793 research outputs found
A Census and Evaluation of Invited Exotic Plants in Owenstown, Andros Island, Bahamas-25 Years after Evacuation
Hydrodynamic simulations with the Godunov SPH
We present results based on an implementation of the Godunov Smoothed
Particle Hydrodynamics (GSPH), originally developed by Inutsuka (2002), in the
GADGET-3 hydrodynamic code. We first review the derivation of the GSPH
discretization of the equations of moment and energy conservation, starting
from the convolution of these equations with the interpolating kernel. The two
most important aspects of the numerical implementation of these equations are
(a) the appearance of fluid velocity and pressure obtained from the solution of
the Riemann problem between each pair of particles, and (b the absence of an
artificial viscosity term. We carry out three different controlled
hydrodynamical three-dimensional tests, namely the Sod shock tube, the
development of Kelvin-Helmholtz instabilities in a shear flow test, and the
"blob" test describing the evolution of a cold cloud moving against a hot wind.
The results of our tests confirm and extend in a number of aspects those
recently obtained by Cha (2010): (i) GSPH provides a much improved description
of contact discontinuities, with respect to SPH, thus avoiding the appearance
of spurious pressure forces; (ii) GSPH is able to follow the development of
gas-dynamical instabilities, such as the Kevin--Helmholtz and the
Rayleigh-Taylor ones; (iii) as a result, GSPH describes the development of curl
structures in the shear-flow test and the dissolution of the cold cloud in the
"blob" test.
We also discuss in detail the effect on the performances of GSPH of changing
different aspects of its implementation. The results of our tests demonstrate
that GSPH is in fact a highly promising hydrodynamic scheme, also to be coupled
to an N-body solver, for astrophysical and cosmological applications.
[abridged]Comment: 19 pages, 13 figures, MNRAS accepted, high resolution version can be
obtained at
http://adlibitum.oats.inaf.it/borgani/html/papers/gsph_hydrosim.pd
Analysis and Implementation of Recovery-Based Discontinuous Galerkin for Diffusion
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76575/1/AIAA-2009-3786-303.pd
Comment on Viscous Stability of Relativistic Keplerian Accretion Disks
Recently Ghosh (1998) reported a new regime of instability in Keplerian
accretion disks which is caused by relativistic effects. This instability
appears in the gas pressure dominated region when all relativistic corrections
to the disk structure equations are taken into account. We show that he uses
the stability criterion in completely wrong way leading to inappropriate
conclusions. We perform a standard stability analysis to show that no unstable
region can be found when the relativistic disk is gas pressure dominated.Comment: 9 pages, 4 figures, uses aasms4.sty, submitted for ApJ Letter
Solving One Dimensional Scalar Conservation Laws by Particle Management
We present a meshfree numerical solver for scalar conservation laws in one
space dimension. Points representing the solution are moved according to their
characteristic velocities. Particle interaction is resolved by purely local
particle management. Since no global remeshing is required, shocks stay sharp
and propagate at the correct speed, while rarefaction waves are created where
appropriate. The method is TVD, entropy decreasing, exactly conservative, and
has no numerical dissipation. Difficulties involving transonic points do not
occur, however inflection points of the flux function pose a slight challenge,
which can be overcome by a special treatment. Away from shocks the method is
second order accurate, while shocks are resolved with first order accuracy. A
postprocessing step can recover the second order accuracy. The method is
compared to CLAWPACK in test cases and is found to yield an increase in
accuracy for comparable resolutions.Comment: 15 pages, 6 figures. Submitted to proceedings of the Fourth
International Workshop Meshfree Methods for Partial Differential Equation
A neighbourhood theorem for submanifolds in generalized complex geometry
We study neighbourhoods of submanifolds in generalized complex geometry. Our
first main result provides sufficient criteria for such a submanifold to admit
a neighbourhood on which the generalized complex structure is B-field
equivalent to a holomorphic Poisson structure. This is intimately tied with our
second main result, which is a rigidity theorem for generalized complex
deformations of holomorphic Poisson structures. Specifically, on a compact
manifold with boundary we provide explicit conditions under which any
generalized complex perturbation of a holomorphic Poisson structure is B-field
equivalent to another holomorphic Poisson structure. The proofs of these
results require two analytical tools: Hodge decompositions on almost complex
manifolds with boundary, and the Nash-Moser algorithm. As a concrete
application of these results, we show that on a four-dimensional generalized
complex submanifold which is generically symplectic, a neighbourhood of the
entire complex locus is B-field equivalent to a holomorphic Poisson structure.
Furthermore, we use the neighbourhood theorem to develop the theory of blowing
down submanifolds in generalized complex geometry.Comment: 36 pages, minor change
Stability of the viscously spreading ring
We study analytically and numerically the stability of the pressure-less,
viscously spreading accretion ring. We show that the ring is unstable to small
non-axisymmetric perturbations. To perform the perturbation analysis of the
ring we use a stretching transformation of the time coordinate. We find that to
1st order, one-armed spiral structures, and to 2nd order additionally two-armed
spiral features may appear. Furthermore, we identify a dispersion relation
determining the instability of the ring. The theoretical results are confirmed
in several simulations, using two different numerical methods. These
computations prove independently the existence of a secular spiral instability
driven by viscosity, which evolves into persisting leading and trailing spiral
waves. Our results settle the question whether the spiral structures found in
earlier simulations of the spreading ring are numerical artifacts or genuine
instabilities.Comment: 13 pages, 12 figures; A&A accepte
Numerical simulations of kink instability in line-tied coronal loops
The results from numerical simulations carried out using a new shock-capturing, Lagrangian-remap, 3D MHD code, Lare3d are presented. We study the evolution of the m=1 kink mode instability in a photospherically line-tied coronal loop that has no net axial current. During the non-linear evolution of the kink instability, large current concentrations develop in the neighbourhood of the infinite length mode rational surface. We investigate whether this strong current saturates at a finite value or whether scaling indicates current sheet formation. In particular, we consider the effect of the shear, defined by where is the fieldline twist of the loop, on the current concentration. We also include a non-uniform resistivity in the simulations and observe the amount of free magnetic energy released by magnetic reconnection
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