1,802 research outputs found
Relaxation of writhe and twist of a bi-helical magnetic field
In the past few years suggestions have emerged that the solar magnetic field
might have a bi-helical component with oppositely polarized magnetic fields at
large and small scales, and that the shedding of such fields may be crucial for
the operation of the dynamo. It is shown that, if a bi-helical field is shed
into the solar wind, positive and negative contributions of the magnetic
helicity spectrum tend to mix and decay. Even in the absence of turbulence,
mixing and decay can occur on a time scale faster than the resistive one
provided the two signs of magnetic helicity originate from a single tube. In
the presence of turbulence, positively and negatively polarized contributions
mix rapidly in such a way that the ratio of magnetic helicity to magnetic
energy is largest both at the largest scale and in the dissipation range. In
absolute units the small scale excess of helical fields is however negligible.Comment: 6 pages, 9 figures, to be published in Astron. Astrophy
Anisotropy of Alfv\'enic Turbulence in the Solar Wind and Numerical Simulations
We investigate the anisotropy of Alfv\'enic turbulence in the inertial range
of slow solar wind and in both driven and decaying reduced magnetohydrodynamic
simulations. A direct comparison is made by measuring the anisotropic
second-order structure functions in both data sets. In the solar wind, the
perpendicular spectral index of the magnetic field is close to -5/3. In the
forced simulation, it is close to -5/3 for the velocity and -3/2 for the
magnetic field. In the decaying simulation, it is -5/3 for both fields. The
spectral index becomes steeper at small angles to the local magnetic field
direction in all cases. We also show that when using the global rather than
local mean field, the anisotropic scaling of the simulations cannot always be
properly measured.Comment: 9 pages, 8 figure
Beneficiation of some Egyptian Glass Sands
The attrition-scrubbing of some Egyptian glass sands was necessary for the disintegration of the ferruginous clayey coating on quartz grains. The effects of time of attrition-ing, impeller speed and solid/liquid ration were investi-gated. Classification of the scrubbed sands resulted in the removal of a slimy fraction and yielded a product having 0.065% Fe. Acid attritioning of sand with commer-cial HCL at room temperature did not affect the iron, coat-ing quartz particles
Generation of Magnetic Field by Combined Action of Turbulence and Shear
The feasibility of a mean-field dynamo in nonhelical turbulence with
superimposed linear shear is studied numerically in elongated shearing boxes.
Exponential growth of magnetic field at scales much larger than the outer scale
of the turbulence is found. The charateristic scale of the field is l_B ~
S^{-1/2} and growth rate is gamma ~ S, where S is the shearing rate. This newly
discovered shear dynamo effect potentially represents a very generic mechanism
for generating large-scale magnetic fields in a broad class of astrophysical
systems with spatially coherent mean flows.Comment: 4 pages, 5 figures; replaced with revised version that matches the
published PR
Системный подход к управлению экономикой знаний
Описываются особенности и уровни управления экономикой знаний. Через системный подход в современном менеджменте рассматривается организационное и ресурсное управление экономикой знаний. Обосновывается взаимообусловленность развития современного менеджмента и экономики знаний. Менеджмент постмодерна является драйвером для совершенствования стратегической концепции экономики знаний, и последняя, в свою очередь, вносит коррективы в траекторию динамики методологии современного менеджмента
Exact scaling laws and the local structure of isotropic magnetohydrodynamic turbulence
This paper examines the consistency of the exact scaling laws for isotropic
MHD turbulence in numerical simulations with large magnetic Prandtl numbers Pm
and with Pm=1. The exact laws are used to elucidate the structure of the
magnetic and velocity fields. Despite the linear scaling of certain third-order
correlation functions, the situation is not analogous to the case of Kolmogorov
turbulence. The magnetic field is adequately described by a model of stripy
(folded) field with direction reversals at the resistive scale. At currently
available resolutions, the cascade of kinetic energy is short-circuited by the
direct exchange of energy between the forcing-scale motions and the stripy
magnetic fields. This nonlocal interaction is the defining feature of isotropic
MHD turbulence.Comment: 10 pages, 6 figures. Accepted for publication in Journal of Fluid
Mechanic
Contributor Notes
FOR production of standard grade Ferro-manganese the
iron content in manganese ores should not exceed certain limits. Vast reserves of ferruginous manganese ores exist in several locations in U. A. R., which do not find good markets on account of their high iron content. Preliminary work showed that the iron content could be successfully reduced by magnetic roasting followed by magnetic sepa-ration. As reduction to the magnetic stage takes place readily in the fluidized state`, it was decided to use the fluidization technique in the present study
The Coincidence Problem in Holographic f(R) Gravity
It is well-known that gravity models formulated in Einstein conformal
frame are equivalent to Einstein gravity together with a minimally coupled
scalar field. In this case, the scalar field couples with the matter sector and
the coupling term is given by the conformal factor. We apply the holographic
principle to such interacting models. In a spatially flat universe, we show
that the Einstein frame representation of models leads to a constant
ratio of energy densities of dark matter to dark energy.Comment: 10 pages, no figure
Reluctance of Students to Utilize Virtual Educational Environments in Public Schools: Real World Experiences
This study aimed to explore the perspectives of public-school students who were hesitant to use the Darsak Educational Platform (DEP) during the COVID-19 pandemic in order to understand their real-life experiences. To achieve the aim of the study, the researchers employed a qualitative approach in its phenomenological form. The study sample consisted of 12 male and female students from various public schools in Jordan. The participants were chosen using an intentional method. Semi-structured interviews were used to collect the data. The study’s findings revealed a variety of causes that contributed to students’ resistance to using DEP. These causes include the students’ lack of readiness to learn through DEP, the DEP’s failure to comply with international design standards, the inadequate physical and technical teaching environment, and the limited digital competencies of teachers needed in the virtual learning environment. Given the findings of this research, the researchers advise that additional consideration should be given to the design of DEP. They also advise providing students with appropriate guidance in the usage of educational platforms
The "zeroth law" of turbulence: Isotropic turbulence simulations revisited
The dimensionless kinetic energy dissipation rate C_epsilon is estimated from
numerical simulations of statistically stationary isotropic box turbulence that
is slightly compressible. The Taylor microscale Reynolds number Re_lambda range
is 20 < Re_lambda < 220 and the statistical stationarity is achieved with a
random phase forcing method. The strong Re_lambda dependence of C_epsilon
abates when Re_lambda approx. 100 after which C_epsilon slowly approaches
approx 0.5 a value slightly different to previously reported simulations but in
good agreement with experimental results. If C_epsilon is estimated at a
specific time step from the time series of the quantities involved it is
necessary to account for the time lag between energy injection and energy
dissipation. Also, the resulting value can differ from the ensemble averaged
value by up to +-30%. This may explain the spread in results from previously
published estimates of C_epsilon.Comment: 7 pages, 7 figures. Submitted to Phys. Rev.
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