830 research outputs found
A New Godunov Scheme for MHD, with Application to the MRI in disks
We describe a new numerical scheme for MHD which combines a higher order
Godunov method (PPM) with Constrained Transport. The results from a selection
of multidimensional test problems are presented. The complete test suite used
to validate the method, as well as implementations of the algorithm in both F90
and C, are available from the web. A fully three-dimensional version of the
algorithm has been developed, and is being applied to a variety of
astrophysical problems including the decay of supersonic MHD turbulence, the
nonlinear evolution of the MHD Rayleigh-Taylor instability, and the saturation
of the magnetorotational instability in the shearing box. Our new simulations
of the MRI represent the first time that a higher-order Godunov scheme has been
applied to this problem, providing a quantitative check on the accuracy of
previous results computed with ZEUS; the latter are found to be reliable.Comment: 11 pages, style files included, Conference Proceedings: "Magnetic
Fields in the Universe: from Laboratory and Stars to Primordial Structures",
More information on Athena can be found at
http://www.astro.princeton.edu/~jstone/athena.htm
Energetics in MRI driven Turbulence
In these proceedings we present recent efforts to understand the energetics
of magnetohydrodynamic (MHD) turbulence driven by the magneto-rotational
instability (MRI). These studies are carried out in the local (shearing box)
approximation using the Athena simulation code. Athena is a higher order
Godunov algorithm based on the piecewise parabolic method (PPM), the corner
transport upwind (CTU) integration algorithm, and the constrained transport
(CT) algorithm for evolving the magnetic field. This algorithm is particularly
suited for these studies owing to the conservation properties of a Godunov
scheme and the particular implementation of the shearing box source terms used
here. We present a variety of calculations which may be compared directly to
previously published results and discuss them in some detail. The only
significant discrepancy found between the results presented here and in the
published literature involves the turbulent heating rate. We observe the
presence of recurrent channel solutions in calculations involving a mean
vertical magnetic field and the associated time lag between the energy
injection and thermalization rate. We also present the results of a shearing
box calculation which includes an optically thin radiative term with a cooling
rate selected to match the turbulent heating rate. Some properties which we
find uniformly present in all of the calculations presented here are
compressible fluctuations, spiral waves and weak shocks. It is found that these
compressible modes dominate the temporal fluctuations in the probability
distribution functions for most of the thermodynamic variables; only the
specific entropy is relatively immune to their effects.Comment: 14 pages, Conference Proceedings: "Magnetic Fields in the Universe:
from Laboratory and Stars to Primordial Structures
Fourier's Law for Quasi One--Dimensional Chaotic Quantum Systems
We derive Fourier's law for a completely coherent quasi one--dimensional
chaotic quantum system coupled locally to two heat baths at different
temperatures. We solve the master equation to first order in the temperature
difference. We show that the heat conductance can be expressed as a
thermodynamic equilibrium coefficient taken at some intermediate temperature.
We use that expression to show that for temperatures large compared to the mean
level spacing of the system, the heat conductance is inversely proportional to
the level density and, thus, inversely proportional to the length of the
system
Target search on a dynamic DNA molecule
We study a protein-DNA target search model with explicit DNA dynamics
applicable to in vitro experiments. We show that the DNA dynamics plays a
crucial role for the effectiveness of protein "jumps" between sites distant
along the DNA contour but close in 3D space. A strongly binding protein that
searches by 1D sliding and jumping alone, explores the search space less
redundantly when the DNA dynamics is fast on the timescale of protein jumps
than in the opposite "frozen DNA" limit. We characterize the crossover between
these limits using simulations and scaling theory. We also rationalize the slow
exploration in the frozen limit as a subtle interplay between long jumps and
long trapping times of the protein in "islands" within random DNA
configurations in solution.Comment: manuscript and supplementary material combined into a single documen
Effect of the Coriolis Force on the Hydrodynamics of Colliding Wind Binaries
Using fully three-dimensional hydrodynamic simulations, we investigate the
effect of the Coriolis force on the hydrodynamic and observable properties of
colliding wind binary systems. To make the calculations tractable, we assume
adiabatic, constant velocity winds. The neglect of radiative driving,
gravitational deceleration, and cooling limit the application of our models to
real systems. However, these assumptions allow us to isolate the effect of the
Coriolis force, and by simplifying the calculations, allow us to use a higher
resolution (up to 640^3) and to conduct a larger survey of parameter space. We
study the dynamics of collidng winds with equal mass loss rates and velocities
emanating from equal mass stars on circular orbits, with a range of values for
the ratio of the wind to orbital velocity. We also study the dynamics of winds
from stars on elliptical orbits and with unequal strength winds. Orbital motion
of the stars sweeps the shocked wind gas into an Archimedean spiral, with
asymmetric shock strengths and therefore unequal postshock temperatures and
densities in the leading and trailing edges of the spiral. We observe the
Kelvin-Helmholtz instability at the contact surface between the shocked winds
in systems with orbital motion even when the winds are identical. The change in
shock strengths caused by orbital motion increases the volume of X-ray emitting
post-shock gas with T > 0.59 keV by 63% for a typical system as the ratio of
wind velocity to orbital velocity decreases to V_w/V_o = 2.5. This causes
increased free-free emission from systems with shorter orbital periods and an
altered time-dependence of the wind attenuation. We comment on the importance
of the effects of orbital motion on the observable properties of colliding wind
binaries.Comment: 12 pages, 17 figures, accepted for publication in Ap
An Unsplit Godunov Method for Ideal MHD via Constrained Transport in Three Dimensions
We present a single step, second-order accurate Godunov scheme for ideal MHD
which is an extension of the method described by Gardiner & Stone (2005) to
three dimensions. This algorithm combines the corner transport upwind (CTU)
method of Colella for multidimensional integration, and the constrained
transport (CT) algorithm for preserving the divergence-free constraint on the
magnetic field. We describe the calculation of the PPM interface states for 3D
ideal MHD which must include multidimensional ``MHD source terms'' and
naturally respect the balance implicit in these terms by the condition. We compare two different forms for the CTU integration
algorithm which require either 6- or 12-solutions of the Riemann problem per
cell per time-step, and present a detailed description of the 6-solve
algorithm. Finally, we present solutions for test problems to demonstrate the
accuracy and robustness of the algorithm.Comment: Extended version of the paper accepted for publication in JC
Measurement-based approach to entanglement generation in coupled quantum dots
Measurements provide a novel mechanism for generating the entanglement
resource necessary for performing scalable quantum computation. Recently, we
proposed a method for performing parity measurements in a coupled quantum dot
system. In this paper we generalise this scheme and perform a comprehensive
analytic and numerical study of environmental factors. We calculate the effects
of possible error sources including non-ideal photon detectors, ineffective
spin-selective excitation and dot distinguishability (both spatial and
spectral). Furthermore, we present an experimental approach for verifying the
success of the parity measurement
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