1,568 research outputs found
Dust distribution in protoplanetary disks - Vertical settling and radial migration
We present the results of a three dimensional, locally isothermal,
non-self-gravitating SPH code which models protoplanetary disks with two
fluids: gas and dust. We ran simulations of a 1 Msun star surrounded by a 0.01
Msun disk comprising 99% gas and 1% dust in mass and extending from 0.5 to ~300
AU. The grain size ranges from 0.001 mm to 10 m for the low resolution (~25 000
SPH particles) simulations and from 0.1 mm to 10 cm for the high resolution
(~160 000 SPH particles) simulations. Dust grains are slowed down by the
sub-Keplerian gas and lose angular momentum, forcing them to migrate towards
the central star and settle to the midplane. The gas drag efficiency varies
according to the grain size, with the larger bodies being weakly influenced and
following marginally perturbed Keplerian orbits, while smaller grains are
strongly coupled to the gas. For intermediate sized grains, the drag force
decouples the dust and gas, allowing the dust to preferentially migrate
radially and efficiently settle to the midplane. The resulting dust
distributions for each grain size will indicate, when grain growth is added,
the regions when planets are likely to form.Comment: Accepted for publication in Astronomy & Astrophysics. 11 pages, 6
figure
New parton distributions from large-x and low-Q^2 data
We report results of a new global next-to-leading order fit of parton
distribution functions in which cuts on W and Q are relaxed, thereby including
more data at high values of x. Effects of target mass corrections (TMCs),
higher twist contributions, and nuclear corrections for deuterium data are
significant in the large-x region. The leading twist parton distributions are
found to be stable to TMC model variations as long as higher twist
contributions are also included. The behavior of the d quark as x-->1 is
particularly sensitive to the deuterium corrections, and using realistic
nuclear smearing models the d-quark distribution at large x is found to be
softer than in previous fits performed with more restrictive cuts.Comment: 31 pages, 8 figures. Minor corrections. References added. To appear
in Phys.Rev.
Gravitational Radiation from Nonaxisymmetric Instability in a Rotating Star
We present the first calculations of the gravitational radiation produced by
nonaxisymmetric dynamical instability in a rapidly rotating compact star. The
star deforms into a bar shape, shedding of its mass and
of its angular momentum. The gravitational radiation is calculated in the
quadrupole approximation. For a mass M and radius km, the gravitational waves have frequency kHz and amplitude
at the distance of the Virgo Cluster. They carry off
energy and radiate angular momentum .Comment: 16 pages, LaTeX with REVTEX macros, reprints available - send mailing
address to [email protected]. Published: PRL 72, 1314 (1994
The construction of non-spherical models of quasi-relaxed stellar systems
Spherical models of collisionless but quasi-relaxed stellar systems have long
been studied as a natural framework for the description of globular clusters.
Here we consider the construction of self-consistent models under the same
physical conditions, but including explicitly the ingredients that lead to
departures from spherical symmetry. In particular, we focus on the effects of
the tidal field associated with the hosting galaxy. We then take a stellar
system on a circular orbit inside a galaxy represented as a "frozen" external
field. The equilibrium distribution function is obtained from the one
describing the spherical case by replacing the energy integral with the
relevant Jacobi integral in the presence of the external tidal field. Then the
construction of the model requires the investigation of a singular perturbation
problem for an elliptic partial differential equation with a free boundary, for
which we provide a method of solution to any desired order, with explicit
solutions to two orders. We outline the relevant parameter space, thus opening
the way to a systematic study of the properties of a two-parameter family of
physically justified non-spherical models of quasi-relaxed stellar systems. The
general method developed here can also be used to construct models for which
the non-spherical shape is due to internal rotation. Eventually, the models
will be a useful tool to investigate whether the shapes of globular clusters
are primarily determined by internal rotation, by external tides, or by
pressure anisotropy.Comment: AASTeX v5.2, 37 pages with 2 figures, accepted for publication in The
Astrophysical Journa
Perturbative Analysis of Adaptive Smoothing Methods in Quantifying Large-Scale Structure
Smoothing operation to make continuous density field from observed point-like
distribution of galaxies is crucially important for topological or
morphological analysis of the large-scale structure, such as, the genus
statistics or the area statistics (equivalently the level crossing statistics).
It has been pointed out that the adaptive smoothing filters are more efficient
tools to resolve cosmic structures than the traditional spatially fixed
filters. We study weakly nonlinear effects caused by two representative
adaptive methods often used in smoothed hydrodynamical particle (SPH)
simulations. Using framework of second-order perturbation theory, we calculate
the generalized skewness parameters for the adaptive methods in the case of
initially power-law fluctuations.
