1,436 research outputs found
Thermal and electrical interaction of tantalum with a low temperature chemically active plasma flow
The paper deals with an experimental study of radiative heat transfer and charge transfer processes from the surface of tantalum plates under conditions of unsteady high-temperature heating and oxidation. It is shown that at plate temperatures of 1800 K, the heat flux may be as high as 400 kW/sq m. Heating is shown to stimulate the emissivity of tantalum and the temperature of the free electrons which surface, through a gas boundary layer, from the plasma onto the metal
A-Model Correlators from the Coulomb Branch
We compute the contribution of discrete Coulomb vacua to A-Model correlators
in toric Gauged Linear Sigma Models. For models corresponding to a compact
variety, this determines the correlators at arbitrary genus. For non-compact
examples, our results imply the surprising conclusion that the quantum
cohomology relations break down for a subset of the correlators.Comment: 27 pages, 1 xy-pic figur
Ordering of small particles in one-dimensional coherent structures by time-periodic flows
Small particles transported by a fluid medium do not necessarily have to
follow the flow. We show that for a wide class of time-periodic incompressible
flows inertial particles have a tendency to spontaneously align in
one-dimensional dynamic coherent structures. This effect may take place for
particles so small that often they would be expected to behave as passive
tracers and be used in PIV measurement technique. We link the particle tendency
to form one-dimensional structures to the nonlinear phenomenon of phase
locking. We propose that this general mechanism is, in particular, responsible
for the enigmatic formation of the `particle accumulation structures'
discovered experimentally in thermocapillary flows more than a decade ago and
unexplained until now
Models of G time variations in diverse dimensions
A review of different cosmological models in diverse dimensions leading to a
relatively small time variation of the effective gravitational constant G is
presented. Among them: 4-dimensional general scalar-tensor model,
multidimensional vacuum model with two curved Einstein spaces, multidimensional
model with multicomponent anisotropic "perfect fluid", S-brane model with
scalar fields and two form field etc. It is shown that there exist different
possible ways of explanation of relatively small time variation of the
effective gravitational constant G compatible with present cosmological data
(e.g. acceleration): 4-dimensional scalar-tensor theories or multidimensional
cosmological models with different matter sources. The experimental bounds on
G-dot may be satisfied ether in some restricted interval or for all allowed
values of the synchronous time variable.Comment: 27 pages, Late
Lidar investigations of M-zone
The creation of pulse dye lasers tuned to resonant line of meteor produced admixtures of atmospheric constituents has made it possible to begin lidar investigations of the vertical distribution of mesospheric sodium concentration and its dynamics in the upper atmosphere. The observed morning increase of sodium concentration in the vertical column is probably caused by diurnal variations of sporadic meteors. The study of the dynamics of the sodium column concentration in the period of meteor streams activity confirms the suggestion of cosmic origin of these atoms. The short lived increase of sodium concentration brought about by a meteor stream, however, exceeds by one order the level of the sporadic background
Spatially resolved microwave pulsations of a flare loop
A microwave burst with quasi-periodic pulsations was studied with high spatial resolution using observations with
the Nobeyama Radioheliograph (NoRH). We found that the time profiles of the microwave emission at 17 and 34 GHz exhibit
quasi-periodic (with two well defined periods P 1 = 14â17 s and P 2 = 8â11 s) variations of the intensity at diïŹerent parts of an observed flaring loop. Detailed Fourier analysis shows the P 1 spectral component to be dominant at the top, while the P 2 one near the feet of the loop. The 14â17 s pulsations are synchronous at the top and in both legs of the loop. The 8â11 s pulsations at the legs are well correlated with each other but the correlation is not so obvious with the pulsations at the loop top. For this P 2
spectral component, a definite phase shift, P 2 /4 â 2.2 s, between pulsations in the northern leg and loop top parts of the loop have been found. The length of the flaring loop is estimated as L = 25 Mm (â34 ) and its average width at half intensity at 34 GHz as about 6 Mm (â8 ). Microwave diagnostics shows the loop to be filled with a dense plasma with the number density n 0 â 10 11 cm â3, penetrated by the magnetic field changing from B 0 â 100 G near the loop top up to B 0 â 200 G near the north footpoint. A comparative analysis of diïŹerent MHD modes of the loop demonstrates the possibility of the simultaneous existence of two modes of oscillations in the loop: the global sausage mode, with the period P 1 = 14â17 s and the nodes at the footpoints, and a higher harmonics mode (possibly with the radial wave number l > 1), with P 2 = 8â11 s
A Jupiter-mass planet around the K0 giant HD 208897
For over 10 years, we have carried out a precise radial velocity (RV) survey
to find substellar companions around evolved G,K-type stars to extend our
knowledge of planet formation and evolution. We performed high precision RV
measurements for the giant star HD 208897 using an iodine (I2) absorption cell.
The measurements were made at T\"UB\.ITAK National Observatory (TUG, RTT150)
and Okayama Astrophysical Observatory (OAO). For the origin of the periodic
variation seen in the RV data of the star, we adopted a Keplerian motion caused
by an unseen companion. We found that the star hosts a planet with a minimum
mass of m2sini=1.40MJ, which is relatively low compared to those of known
planets orbiting evolved intermediate-mass stars. The planet is in a nearly
circular orbit with a period of P=353 days at about 1 AU distance from the host
star. The star is metal rich and located at the early phase of ascent along the
red giant branch. The photometric observations of the star at Ankara University
Kreiken Observatory (AUKR) and the HIPPARCOS photometry show no sign of
variation with periods associated with the RV variation. Neither bisector
velocity analysis nor analysis of the Ca II and Halpha lines shows any
correlation with the RV measurements
Front Propagation of Spatio-temporal Chaos
We study the dynamics of the front separating a spatio-temporally chaotic
region from a stable steady region using a simple model applicable to
periodically forced systems. In particular, we investigate both the coarsening
of the front induced by the inherent `noise' of the chaotic region, and the
long wavelength dynamics causing the front to develop cusps
Resonance Zones and Lobe Volumes for Volume-Preserving Maps
We study exact, volume-preserving diffeomorphisms that have heteroclinic
connections between a pair of normally hyperbolic invariant manifolds. We
develop a general theory of lobes, showing that the lobe volume is given by an
integral of a generating form over the primary intersection, a subset of the
heteroclinic orbits. Our definition reproduces the classical action formula in
the planar, twist map case. For perturbations from a heteroclinic connection,
the lobe volume is shown to reduce, to lowest order, to a suitable integral of
a Melnikov function.Comment: ams laTeX, 8 figure
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