238 research outputs found
Viscosity in spherically symmetric accretion
The influence of viscosity on the flow behaviour in spherically symmetric
accretion, has been studied here. The governing equation chosen has been the
Navier-Stokes equation. It has been found that at least for the transonic
solution, viscosity acts as a mechanism that detracts from the effectiveness of
gravity. This has been conjectured to set up a limiting scale of length for
gravity to bring about accretion, and the physical interpretation of such a
length-scale has been compared with the conventional understanding of the
so-called "accretion radius" for spherically symmetric accretion. For a
perturbative presence of viscosity, it has also been pointed out that the
critical points for inflows and outflows are not identical, which is a
consequence of the fact that under the Navier-Stokes prescription, there is a
breakdown of the invariance of the stationary inflow and outflow solutions --
an invariance that holds good under inviscid conditions. For inflows, the
critical point gets shifted deeper within the gravitational potential well.
Finally, a linear stability analysis of the stationary inflow solutions, under
the influence of a perturbation that is in the nature of a standing wave, has
indicated that the presence of viscosity induces greater stability in the
system, than has been seen for the case of inviscid spherically symmetric
inflows.Comment: 7 pages. Minor changes made in the version published in MNRA
Патогенетические механизмы повреждений бета-клеток панкреатических островков при диабете и влияние прерывистой гипоксии
У щурів з експериментальним цукровим діабетом та впливом на них переривчастої гіпоксії вивчені особливості синтезу інсуліну, цитоархітектоніки бета-клітин та експресія маркерів апоптозу та проліферації. Кількість бета-клітин підвищується, концентрація інсуліну залишається в межах інтактного показника. Значною мірою підвищується вироблення антиапоптотичного білку Bcl2. Гіпокситерапія призводить до суттєвого підвищення проліферативної активності ендокриноцитів панкреатичних острівців.At rats with an experimental diabetes and influence on intermittent hypoxia, features of synthesis of insulin, cells-architectonics beta-cells and an expression of markers apoptotic and proliferative are studied. Amount beta-cells increase, and concentration of insulin remains border of a control indicator. The production of anti-apoptotic protein Bcl2 increase to a considerable extent. Intermittent hypoxia significantly increase the proliferative activity endokrinotsitis pancreatic islets
Singular Potentials and Limit Cycles
We show that a central singular potential (with ) is
renormalized by a one-parameter square-well counterterm; low-energy observables
are made independent of the square-well width by adjusting the square-well
strength. We find a closed form expression for the renormalization-group
evolution of the square-well counterterm.Comment: 15 pages LaTex, 5 eps figures, error in figures and text correcte
Gauge Theory on Fuzzy S^2 x S^2 and Regularization on Noncommutative R^4
We define U(n) gauge theory on fuzzy S^2_N x S^2_N as a multi-matrix model,
which reduces to ordinary Yang-Mills theory on S^2 x S^2 in the commutative
limit N -> infinity. The model can be used as a regularization of gauge theory
on noncommutative R^4_\theta in a particular scaling limit, which is studied in
detail. We also find topologically non-trivial U(1) solutions, which reduce to
the known "fluxon" solutions in the limit of R^4_\theta, reproducing their full
moduli space. Other solutions which can be interpreted as 2-dimensional branes
are also found. The quantization of the model is defined non-perturbatively in
terms of a path integral which is finite. A gauge-fixed BRST-invariant action
is given as well. Fermions in the fundamental representation of the gauge group
are included using a formulation based on SO(6), by defining a fuzzy Dirac
operator which reduces to the standard Dirac operator on S^2 x S^2 in the
commutative limit. The chirality operator and Weyl spinors are also introduced.Comment: 39 pages. V2-4: References added, typos fixe
Effects of non-invasive brain stimulation in multiple sclerosis: systematic review and meta-analysis
Objective: The objective of this meta-analysis was to summarize evidence on the therapeutic effects of non-invasive brain stimulation (NIBS) on core symptoms of multiple sclerosis (MS). Specifically, findings from studies deploying transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS) protocols were summarized in this review. Methods: We systematically searched articles published in four databases, until 31 May 2021, which compared the effects of active tDCS or rTMS with sham intervention in MS patients. We used a random-effects model for this meta-analysis. Meta-regression and subgroup meta-analysis were used to examine the effects of stimulation dose and different stimulation protocols, respectively. Results: Twenty-five randomized controlled trials (RCTs) were included in this review, consisting of 19 tDCS and 6 rTMS studies. tDCS led to a significant and immediate reduction of fatigue with a large effect size (Hedges’s g = −0.870, 95% confidence intervals (CI) = [−1.225 to −0.458], number needed to treat (NNT) = 2). Particularly, a subgroup analysis showed that applying tDCS over the left DLPFC and bilateral S1 led to fatigue reductions compared to sham stimulation. Furthermore, tDCS had favorable effects on fatigue in MS patients with low physical disability but not those with high physical disability, and additionally improved cognitive function. Finally, whereas rTMS was observed to reduce muscle spasticity, these NIBS protocols showed no further effect on MS-associated pain and mood symptoms. Conclusion: tDCS in MS alleviates fatigue and improves cognitive function whereas rTMS reduces muscle spasticity. More high-quality studies are needed to substantiate the therapeutic effects of different NIBS protocols in MS
