active, and the two forbidden quantum transitions from the ground state of the crystal to the magnetoexciton states. The interaction of the four optical-active magnetoexciton states with the cavity-mode photons with a given circular polarization and with well-defined incidence direction leads to the creation of five magnetoexciton-polariton branches. The fifth-order dispersion equation is examined by using numerical calculations and the second-order dispersion equation is solved analytically, taking into account only one dipole-active magnetoexciton state in the point of the in-plane wave vector (k) over right arrow || = 0. The effective polariton mass on the lower polariton branch, the Rabi frequency, and the corresponding Hopfield coefficients are determined in dependence on the magnetic-field strength, the Rashba spin-orbit coupling parameters, and the electron and hole g-factors. (C) 2016 Society of Photo-Optical Instrumentation Engineers (SPIE).

  • 14. Qian, Chengeng
    et al.
    Wang, Cheng
    Liu, JianNan
    Brandenburg, Axel
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA.
    Haugen, Nils E. L.
    Liberman, Mikhael A.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Convergence properties of detonation simulations2020In: Geophysical and Astrophysical Fluid Dynamics, ISSN 0309-1929, E-ISSN 1029-0419, Vol. 114, no 1-2, p. 58-76Article in journal (Refereed)
    Abstract [en]

    We present a high-resolution convergence study of detonation initiated by a temperature gradient in a stoichiometric hydrogen-oxygen mixture using the PENCIL CODE and compare with a code that employs a fifth order weighted essentially non-oscillating (WENO) scheme. With Mach numbers reaching 10-30, a certain amount of shock viscosity is needed in the PENCIL CODE to remove or reduce numerical pressure oscillations on the grid scale at the position of the shock. Detonation is found to occur for intermediate values of the shock viscosity parameter. At fixed values of this parameter, the numerical error associated with those small wiggles in the pressure profile is found to decrease with decreasing mesh width like down to. With the WENO scheme, solutions are smooth at, but no detonation is obtained for. This is argued to be an artifact of a decoupling between pressure and reaction fronts.

  • 15. Wang, Cheng
    et al.
    Qian, Chengeng
    Liu, JianNan
    Liberman, Mikhail A.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Influence of chemical kinetics on detonation initiating by temperature gradients in methane/air2018In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 197, p. 400-415Article in journal (Refereed)
    Abstract [en]

    Different simplified and detailed chemical models and their impact on simulations of combustion regimes initiating by the initial temperature gradient in methane/air mixtures are studied. The limits of the regimes of reaction wave propagation depend upon the spontaneous wave speed and the characteristic velocities of the problem. The present study mainly focus to identify conditions required for the development a detonation and to compare the difference between simplified chemical models and detailed chemistry. It is shown that a widely used simplified chemical schemes, such as one-step, two-step and other simplified models, do not reproduce correctly the ignition process in methane/air mixtures. The ignition delay times calculated using simplified models are in orders of magnitude shorter than the ignition delay times calculated using detailed chemical models and measured experimentally. This results in considerably different times when the exothermic reaction affects significantly the ignition, evolution, and coupling of the spontaneous reaction wave and pressure waves. We show that the temperature gradient capable to trigger detonation calculated using detailed chemical models is much shallower (the size of the hot spot is much larger) than that, predicted by simulations with simplified chemical models. These findings suggest that the scenario leading to the deflagration to detonation transition (DDT) may depend greatly on the chemical model used in simulations and that the Zel'dovich gradient mechanism is not necessary a universal mechanism triggering DDT. The obtained results indicate that the conclusions derived from the simulations of DDT with simplified chemical models should be viewed with great caution.

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