2,732 research outputs found
Anisotropy of Alfvénic Turbulence in the Solar Wind and Numerical Simulations
No full text10.12.13 KB. Ok to add published version to spiral, RAS/OUP polic
Tearing-mediated Alfvénic Turbulence in a Relativistic Plasma
No full textIn astrophysical relativistic plasmas, Alfvénic turbulence exists across a wide range of scales. Similar to a nonrelativistic case, this turbulence can be influenced by the tearing instability. We argue that in an ultrarelativistic pair plasma the tearing effects become significant at the critical scale given by , where L _⊥ represents the outer scale of the turbulence and d _rel the relativistic electron inertial scale. Here n = 1 corresponds to the Harris magnetic configurations, while n = 2 applies to localized (non-Harris) magnetic profiles of turbulence-generated current sheets. Below the critical scale, the energy spectrum of magnetic fluctuations steepens to W ( k ) dk ∝ k ^−(7 ^n ^+2)/3 ^n dk . However, this happens only when , where is the magnetization parameter based on the magnetic fluctuations. If the opposite inequality holds, the Alfvénic cascade is instead affected by charge starvation at scales below , and the energy spectrum becomes steeper than W ( k ) dk ∝ k ^−3 dk . When d _rel is replaced by its nonrelativistic equivalent, our results agree with the nonrelativistic analysis of N. F. Loureiro & S. Boldyrev (2018). In the Harris case n = 1, they agree with the recent relativistic studies by I. Demidov & Y. Lyubarsky
Numerical Study of Inertial Kinetic-Alfven Turbulence
No full textRecent observational and analytical studies suggested that a new regime of kinetic turbulence may exist in plasma environments with low electron beta (Chen & Boldyrev 2017). Such a regime, termed inertial kinetic-Alfvén turbulence (iKAW), is relevant for the solar corona, Earth's magnetosheath, and other astrophysical systems where the electron and ion plasma beta parameters satisfy the condition β e ≪ β i ≲ 1. In this paper we present kinetic numerical simulations that confirm the existence of the iKAW regime. Specifically, the simulations demonstrate a transition at scales below electron inertial length d e when β e ≪ β i ≲ 1. Spectral slopes and other statistical properties of turbulence at sub-d e scales are consistent with the phenomenological theory of inertial kinetic-Alfvén turbulence proposed by Chen & Boldyrev (2017) and with the recent observations in the Earth's magnetosheath. ©2019NSF CAREER (award no. 1654168
TRANSITION-METAL-DOPED PLANAR BORON CLUSTERS: A NEW CLASS OF AROMATIC COMPOUNDS WITH HIGH COORDINATION
No full textAuthor Institution: Department of Chemistry, Brown University, Providence, Rhode IslandPhotoelectron spectroscopy in combination with computational studies over the past decade has shown that boron clusters possess planar or quasi-planar structures, in contrast to that of bulk boron, which is dominated by three-dimensional cage-like building blocks. All planar or quasi-planar boron clusters are observed to consist of a monocyclic circumference with one or more interior atoms. The propensity for planarity has been found to be due to both and electron delocalization throughout the molecular plane, giving rise to concepts of and double aromaticity. We have found further that the central boron atoms can be substituted by transition metal atoms to form a new class of aromatic compounds, which consist of a central metal atom and a monocyclic boron ring (Mopyright B). Eight-, nine-, and ten-membered rings of boron have been observed, giving rise to octa-, ennea-, and deca-coordinated aromatic transition metal compounds [1-3]. References: [1]"Aromatic Metal-Centered Monocyclic Boron Rings: Coopyright B and Ruopyright B" (Constantin Romanescu, Timur R. Galeev, Wei-Li Li, A. I. Boldyrev, and L. S. Wang), Angew. Chem. Int. Ed. \textbf{50}, 9334-9337 (2011). [2]"Transition-Metal-Centered Nine-Membered Boron Rings: Mopyright B and Mopyright B (M = Rh, Ir)" (Wei-Li Li, Constantin Romanescu, Timur R. Galeev, Zachary Piazza, A. I. Boldyrev, and L. S. Wang), J. Am. Chem. Soc. \textbf{134}, 165-168 (2012). [3]"Observation of the Highest Coordination Number in Planar Species: Decacoordinated Taopyright B and Nbopyright B Anions" (Timur R. Galeev, Constantin Romanescu, Wei-Li Li, L. S. Wang, and A. I. Boldyrev), Angew. Chem. Int. Ed. \textbf{51}, 2101-2105 (2012)
Localized vibrational modes of GeV-centers in diamond: Photoluminescence and first-principles phonon study
No full textThe vibrational behaviour of the germanium-vacancy (GeV) in diamond is studied through its photoluminescence spectrum and first-principles modelled partial phonon density of states. The former is measured in a region below 600 cm(-1). The latter is calculated for the GeV center in its neutral, charged, and excited state. The photoluminescence spectrum presents a previously unobserved feature at 248 cm(-1) in addition to the wellknown peak at 365 cm(-1). In our calculations, two localized modes, associated with the GeV center and six nearest carbon atoms (GeC6 cluster) are identified. These correspond to one vibration of the Ge ion along with the [111] orientation of the crystal and one perpendicular to this direction. We propose these modes to be assigned to the two features observed in the photoluminescence spectrum. The dependence of the energies of the localized modes on the GeV-center and their manifestation in experimental optical spectra is discussed.Calculations were carried out using a supercomputer at the Joint Supercomputer Center of the Russian Academy of Sciences. This work was supported by the grant of the Russian Science Foundation No. 19- 72-10132. Reference
Magnetohydrodynamic Turbulence Mediated by Reconnection
