6,241 research outputs found
NASTRAN thermal analyzer in a unified finite-element treatment of thermo-structural analyses
The NASTRAN thermal analyzer (NTA) which performs large-scale unified thermo-structural analyses with the NASTRAN (NASA structural analysis) computer program is described. The mathematical similitude between these two distinct disciplines of thermal and structure is examined. It serves as the theoretical basis upon which the implementation of the thermal capability in NASTRAN was accomplished. The program structure, the functional flow, the solution algorithms, the organization of an input data deck and the solution capabilities of NTA are summarized. Emphasis is placed on the interface of the unified approach in thermo-structural analyses where stresses, deflections, vibrations and bucklings induced by the effect of temperature change are of concern. Attentions are also directed to the preprocessor and post processors. As a specially designed preprocessor, the VIEW program is capable of generating exchange factors which can be output, at user's option, in formats compatible with that required by NTA. Two post processors that serve specific objectives are included. They are the thermal variance analysis and the graphical displaying capability of temperatures in color or black and white
Interaction Between Convection and Pulsation
This article reviews our current understanding of modelling convection
dynamics in stars. Several semi-analytical time-dependent convection models
have been proposed for pulsating one-dimensional stellar structures with
different formulations for how the convective turbulent velocity field couples
with the global stellar oscillations. In this review we put emphasis on two,
widely used, time-dependent convection formulations for estimating pulsation
properties in one-dimensional stellar models. Applications to pulsating stars
are presented with results for oscillation properties, such as the effects of
convection dynamics on the oscillation frequencies, or the stability of
pulsation modes, in classical pulsators and in stars supporting solar-type
oscillations.Comment: Invited review article for Living Reviews in Solar Physics. 88 pages,
14 figure
A Turbulent Model for the Interstellar Medium. II. Magnetic Fields and Rotation
We present results from two-dimensional numerical simulations of a supersonic
turbulent flow in the plane of the galactic disk, incorporating shear,
thresholded and discrete star formation (SF), self-gravity, rotation and
magnetic fields. A test of the model in the linear regime supports the results
of the linear theory of Elmegreen (1991). In the fully nonlinear turbulent
regime, while some results of the linear theory persist, new effects also
emerge. Some exclusively nonlinear effects are: a) Even though there is no
dynamo in 2D, the simulations are able to maintain or increase their net
magnetic energy in the presence of a seed uniform azimuthal component. b) A
well-defined power-law magnetic spectrum and an inverse magnetic cascade are
observed in the simulations, indicating full MHD turbulence. Thus, magnetic
field energy is generated in regions of SF and cascades up to the largest
scales. c) The field has a slight but noticeable tendency to be aligned with
density features. d) The magnetic field prevents HII regions from expanding
freely, as in the recent results of Slavin \& Cox (1993). e) A tendency to
exhibit {\it less} filamentary structures at stronger values of the uniform
component of the magnetic field is present in several magnetic runs. f) For
fiducial values of the parameters, the flow in general appears to be in rough
equipartition between magnetic and kinetic energy. There is no clear domination
of either the magnetic or the inertial forces. g) A median value of the
magnetic field strength within clouds is G, while for the
intercloud medium a value of G is found. Maximum contrasts of up to
a factor of are observed.Comment: Plain TeX file, 25 pages. Gzipped, tarred set of Tex file plus 17
figures and 3 tables (Postscript) available at
ftp://kepler.astroscu.unam.mx/incoming/enro/papers/mhdgturb.tar.g
Ignition of Deflagration and Detonation Ahead of the Flame due to Radiative Preheating of Suspended Micro Particles
We study a flame propagating in the gaseous combustible mixture with
suspended inert particles. The gas is assumed to be transparent for the
radiation emitted by the combustion products, while particles absorb and
re-emit the radiation. Thermal radiation heats the particles, which in turn
transfer the heat to the surrounding gaseous mixture by means of heat
conduction, so that the gas temperature lags that of the particles. We consider
different scenarios depending on the spatial distribution of the particles,
their size and the number density. In the case of uniform distribution of the
particles the radiation causes a modest increase of the temperature ahead of
the flame and the corresponding increase of the flame velocity. The effects of
radiation preheating is stronger for a flame with smaller normal velocity. In
the case of non-uniform distribution of the particles, such that the particles
number density is smaller just ahead of the flame and increases in the distant
region ahead of the flame, the preheating caused by the thermal radiation may
trigger additional independent source of ignition. This scenario requires the
formation of a temperature gradient with the maximum temperature sufficient for
ignition in the region of denser particles cloud ahead of the advancing flame.
Depending on the steepness of the temperature gradient formed in the unburned
mixture, either deflagration or detonation can be initiated via the Zeldovich's
gradient mechanism. The ignition and the resulting combustion regimes depend on
the temperature profile which is formed in effect of radiation absorption and
gas-dynamic expansion. In the case of coal dust flames propagating through a
layered dust cloud the effect of radiation heat transfer can result in the
propagation of combustion wave with velocity up to 1000m/s and can be a
plausible explanation of the origin of dust explosion in coal mines.Comment: 45 pages, 14 figures. Accepted for publication Combustion and Flame
29 June 201
Stability of metal-rich very massive stars
We revisit the stability of very massive nonrotating main-sequence stars at
solar metallicity, with the goal of understanding whether radial pulsations set
a physical upper limit to stellar mass. Models of up to 938 solar masses are
constructed with the Mesa code, and their linear stability in the fundamental
mode, assumed to be the most dangerous, is analysed with a fully nonadiabatic
method. Models above 100 MSun have extended tenuous atmospheres ("shelves")
that affect the stability of the fundamental. Even when positive, this growth
rate is small, in agreement with previous results. We argue that small growth
rates lead to saturation at small amplitudes that are not dangerous to the
star. A mechanism for saturation is demonstrated involving nonlinear parametric
coupling to short-wavelength g modes and the damping of the latter by radiative
diffusion. The shelves are subject to much more rapidly growing strange modes.
This also agrees with previous results but is extended here to higher masses.
The strange modes probably saturate via shocks rather than mode coupling but
have very small amplitudes in the core, where almost all of the stellar mass
resides. Although our stellar models are hydrostatic, the structure of their
outer parts suggests that optically thick winds, driven by some combination of
radiation pressure, transsonic convection, and strange modes, are more likely
than pulsation in the fundamental mode to limit the main-sequence lifetime.Comment: 14 pages, 8 figures, 1 appendix; this version to be published in
MNRA
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