3,717,552 research outputs found
Constrained Hyperbolic Divergence Cleaning for Smoothed Particle Magnetohydrodynamics
We present a constrained formulation of Dedner et al's hyperbolic/parabolic
divergence cleaning scheme for enforcing the \nabla\dot B = 0 constraint in
Smoothed Particle Magnetohydrodynamics (SPMHD) simulations. The constraint we
impose is that energy removed must either be conserved or dissipated, such that
the scheme is guaranteed to decrease the overall magnetic energy. This is shown
to require use of conjugate numerical operators for evaluating \nabla\dot B and
\nabla{\psi} in the SPMHD cleaning equations. The resulting scheme is shown to
be stable at density jumps and free boundaries, in contrast to an earlier
implementation by Price & Monaghan (2005). Optimal values of the damping
parameter are found to be {\sigma} = 0.2-0.3 in 2D and {\sigma} = 0.8-1.2 in
3D. With these parameters, our constrained Hamiltonian formulation is found to
provide an effective means of enforcing the divergence constraint in SPMHD,
typically maintaining average values of h |\nabla\dot B| / |B| to 0.1-1%, up to
an order of magnitude better than artificial resistivity without the associated
dissipation in the physical field. Furthermore, when applied to realistic, 3D
simulations we find an improvement of up to two orders of magnitude in momentum
conservation with a corresponding improvement in numerical stability at
essentially zero additional computational expense.Comment: 28 pages, 25 figures, accepted to J. Comput. Phys. Movies at
http://www.youtube.com/playlist?list=PL215D649FD0BDA466 v2: fixed inverted
figs 1,4,6, and several color bar
Letter to Secretary of Energy Bodman and Report
Letter to Secretary of Energy Bodman and report – December 18, 2005. Regards Bakken crude oil resource of the Williston Basin and calling for a fresh look at resources in place, recovery factors, state-of-the-art best practices, and how to push them forward to the next logical step
Citizenship, Gender, and Racial Differences in the Publishing Success Of Graduate Students and Young Academics
Although extensive research exists on the publishing success of academics, few studies have examined factors influencing the publishing success of graduate students and young academics. Data from a survey of 12,000 graduate students in the Humanities and related social sciences was used to examine citizenship, gender and racial/ethnic differences in publishing success during graduate school and the first three years after graduation. The results of this analysis indicate that international students have the highest publication rates during graduate school as well as in the first three years following receipt of degree. Results also indicate that female graduate students are less likely than male graduate students to publish, a gap that remains in the years following graduate school. Finally, results indicate that U.S. citizen minority students exhibit lower levels of publishing success compared with non-minority students during graduate school, but that this gap that disappears within the first few years after graduate school
Does Time-Symmetry Imply Retrocausality? How the Quantum World Says "Maybe"
It has often been suggested that retrocausality offers a solution to some of
the puzzles of quantum mechanics: e.g., that it allows a Lorentz-invariant
explanation of Bell correlations, and other manifestations of quantum
nonlocality, without action-at-a-distance. Some writers have argued that
time-symmetry counts in favour of such a view, in the sense that retrocausality
would be a natural consequence of a truly time-symmetric theory of the quantum
world. Critics object that there is complete time-symmetry in classical
physics, and yet no apparent retrocausality. Why should the quantum world be
any different? This note throws some new light on these matters. I call
attention to a respect in which quantum mechanics is different, under some
assumptions about quantum ontology. Under these assumptions, the combination of
time-symmetry without retrocausality is unavailable in quantum mechanics, for
reasons intimately connected with the differences between classical and quantum
physics (especially the role of discreteness in the latter). Not all
interpretations of quantum mechanics share these assumptions, however, and in
those that do not, time-symmetry does not entail retrocausality.Comment: 22 pages, 6 figures; significant revision
Letter to Acting USGS Director Leahy and attachment
Letter to Acting USGS Director Leahy and attachment – February 24, 2006. Request that USGS release unpublished Leigh Price paper to the public
Correspondence with EIA Administrator, The Honorable Guy F. Caruso
Correspondence with the EIA Administrator, The Honorable Guy F. Caruso, on the subject of How Will EIA Capture and Report Information as to Bakken Crude Oil
U.S. Department of Energy Correspondence Close-Out
April 12, 1993, concerning the O\u27Leary letter
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