No full textMagnetic field fluctuations in MHD turbulence can be viewed as current sheets that are progressively more anisotropic at smaller scales. As suggested by Loureiro & Boldyrev 2017 and Mallet et al. 2017, below a certain critical thickness such current sheets become tearing-unstable. We propose that the tearing instability changes the effective alignment of the magnetic field lines in such a way as to balance the eddy turnover rate at all scales smaller than . As a result, turbulent fluctuations become progressively less anisotropic at smaller scales, with the alignment angle increasing as , where is the resistive dissipation scale. Here is the outer scale of the turbulence, is the corresponding Lundquist number, and {} is a parameter. The resulting Fourier energy spectrum is , where is the wavenumber normal to the local mean magnetic field, and the critical scale is . The simplest model corresponds to , in which case the predicted scaling formally agrees with one of the solutions obtained in (Mallet et al. 2017) from a discrete hierarchical model of abruptly collapsing current sheets, an approach different and complementary to ours. We also show that the reconnection-mediated interval is non-universal with respect to the dissipation mechanism. Hyper-resistivity of the form leads (in the simplest case of ) to the different transition scale and the energy spectrum , where is the corresponding hyper-resistive Lundquist number
Magnetohydrodynamic Turbulence Mediated by Reconnection
No full textMagnetic field fluctuations in MHD turbulence can be viewed as current sheets that are progressively more anisotropic at smaller scales. As suggested by Loureiro & Boldyrev 2017 and Mallet et al. 2017, below a certain critical thickness such current sheets become tearing-unstable. We propose that the tearing instability changes the effective alignment of the magnetic field lines in such a way as to balance the eddy turnover rate at all scales smaller than . As a result, turbulent fluctuations become progressively less anisotropic at smaller scales, with the alignment angle increasing as , where is the resistive dissipation scale. Here is the outer scale of the turbulence, is the corresponding Lundquist number, and {} is a parameter. The resulting Fourier energy spectrum is , where is the wavenumber normal to the local mean magnetic field, and the critical scale is . The simplest model corresponds to , in which case the predicted scaling formally agrees with one of the solutions obtained in (Mallet et al. 2017) from a discrete hierarchical model of abruptly collapsing current sheets, an approach different and complementary to ours. We also show that the reconnection-mediated interval is non-universal with respect to the dissipation mechanism. Hyper-resistivity of the form leads (in the simplest case of ) to the different transition scale and the energy spectrum , where is the corresponding hyper-resistive Lundquist number
Finite element method calculation of free orthogonal cutting of composite materials
No full textЩуров Игорь Алексеевич. Доктор технических наук, профессор, заведующий кафедрой «Станки и инструмент», Южно-Уральский государственный университет. Область научных интересов – математическое моделирование резьбообработки, обработки композитных материалов и обработки на станках с ЧПУ. Тел.: (351) 267-91-11; e-mail: [email protected]
Igor A. Shchurov. Doctor of engineering science, professor, head of Machine and cutting tools department, South Ural state university. The area of scientific interests – mathematical modeling of threadcutting, composites and CNC machining. Tel.: (351) 267-91-11; e-mail: [email protected]
Болдырев Игорь Станиславович. Кандидат технических наук, доцент кафедры «Станки и инструмент», Южно-Уральский государственный университет. Область научных интересов – математическое моделирование резьбообработки и резания композиционных материалов. Тел.: (351) 267-93-43; e-mail: [email protected]
Igor S. Boldyrev. Сandidate of engineering science, associate professor of Machine and cutting tools department, South Ural state university. The area of scientific interests – mathematical modeling of threadcutting and composites machining. Tel.: (351) 267-93-43; e-mail: [email protected]Приведены результаты расчета процесса свободного резания заготовки из композитного материала методом конечных элементов. В результате расчета определено напряженно-деформированное состояние фрагмента заготовки, описаны начальные и граничные условия. Результаты расчета имеют качественную сходимость с экспериментальными данными. The paper describes finite elements method calculation of free orthogonal cutting of composite materials. As a result, stress-deformed state was defined and initial and boundary conditions stated. Calculation results are similar to those of experimental data
Modeling of dynamics of vapor compression cooling system
No full text© Published under licence by IOP Publishing Ltd. A mathematical model of cooling system with a vapor-liquid compression unit is presented. The modeling of dynamics of the parameters of the vapor compression system during the system start-up in the cooling mode was carried out. It is noted that with the accepted assumptions the evaporation and condensation temperatures stabilize fast enough: in the evaporator - in 0.5 s, in the condenser - in 2 s
Modeling of dynamics of vapor compression cooling system
No full text© Published under licence by IOP Publishing Ltd. A mathematical model of cooling system with a vapor-liquid compression unit is presented. The modeling of dynamics of the parameters of the vapor compression system during the system start-up in the cooling mode was carried out. It is noted that with the accepted assumptions the evaporation and condensation temperatures stabilize fast enough: in the evaporator - in 0.5 s, in the condenser - in 2 s
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