3,243 research outputs found
Thermodynamic analysis of new cycles for liquid-metal MHD generators
Acceleration devices for liquid metal magnetohydrodynamic generator
On weak and strong magnetohydrodynamic turbulence
Recent numerical and observational studies contain conflicting reports on the
spectrum of magnetohydrodynamic turbulence. In an attempt to clarify the issue
we investigate anisotropic incompressible magnetohydrodynamic turbulence with a
strong guide field . We perform numerical simulations of the reduced MHD
equations in a special setting that allows us to elucidate the transition
between weak and strong turbulent regimes. Denote ,
characteristic field-parallel and field-perpendicular wavenumbers of the
fluctuations, and the fluctuating field at the scale . We find that when the critical balance condition, , is satisfied, the turbulence is strong, and the energy
spectrum is . As the width of
the spectrum increases, the turbulence rapidly becomes weaker, and in the limit
, the spectrum approaches
. The observed sensitivity of the spectrum
to the balance of linear and nonlinear interactions may explain the conflicting
numerical and observational findings where this balance condition is not well
controlled.Comment: 4 pages, 2 figure
Turbulence without pressure in d dimensions
The randomly driven Navier-Stokes equation without pressure in d-dimensional
space is considered as a model of strong turbulence in a compressible fluid. We
derive a closed equation for the velocity-gradient probability density
function. We find the asymptotics of this function for the case of the gradient
velocity field (Burgers turbulence), and provide a numerical solution for the
two-dimensional case. Application of these results to the velocity-difference
probability density function is discussed.Comment: latex, 5 pages, revised and enlarge
Integer Quantum Hall Transition and Random SU(N) Rotation
We reduce the problem of integer quantum Hall transition to a random rotation
of an N-dimensional vector by an su(N) algebra, where only N specially selected
generators of the algebra are nonzero. The group-theoretical structure revealed
in this way allows us to obtain a new series of conservation laws for the
equation describing the electron density evolution in the lowest Landau level.
The resulting formalism is particularly well suited to numerical simulations,
allowing us to obtain the critical exponent \nu numerically in a very simple
way. We also suggest that if the number of nonzero generators is much less than
N, the same model, in a certain intermediate time interval, describes
percolating properties of a random incompressible steady two-dimensional flow.
In other words, quantum Hall transition in a very smooth random potential
inherits certain properties of percolation.Comment: 4 pages, 1 figur
Ferromagnetic Detectors of Axions in RF (S - X) Band
The (pseudo) Goldstone bosons arise naturally in many modern theories such as
supergravity, superstring theory and variants of general relativity with
torsion. By the other hand, there are well known indications that a large part
of the Universe mass exists in a form of dark matter. The most attractive model
of the dark matter is non-relativistic gas of the light elementary particles
weakly interacting with the "usual" matter \cite{b2} - \cite{b4}. We describe
ferromagnetic detectors, for search of arion(axion), where a high-sensitive
two-channel SHF receiver is used. Its sensitivity reaches to ,
with time of accumulation . Fourier analysis of signal provides a
survey in zone up to with spectral resolution .
There was applied a high sensitive SHF receiver based on a special computer
method of coherent accumulation of signals. It is possible to use the receiver
in other precise experiments: measuring of electron/positron beams polarization
in storage rings, investigation of parity violation, investigation of
atmosphere with radars etc.Comment: 6 pages, LaTeX, no figure
Multiple Local σ-aromaticity of Nonagermanide Clusters
Nonagermanide clusters are widely used in inorganic synthesis and are actively studied by experimentalists and theoreticians. However, chemical bonding of such versatile species is still not completely understood. In our work we deciphered a bonding pattern for various experimentally obtained nonagermanide species. We localized the electron density via the AdNDP algorithm for the model structures ([Ge9]4−, [Ge9{P(NH2)2}3]−, Cu[Ge9{P(NH2)2}3] and Cu(NHC)[Ge9{P(NH2)2}3]) and obtained a simple and chemically intuitive bonding pattern which can explain the variety of active sites and the existence of both D3h and C4v geometries for such clusters. Moreover, the [Ge9]4− core is found to be a unique example of an inorganic Zintl cluster with multiple local σ-aromaticity
Influence of a large-scale field on energy dissipation in magnetohydrodynamic turbulence
This is the author accepted manuscript. The final version is available from OUP via the DOI in this record.In magnetohydrodynamic (MHD) turbulence, the large-scale magnetic field sets a preferred local direction for the small-scale dynamics, altering the statistics of turbulence from the isotropic case. This happens even in the absence of a total magnetic flux, since MHD turbulence forms randomly oriented large-scale domains of strong magnetic field. It is therefore customary to study small-scale magnetic plasma turbulence by assuming a strong background magnetic field relative to the turbulent fluctuations. This is done, for example, in reduced models of plasmas, such as reduced MHD, reduced-dimension kinetic models, gyrokinetics, etc., which make theoretical calculations easier and numerical computations cheaper. Recently, however, it has become clear that the turbulent energy dissipation is concentrated in the regions of strong magnetic field variations. A significant fraction of the energy dissipation may be localized in very small volumes corresponding to the boundaries between strongly magnetized domains. In these regions, the reduced models are not applicable. This has important implications for studies of particle heating and acceleration in magnetic plasma turbulence. The goal of this work is to systematically investigate the relationship between local magnetic field variations and magnetic energy dissipation, and to understand its implications for modelling energy dissipation in realistic turbulent plasmas.The authors thank the referee, Alexander Schekochihin,
for helpful comments. VZ acknowledges support from NSF
grant AST-1411879. SB is partly supported by the National
Science Foundation under the grant NSF AGS-1261659 and
by the Vilas Associates Award from the University of Wis-
consin - Madison. JM acknowledges the support of the
EPSRC, through grant EP/M004546/1. We acknowledge
PRACE for awarding us access to resource FERMI based
in Italy at CINECA, and the STFC DiRAC HPC Facility
for access to the COSMA Data Centric system at Durham
University and MINERVA at the University of Warwick
- …