600 research outputs found
An adaptive computation mesh for the solution of singular perturbation problems
In singular perturbation problems, control of zone size variation can affect the effort required to obtain accurate, numerical solutions of finite difference equations. The mesh is generated by the solution of potential equations. Numerical results for a singular perturbation problem in two dimensions are presented. The mesh was used in calculations of resistive magnetohydrodynamic flow in two dimensions
Advanced Conducting Project
Chapter 1: Celebration Tribalesque, by Randall D. Standridge, pg. 2 Chapter 2: Nimrod from Enigma Variations, by Edward Elgar, arr. Alfred Reed, pg. 18 Chapter 3: Daedalus\u27 Labyrinth, by Sean O\u27Loughlin, pg. 37 Chapter 4: Lightning Field, by John Mackey, pg. 56 Chapter 5: Mambo from West Side Story, by Leonard Bernstein, arr. Michael Sweeney, pg. 7 4 Chapter 6: Prairie Dances, by David R. Holsinger, pg. 91 Chapter 7: Alchemy, by Andrew Boysen, Jr., pg. 110 Chapter 8: Symphonic Suite, by James Clifton Williams, pg. 130 Chapter 9: Vesuvius, by Frank Ticheli, pg. 16
An Intercomparison Between Divergence-Cleaning and Staggered Mesh Formulations for Numerical Magnetohydrodynamics
In recent years, several different strategies have emerged for evolving the
magnetic field in numerical MHD. Some of these methods can be classified as
divergence-cleaning schemes, where one evolves the magnetic field components
just like any other variable in a higher order Godunov scheme. The fact that
the magnetic field is divergence-free is imposed post-facto via a
divergence-cleaning step. Other schemes for evolving the magnetic field rely on
a staggered mesh formulation which is inherently divergence-free. The claim has
been made that the two approaches are equivalent. In this paper we
cross-compare three divergence-cleaning schemes based on scalar and vector
divergence-cleaning and a popular divergence-free scheme. All schemes are
applied to the same stringent test problem. Several deficiencies in all the
divergence-cleaning schemes become clearly apparent with the scalar
divergence-cleaning schemes performing worse than the vector
divergence-cleaning scheme. The vector divergence-cleaning scheme also shows
some deficiencies relative to the staggered mesh divergence-free scheme. The
differences can be explained by realizing that all the divergence-cleaning
schemes are based on a Poisson solver which introduces a non-locality into the
scheme, though other subtler points of difference are also catalogued. By using
several diagnostics that are routinely used in the study of turbulence, it is
shown that the differences in the schemes produce measurable differences in
physical quantities that are of interest in such studies
Plasma sheet structure in the magnetotail: kinetic simulation and comparison with satellite observations
We use the results of a three-dimensional kinetic simulation of an Harris
current sheet to propose an explanation and to reproduce the ISEE-1/2, Geotail,
and Cluster observations of the magnetotail current sheet structure. Current
sheet flapping, current density bifurcation, and reconnection are explained as
the results of the kink and tearing instabilities, which dominate the current
sheet evolution.Comment: Submitted to Geophys. Res. Lett. (2003
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Experimental evidence for tipping points in social convention
Theoretical models of critical mass have shown how minority groups can initiate social change dynamics in the emergence of new social conventions. Here we study an artificial system of social conventions in which human subjects interact to establish a new coordination equilibrium. The findings provide direct empirical demonstration of the existence of a tipping point in the dynamics of changing social conventions. When minority groups reached the critical mass –that is, the critical group size for initiating social change –they were consistently able to overturn the established behavior. The size of the required critical mass is expected to vary based on theoretically identifiable features of a social setting. Our results show that the theoretically predicted dynamics of critical mass do in fact emerge as expected within an empirical system of social coordination
Influence of the Lower Hybrid Drift Instability on the onset of Magnetic Reconnection
Two-dimensional and three-dimensional kinetic simulation results reveal the
importance of the Lower-Hybrid Drift Instability LHDI to the onset of magnetic
reconnection. Both explicit and implicit kinetic simulations show that the LHDI
heats electrons anisotropically and increases the peak current density. Linear
theory predicts these modifications can increase the growth rate of the tearing
instability by almost two orders of magnitude and shift the fastest growing
modes to significantly shorter wavelengths. These predictions are confirmed by
nonlinear kinetic simulations in which the growth and coalescence of small
scale magnetic islands leads to a rapid onset of large scale reconnection
3D reconnection due to oblique modes: a simulation of Harris current sheets
International audienceSimulations in three dimensions of a Harris current sheet with mass ratio, mi/me = 180, and current sheet thickness, pi/L = 0.5, suggest the existence of a linearly unstable oblique mode, which is independent from either the drift-kink or the tearing instability. The new oblique mode causes reconnection independently from the tearing mode. During the initial linear stage, the system is unstable to the tearing mode and the drift kink mode, with growth rates that are accurately described by existing linear theories. How-ever, oblique modes are also linearly unstable, but with smaller growth rates than either the tearing or the drift-kink mode. The non-linear stage is first reached by the drift-kink mode, which alters the initial equilibrium and leads to a change in the growth rates of the tearing and oblique modes. In the non-linear stage, the resulting changes in magnetic topology are incompatible with a pure tearing mode. The oblique mode is shown to introduce a helical structure into the magnetic field lines
Hydraulics and heat transfer in the IFMIF liquid lithium target: CFD calculations
CFD (Computational fluid dynamics) calculation turns out to be a good approximation to the real behavior of the lithium (Li) flow of the target of the international fusion materials irradiation facility (IFMIF). A three-dimensional (3D) modelling of the IFMIF design Li target assembly, made with the CFD commercial code ANSYS-FLUENT has been carried out. The simulation by a structural mesh is focused on the thermal-hydraulic analysis inside the Li jet flow. For, this purpose, the two deuteron beams energy deposition profile is modelled as an energy source term inside the volume of liquid affected. Turbulence is estimated using the RNG k– model, and a surface-tracking technique applied to a fixed Eulerian mesh called volume of fluid (VOF) is used to determine the position of the free surface. Calculations varying the jet velocity from a range of 10–20 m/s, show that maximum calculated temperatures are still below the lithium's boiling point, due to the increase of the pressure induced by centrifugal forc
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