Then we apply the multidimensional Edgeworth expansion method and investigate
weakly nonlinear evolution of the genus statistics and the area statistics.
Isodensity contour surfaces are often parameterized by the volume fraction of
the regions above a given density threshold. We also discuss this
parameterization method in perturbative manner.Comment: 42 pages including 9 figure, ApJ 537 in pres
Gravitational Radiation from Coalescing Binary Neutron Stars
We calculate the gravitational radiation produced by the merger and
coalescence of inspiraling binary neutron stars using 3-dimensional numerical
simulations. The stars are modeled as polytropes and start out in the
point-mass limit at wide separation. The hydrodynamic integration is performed
using smooth particle hydrodynamics (SPH) with Newtonian gravity, and the
gravitational radiation is calculated using the quadrupole approximation. We
have run several simulations, varying both the neutron star radius and the
equation of state. The resulting gravitational wave energy spectra are
rich in information about the hydrodynamics of merger and coalescence. In
particular, our results demonstrate that detailed information on both
and the equation of state can in principle be extracted from the spectrum.Comment: 33 pages, LaTex with RevTex macros; 21 figures available in
compressed PostScript format via anonymous ftp to
ftp://zonker.drexel.edu/papers/ns_coll_1 ; in press, Phys. Rev. D (Nov 15,
1994 issue
Gravitational Radiation from the Coalescence of Binary Neutron Stars: Effects Due to the Equation of State, Spin, and Mass Ratio
We calculate the gravitational radiation produced by the coalescence of
inspiraling binary neutron stars in the Newtonian regime using 3-dimensional
numerical simulations. The stars are modeled as polytropes and start out in the
point-mass regime at wide separation. The hydrodynamic integration is performed
using smooth particle hydrodynamics (SPH) with Newtonian gravity, and the
gravitational radiation is calculated using the quadrupole approximation. We
have run a number of simulations varying the neutron star radii, equations of
state, spins, and mass ratio. The resulting gravitational waveforms and spectra
are rich in information about the hydrodynamics of coalescence, and show
characteristic dependence on GM/Rc^2, the equation of state, and the mass
ratio.Comment: 39 pages, uses Latex 2.09. To be published in the Dec. 15, 1996 issue
of Physical Review D. 16 Figures (bitmapped). Originals available in
compressed Postscript format at ftp://zonker.drexel.edu/papers/PAPER2
IMECE2009-12985 REDUCED ORDER MODELING OF ENTRAINED FLOW SOLID FUEL GASIFICATION
ABSTRACT Reduced order models that accurately predict the operation of entrained flow gasifiers as components within integrated gasification combined cycle (IGCC) or polygeneration plants are essential for greater commercialization of gasification-based energy systems. A reduced order model, implemented in Aspen Custom Modeler, for entrained flow gasifiers that incorporates mixing and recirculation, rigorously calculated char properties, drying and devolatilization, chemical kinetics, simplified fluid dynamics, heat transfer, slag behavior and syngas cooling is presented. The model structure and submodels are described. Results are presented for the steady-state simulation of a two-metric-tonne-per-day (2 tpd) laboratory-scale Mitsubishi Heavy Industries (MHI) gasifier, fed by two different types of coal. Improvements over the state-of-the-art for reduced order modeling include the ability to incorporate realistic flow conditions and hence predict the gasifier internal and external temperature profiles, the ability to easily interface the model with plant-wide flowsheet models, and the flexibility to apply the same model to a variety of entrained flow gasifier designs. Model validation shows satisfactory agreement with measured values and computational fluid dynamics (CFD) results for syngas temperature profiles, syngas composition, carbon conversion, char flow rate, syngas heating value and cold gas efficiency. Analysis of the results shows the accuracy of the reduced order model to be similar to that of more detailed models that incorporate CFD. Next steps include the activation of pollutant chemistry and slag submodels, application of the reduced order model to other gasifier designs, parameter studies and uncertainty analysis of unknown and/or assumed physical and modeling parameters, and activation of dynamic simulation capability
Conformally Flat Smoothed Particle Hydrodynamics: Application to Neutron Star Mergers
We present a new 3D SPH code which solves the general relativistic field +
hydrodynamics equations in the conformally flat approximation. Several test
cases are considered to test different aspects of the code. We finally apply
then the code to the coalescence of a neutron star binary system. The neutron
stars are modeled by a polytropic equation of state (EoS) with adiabatic
indices , and . We calculate the
gravitational wave signals, luminosities and frequency spectra by employing the
quadrupole approximation for emission and back reaction in the slow motion
limit. In addition, we consider the amount of ejected mass.Comment: 23 pages, 12 figures. Accepted for publication in Phys. Rev. D. v3:
Final Versio
- …