3,025 research outputs found
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
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
Role of cross helicity in magnetohydrodynamic turbulence
Strong incompressible three-dimensional magnetohydrodynamic turbulence is
investigated by means of high resolution direct numerical simulations. The
simulations show that the configuration space is characterized by regions of
positive and negative cross-helicity, corresponding to highly aligned or
anti-aligned velocity and magnetic field fluctuations, even when the average
cross-helicity is zero. To elucidate the role of cross-helicity, the spectra
and structure of turbulence are obtained in imbalanced regions where
cross-helicity is non-zero. When averaged over regions of positive and negative
cross-helicity, the result is consistent with the simulations of balanced
turbulence. An analytical explanation for the obtained results is proposed.Comment: 4 pages, 4 figure
A note on Burgers' turbulence
In this note the Polyakov equation [Phys. Rev. E {\bf 52} (1995) 6183] for
the velocity-difference PDF, with the exciting force correlation function
is analyzed. Several solvable cases are
considered, which are in a good agreement with available numerical results.
Then it is shown how the method developed by A. Polyakov can be applied to
turbulence with short-scale-correlated forces, a situation considered in models
of self-organized criticality.Comment: 11 pages, Late
Dynamical Anomalies and Intermittency in Burgers Turbulence
We analyze the field theory of fully developed Burgers turbulence. Its key
elements are shock fields, which characterize the singularity statistics of the
velocity field. The shock fields enter an operator product expansion describing
intermittency. The latter is found to be constrained by dynamical anomalies
expressing finite dissipation in the inviscid limit. The link between dynamical
anomalies and intermittency is argued to be important in a wider context of
turbulence.Comment: revised version, 4 pp., 1 fig., to appear in PR
THE STATE POLICY OF THE RUSSIAN FEDERATION ON THE NORTHERN AND ARCTIC TERRITORIES IN THE 1990S: STAGES, PRIORITIES, MECHANISMS, AND RESULTS
Purpose: The purpose of the study is to determine the state policy of the Russian Federation on the Northern and Arctic territories in the 1990s.
Methodology: This is analytical-logical research that has been done through content analysis and documentary and library research.
Result: Results showed that First of all Russia abruptly changed the priorities of its policy to the Far North setting the course to leave the region in all spheres. Dozens of polar expeditions from the USA, Norway, and Germany were sent to the Russian sector of polar lands. Russia itself rapidly turned off its economic presence in the North. “The North Pole-31” (drifting polar station) was stopped for 12 years on June 25, 1991.
Applications: This research can be used for universities, teachers, and students.
Novelty/Originality: In this research, the model of the state policy of the Russian Federation on the northern and arctic territories in the 1990s is presented in a comprehensive and complete manner
Symmetries of the stochastic Burgers equation
All Lie symmetries of the Burgers equation driven by an external random force
are found. Besides the generalized Galilean transformations, this equation is
also invariant under the time reparametrizations. It is shown that the Gaussian
distribution of a pumping force is not invariant under the symmetries and
breaks them down leading to the nontrivial vacuum (instanton). Integration over
the volume of the symmetry groups provides the description of fluctuations
around the instanton and leads to an exactly solvable quantum mechanical
problem.Comment: 4 pages, REVTeX, replaced with published versio
Mechanochemistry and solubilization of drugs
The results on solubilization of poorly water-soluble drugs,
indomethacin, piroxicam, and meloxicam, with the help of
mechanochemical methods are presented. The mechanocomposites
of the drugs with various excipients, including soluble
polymers and non-soluble fine porous inorganic oxides, were
obtained by co-grinding using vibrational and planetary
centrifugal mills. The samples obtained revealed higher release rate and apparent solubility of the drugs with respect to the initial ones. Nevertheless, in some cases, e.g. for composites of piroxicam with alumina and ferric oxide, decreasing the release rate and solubility of the drug was observed suggesting the formation of poorly soluble strong drug - carrier associates
Spectral Properties of Compressible Magnetohydrodynamic Turbulence from Numerical Simulations
We analyze the spectral properties of driven, supersonic compressible
magnetohydrodynamic (MHD) turbulence obtained via high-resolution numerical
experiments, for application to understanding the dynamics of giant molecular
clouds. Via angle-averaged power spectra, we characterize the transfer of
energy from the intermediate, driving scales down to smaller dissipative
scales, and also present evidence for inverse cascades that achieve
modal-equipartition levels on larger spatial scales. Investigating compressive
versus shear modes separately, we evaluate their relative total power, and find
that as the magnetic field strength decreases, (1) the shear fraction of the
total kinetic power decreases, and (2) slopes of power-law fits over the
inertial range steepen. To relate to previous work on incompressible MHD
turbulence, we present qualitative and quantitative measures of the
scale-dependent spectral anisotropy arising from the shear-Alfv\'{e}n cascade,
and show how these vary with changing mean magnetic field strength. Finally, we
propose a method for using anisotropy in velocity centroid maps as a diagnostic
of the mean magnetic field strength in observed cloud cores.Comment: 22 pages, 11 figures; Ap.J., accepte
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