88 research outputs found
Angular momentum transport in convectively unstable shear flows
Angular momentum transport owing to hydrodynamic turbulent convection is
studied using local three dimensional numerical simulations employing the
shearing box approximation. We determine the turbulent viscosity from
non-rotating runs over a range of values of the shear parameter and use a
simple analytical model in order to extract the non-diffusive contribution
(Lambda-effect) to the stress in runs where rotation is included. Our results
suggest that the turbulent viscosity is of the order of the mixing length
estimate and weakly affected by rotation. The Lambda-effect is non-zero and a
factor of 2-4 smaller than the turbulent viscosity in the slow rotation regime.
We demonstrate that for Keplerian shear, the angular momentum transport can
change sign and be outward when the rotation period is greater than the
turnover time, i.e. when the Coriolis number is below unity. This result seems
to be relatively independent of the value of the Rayleigh number.Comment: 10 pages, 12 figures, published version. Version with higher
resolution figures can be found at http://www.helsinki.fi/~kapyla/publ.htm
Near-surface shear layer dynamics
The outer surface layers of the sun show a clear deceleration at low
latitudes. This is generally thought to be the result of a strong dominance of
vertical turbulent motions associated with strong downdrafts. This strong
negative radial shear should not only contribute to amplifying the toroidal
field locally and to expelling magnetic helicity, but it may also be
responsible for producing a strong prograde pattern speed in the
supergranulation layer. Using simulations of rotating stratified convection in
cartesian boxes located at low latitudes around the equator it is shown that in
the surface layers patterns move in the prograde direction on top of a
retrograde mean background flow. These patterns may also be associated with
magnetic tracers and even sunspot proper motions that are known to be prograde
relative to the much slower surface plasma.Comment: 11 pages, 8 figures, to appear in Highlights of Astronomy, ed. F.
Kupka, I.W. Roxburgh, and K.L. Chan, Astron. Soc. Pac. Conf. Se
Magnetic systems at criticality: different signatures of scaling
Different aspects of critical behaviour of magnetic materials are presented
and discussed. The scaling ideas are shown to arise in the context of purely
magnetic properties as well as in that of thermal properties as demonstrated by
magnetocaloric effect or combined scaling of excess entropy and order
parameter. Two non-standard approaches to scaling phenomena are described. The
presented concepts are exemplified by experimental data gathered on four
representatives of molecular magnets.Comment: 33 pages, 16 figure
Turbulent Mixing in the Surface Layers of Accreting Neutron Stars
During accretion a neutron star (NS) is spun up as angular momentum is
transported through its surface layers. We study the resulting differentially
rotating profile, focusing on the impact this has for type I X-ray bursts. The
predominant viscosity is likely provided by the Tayler-Spruit dynamo. The
radial and azimuthal magnetic field components have strengths of ~10^5 G and
~10^10 G, respectively. This leads to nearly uniform rotation at the depths of
interest for X-ray bursts. A remaining small shear transmits the accreted
angular momentum inward to the NS interior. Though this shear gives little
viscous heating, it can trigger turbulent mixing. Detailed simulations will be
required to fully understand the consequences of mixing, but our models
illustrate some general features. Mixing has the greatest impact when the
buoyancy at the compositional discontinuity between accreted matter and ashes
is overcome. This occurs at high accretion rates, at low spin frequencies, or
may depend on the ashes from the previous burst. We then find two new regimes
of burning. The first is ignition in a layer containing a mixture of heavier
elements from the ashes. If ignition occurs at the base of the mixed layer,
recurrence times as short as ~5-30 minutes are possible. This may explain the
short recurrence time of some bursts, but incomplete burning is still needed to
explain these bursts' energetics. When mixing is sufficiently strong, a second
regime is found where accreted helium mixes deep enough to burn stably,
quenching X-ray bursts. We speculate that the observed change in X-ray burst
properties near one-tenth the Eddington accretion rate is from this mechanism.