The Cerenkov effect revisited: from swimming ducks to zero modes in gravitational analogs
We present an interdisciplinary review of the generalized Cerenkov emission
of radiation from uniformly moving sources in the different contexts of
classical electromagnetism, superfluid hydrodynamics, and classical
hydrodynamics. The details of each specific physical systems enter our theory
via the dispersion law of the excitations. A geometrical recipe to obtain the
emission patterns in both real and wavevector space from the geometrical shape
of the dispersion law is discussed and applied to a number of cases of current
experimental interest. Some consequences of these emission processes onto the
stability of condensed-matter analogs of gravitational systems are finally
illustrated.Comment: Lecture Notes at the IX SIGRAV School on "Analogue Gravity" in Como,
Italy from May 16th-21th, 201
Defects and glassy dynamics in solid He-4: Perspectives and current status
We review the anomalous behavior of solid He-4 at low temperatures with
particular attention to the role of structural defects present in solid. The
discussion centers around the possible role of two level systems and structural
glassy components for inducing the observed anomalies. We propose that the
origin of glassy behavior is due to the dynamics of defects like dislocations
formed in He-4. Within the developed framework of glassy components in a solid,
we give a summary of the results and predictions for the effects that cover the
mechanical, thermodynamic, viscoelastic, and electro-elastic contributions of
the glassy response of solid He-4. Our proposed glass model for solid He-4 has
several implications: (1) The anomalous properties of He-4 can be accounted for
by allowing defects to freeze out at lowest temperatures. The dynamics of solid
He-4 is governed by glasslike (glassy) relaxation processes and the
distribution of relaxation times varies significantly between different
torsional oscillator, shear modulus, and dielectric function experiments. (2)
Any defect freeze-out will be accompanied by thermodynamic signatures
consistent with entropy contributions from defects. It follows that such
entropy contribution is much smaller than the required superfluid fraction, yet
it is sufficient to account for excess entropy at lowest temperatures. (3) We
predict a Cole-Cole type relation between the real and imaginary part of the
response functions for rotational and planar shear that is occurring due to the
dynamics of defects. Similar results apply for other response functions. (4)
Using the framework of glassy dynamics, we predict low-frequency yet to be
measured electro-elastic features in defect rich He-4 crystals. These
predictions allow one to directly test the ideas and very presence of glassy
contributions in He-4.Comment: 33 pages, 13 figure
Developmental Links between Teacher-Child Closeness and Disobedience for Boys Placed in Special Education
The aim of this study was to examine developmental links between disobedience and teacher-child closeness in a sample of boys with psychiatric disorders (i.e., emotional and behavioral disorders (EBD) and autism spectrum disorders (ASD)) and special educational needs who are placed in special education. More specifically, this study examined whether developmental links were different between boys with EBD (n = 150) versus boys with ASD (n = 122). Developmental links between disobedience and teacher-child closeness were investigated by incorporating a multi-informant perspective using teacher, child, and peer ratings and analyzed using autoregressive cross-lagged models across three waves within one school year. Results showed that in general, developmental links between teacher-child closeness and disobedience were stronger for boys with EBD than for boys with ASD. Specifically, boys with EBD experiencing less teacher-child closeness showed more disobedience, which in turn negatively affected their rela
A Non-Perturbative Study of Gauge Theory on a Non-Commutative Plane
We perform a non-perturbative study of pure gauge theory in a two dimensional
non-commutative (NC) space. On the lattice, it is equivalent to the twisted
Eguchi-Kawai model, which we simulated at N ranging from 25 to 515. We observe
a clear large-N scaling for the 1- and 2-point function of Wilson loops, as
well as the 2-point function of Polyakov lines. The 2-point functions agree
with a universal wave function renormalization. Based on a Morita equivalence,
the large-N double scaling limit corresponds to the continuum limit of NC gauge
theory, so the observed large-N scaling demonstrates the non-perturbative
renormalizability of this NC field theory. The area law for the Wilson loops
holds at small physical area as in commutative 2d planar gauge theory, but at
large areas we find an oscillating behavior instead. In that regime the phase
of the Wilson loop grows linearly with the area. This agrees with the
Aharonov-Bohm effect in the presence of a constant magnetic field, identified
with the inverse non-commutativity parameter.Comment: 18 pages, 6 figures, final version published in JHE
In-medium QCD and Cherenkov gluons
The equations of in-medium gluodynamics are proposed. Their classical lowest
order solution is explicitly shown for a color charge moving with constant
speed. For nuclear permittivity larger than 1 it describes emission of
Cherenkov gluons resembling results of classical electrodynamics. The choice of
nuclear permittivity and Lorentz-invariance of the problem are discussed.
Effects induced by the transversely and longitudinally moving (relative to the
collision axis) partons at LHC energies are described.Comment: 13 p., misprints correcte
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