71,559 research outputs found
Giant Leaps and Minimal Branes in Multi-Dimensional Flux Landscapes
There is a standard story about decay in multi-dimensional flux landscapes:
that from any state, the fastest decay is to take a small step, discharging one
flux unit at a time; that fluxes with the same coupling constant are
interchangeable; and that states with N units of a given flux have the same
decay rate as those with -N. We show that this standard story is false. The
fastest decay is a giant leap that discharges many different fluxes in unison;
this decay is mediated by a 'minimal' brane that wraps the internal manifold
and exhibits behavior not visible in the effective theory. We discuss the
implications for the cosmological constant.Comment: Minor updates to agree with published version. 9 pages, 4 figure
Revised Results for Non-thermal Recombination Flare Hard X-Ray Emission
Brown and Mallik (BM) recently showed that, for hot sources, recombination of
non-thermal electrons (NTR) onto highly ionised heavy ions is not negligible
compared to non-thermal bremsstrahlung (NTB) as a source of flare hard X-rays
(HXRs) and so should be included in modelling non-thermal HXR flare emission.
In view of major discrepancies between BM results for the THERMAL continua and
those of the Chianti code and of RHESSI solar data, we critically re-examine
and correct the BM analysis and modify the conclusions concerning the
importance of NTR. Although the analytic Kramers expression used by BM is
correct for the purely hydrogenic recombination cross section, the heuristic
expressions used by BM to extend the Kramers expression beyond the `bare
nucleus' case to which it applies had serious errors. BM results have therefore
been recalculated using corrected expressions, which have been validated
against the results of detailed calculations. At T ~ 10-30 MK the dominant ions
are Fe 22+, 23+, 24+ for which BM erroneously overestimated NTR emission by
around an order of magnitude. Contrary to the BM claim, NTR in hot flare
plasmas does NOT dominate over NTB, although in some cases it can be comparable
and so still very important in inversions of photon spectra to derive electron
spectra, especially as NTR includes sharp edge features. The BM claim of
dominance of NTR over NTB in deka-keV emission is incorrect due to a serious
error in their analysis. However, the NTR contribution can still be large
enough to demand inclusion in spectral fitting, the spectral edges having
potentially serious effects on inversion of HXR spectra to infer fast electron
spectra.Comment: 6 pages, 8 figures, 1 tabl
Extension and application of a sequential estimator
Improved sequential estimation technique for nonlinear time varying system
Nonlinear bending-torsional vibration and stability of rotating, pretwisted, preconed blades including Coriolis effects
The coupled bending-bending-torsional equations of dynamic motion of rotating, linearly pretwisted blades are derived including large precone, second degree geometric nonlinearities and Coriolis effects. The equations are solved by the Galerkin method and a linear perturbation technique. Accuracy of the present method is verified by comparisons of predicted frequencies and steady state deflections with those from MSC/NASTRAN and from experiments. Parametric results are generated to establish where inclusion of only the second degree geometric nonlinearities is adequate. The nonlinear terms causing torsional divergence in thin blades are identified. The effects of Coriolis terms and several other structurally nonlinear terms are studied, and their relative importance is examined
An inventory of aeronautical ground research facilities. Volume 1: Wind tunnels
A survey of wind tunnel research facilities in the United States is presented. The inventory includes all subsonic, transonic, and hypersonic wind tunnels operated by governmental and private organizations. Each wind tunnel is described with respect to size, mechanical operation, construction, testing capabilities, and operating costs. Facility performance data are presented in charts and tables
An inventory of aeronautical ground research facilities. Volume 3: Structural
An inventory of test facilities for conducting acceleration, environmental, impact, structural shock, load, heat, vibration, and noise tests is presented. The facility is identified with a description of the equipment, the testing capabilities, and cost of operation. Performance data for the facility are presented in charts and tables
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Dissecting Visual Analytics: Comparing Frameworks for Interpreting and Modelling Observed Visual Analytics Behavior
This paper provides an empirical, comparative exploration of the role of analytic frameworks in interpreting and modelling visual analytics behavior through data gathered in observational studies. The crucial research on understanding the complex and multi-faceted interplay between visual analytics tools and their users is often done through controlled or naturalistic observations of analysts engaging in the visual analytic process, followed by the interpretation of the observation data. The researchers in Human Computer Interaction and Cognitive Sciences have long used structured analytic frameworks for such analyses, where a guiding set of principles and questions direct attention to relevant aspects of the studied behavior, eventually leading to more complete and consistent analyses. Such frameworks are rarely applied in the visualization domain however, and information about how to apply them and their benefits is scarce. With this paper, we contribute a comparative account, grounded in empirical data collected in a user study with 10 participants using Tableau to analyze domain-specific data, of the types of insights we can glean from interpreting observational data using three different frameworks: Joint Action Theory, Distributed Cognition, and Situated Cognition
A new method for ranking academic journals in accounting and finance
Given the many and varied uses to which journal rankings are put, interest in ranking journal 'quality' is likely to persist. Unfortunately, existing methods of constructing such rankings all have inherent limitations. This paper proposes a new (complementary) approach, based on submissions to RAE 2001, which is not restricted to a pre-defined journal set and, importantly, is based on quality choice decisions driven by economic incentives. For three metrics, submissions to RAE 2001 are compared with the available set of publications to provide evidence on the perception of journal quality, a fourth metric is based on the overall RAE grades, and an overall ranking is produced
Gravitational Constraint Combinations Generate a Lie Algebra
We find a first--order partial differential equation whose solutions are all
ultralocal scalar combinations of gravitational constraints with Abelian
Poisson brackets between themselves. This is a generalisation of the Kucha\v{r}
idea of finding alternative constraints for canonical gravity. The new scalars
may be used in place of the hamiltonian constraint of general relativity and,
together with the usual momentum constraints, replace the Dirac algebra for
pure gravity with a true Lie algebra: the semidirect product of the Abelian
algebra of the new constraint combinations with the algebra of spatial
diffeomorphisms.Comment: 10 pages, latex, submitted to Classical and Quantum Gravity. Section
3 is expanded and an additional solution provided, minor errors correcte
Kaon Energies in Dense Matter
We discuss the role of kaon-nucleon and nucleon-nucleon correlations in kaon
condensation in dense matter. Correlations raise the threshold density for kaon
condensation, possibly to densities higher than those encountered in stable
neutron stars.Comment: RevTeX, 11 pages, 2 PostScript figures; manuscript also available, in
PostScript form, at http://www.nordita.dk/locinfo/preprints.htm
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