The carbon-rich material produced by stable helium burning would be important
for triggering and fueling superbursts. (abridged)Comment: Accepted for publication in The Astrophysical Journal, 16 pages, 15
figure
Observation of an unusual field dependent slow magnetic relaxation and two distinct transitions in a family of new complexes
An unusual field dependent slow magnetic relaxation and two distinct
transitions were observed in a family of new rare earth-transition metal
complexes, [Ln (bipy) (HO) M(CN)] 1.5 (bipy) 4HO (bipy = 2,2'-bipyridine; Ln = Gd,Y; M = Fe,
Co). The novel magnetic relaxation, which is quite different from those
in normal spin glasses and superparamagnets but very resembles qualitatively
those in single-molecule magnet Mn-Ac even if they possess different
structures, might be attributed to the presence of frustration that is
incrementally unveiled by the external magnetic field. The two distinct
transitions in [GdFe] were presumed from DC and AC susceptibility as well as
heat capacity measurements.Comment: Revtex, 6 figure
Application of an efficient gradient-based optimization strategy for aircraft wing structures
In this paper, a practical optimization framework and enhanced strategy within an industrial setting are proposed for solving large-scale structural optimization problems in aerospace. The goal is to eliminate the difficulties associated with optimization problems, which are mostly nonlinear with numerous mixed continuous-discrete design variables. Particular emphasis is placed on generating good initial starting points for the search process and in finding a feasible optimum solution or improving the chances of finding a better optimum solution when traditional techniques and methods have failed. The efficiency and reliability of the proposed strategy were demonstrated through the weight optimization of different metallic and composite laminated wingbox structures. The results show the effectiveness of the proposed procedures in finding an optimized solution for high-dimensional search space cases with a given level of accuracy and reasonable computational resources and user efforts. Conclusions are also inferred with regards to the sensitivity of the optimization results obtained with respect to the choice of different starting values for the design variables, as well as different optimization algorithms in the optimization process
Presupernova Evolution of Rotating Massive Stars I: Numerical Method and Evolution of the Internal Stellar Structure
The evolution of rotating stars with zero-age main sequence (ZAMS) masses in
the range 8 to 25 M_sun is followed through all stages of stable evolution. The
initial angular momentum is chosen such that the star's equatorial rotational
velocity on the ZAMS ranges from zero to ~ 70 % of break-up. Redistribution of
angular momentum and chemical species are then followed as a consequence of
rotationally induced circulation and instablities. The effects of the
centrifugal force on the stellar structure are included. Uncertain mixing
efficiencies are gauged by observations. We find, as noted in previous work,
that rotation increases the helium core masses and enriches the stellar
envelopes with products of hydrogen burning. We determine, for the first time,
the angular momentum distribution in typical presupernova stars along with
their detailed chemical structure. Angular momentum loss due to (non-magnetic)
stellar winds and the redistribution of angular momentum during core hydrogen
burning are of crucial importance for the specific angular momentum of the
core. Neglecting magnetic fields, we find angular momentum transport from the
core to the envelope to be unimportant after core helium burning. We obtain
specific angular momenta for the iron core and overlaying material of
1E16...1E17 erg s. These values are insensitive to the initial angular
momentum. They are small enough to avoid triaxial deformations of the iron core
before it collapses, but could lead to neutron stars which rotate close to
break-up. They are also in the range required for the collapsar model of
gamma-ray bursts. The apparent discrepancy with the measured rotation rates of
young pulsars is discussed.Comment: 62 pages, including 7 tables and 19 figures. Accepted by Ap
Loss of the Orphan Nuclear Receptor SHP Is More Pronounced in Fibrolamellar Carcinoma than in Typical Hepatocellular Carcinoma
Hepatocellular carcinoma (HCC) remains a major problem in oncology. The molecular mechanisms which underlie its pathogenesis are poorly understood. Recently the Small Heterodimer Partner (SHP), an orphan nuclear receptor, was suggested to be involved as a tumor suppressor in hepatocellular carcinoma development. To date, there are no such studies regarding fibrolamellar carcinoma, a less common variant of HCC, which usually affects young people and displays distinct morphological features. The aim of our project was to evaluate the SHP levels in typical and fibrolamellar hepatocellular carcinoma with respect to the levels of one of the cell cycle regulators, cyclin D1. We assessed the immunoreactivity levels of SHP and cyclin D1 in 48 typical hepatocellular carcinomas, 9 tumors representing the fibrolamellar variant, 29 non malignant liver tissues and 7 macroregenerative nodules. We detected significantly lower SHP immunoreactivity in hepatocellular carcinoma when compared to non malignant liver tissue. Moreover, we found that SHP immunoreactivity is reduced in fibrolamellar carcinoma when compared to typical hepatocellular carcinoma. We also found that SHP is more commonly lost in HCC which arises in the liver with steatosis. The comparison between the cyclin D1 and SHP expression revealed the negative correlation between these proteins in the high grade HCC. Our results indicate that the impact of loss of SHP protein may be even more pronounced in fibrolamellar carcinoma than in a typical form of HCC. Further investigation of mechanisms through which the loss of SHP function may influence HCC formation may provide important information in order to design more effective HCC therapy
Lattice dynamics of the ethylene crystal with interaction potentials from ab initio calculations
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