75,217 research outputs found
Complex eigenvalues for the stability of Couette flow
The eigenvalue problem for the linear stability of Couette flow between rotating concentric cylinders to axisymmetric disturbances is considered. It is shown by numerical calculations and by formal perturbation methods that when the outer cylinder is at rest there exist complex eigenvalues corresponding to oscillatory damped disturbances. The structure of the first few eigenvalues in the spectrum is discussed. The results do not contradict the principle of exchange of stabilities, namely, for a fixed axial wavenumber the first mode to become unstable as the speed of the inner cylinder is increased is nonoscillatory as the stability boundary is crossed
Supersymmetric Yang-Mills and Supergravity Amplitudes at One Loop
By applying the known expressions for SYM and SUGRA tree amplitudes, we write
generating functions for the NNMHV box coefficients of SYM as well as the MHV,
NMHV, and NNMHV box coefficients for SUGRA. The all-multiplicity generating
functions utilize covariant, on-shell superspace whereby the contribution from
arbitrary external states in the supermultiplet can be extracted by Grassmann
operators. In support of the relation between dual Wilson loops and SYM
scattering amplitudes at weak coupling, the SYM amplitudes are presented in a
manifestly dual superconformal form. We introduce ordered box coefficients for
calculating SUGRA quadruple cuts and prove that ordered coefficients generate
physical cut amplitudes after summing over permutations of the external legs.
The ordered box coefficients are produced by sewing ordered subamplitudes,
previously used in applying on-shell recursion relations at tree level. We
describe our verification of the results against the literature, and a formula
for extracting the contributions from external gluons or gravitons to NNMHV
superamplitudes is presented.Comment: 46 pages, 2 figures, additional references and clarifications
include
On the receptivity and non-parallel stability of travelling disturbances in rotating disk flow
The generation and evolution of small amplitude wavelength traveling disturbances in rotating disk flow is discussed. The steady rotational speed of the disk is perturbed so as to introduce high frequency oscillations in the flow field. Secondly, surface imperfections are introduced on the disk such as roughness elements. The interaction of these two disturbances will generate the instability waves whose evolution is governed by parabolic partial differential equations that are solved numerically. For the class of disturbances considered (wavelength on the order of Reynolds number), it is found that eigensolutions exist which decay or grow algebraically in the radial direction. However, these solutions grow only for frequencies larger than 4.58 times the steady rotational speed of the disk. The computed receptivity coefficient shows that there is an optimum size of roughness for which these modes are excited the most. The width of these roughness elements in the radial direction is about .1 r(sub 0) where r(sub 0) is the radial location of the roughness. It is also found that the receptivity coefficient is larger for a negative spanwise wavenumber than for a positive one. Typical wave angles found for these disturbances are about -26 degrees
Discovery of hydroxyl and water masers in R Aquarii and H1-36 Arae
We present the first results from an all-sky maser-line survey of symbiotic
Miras. Interferometric spectral-line observations of R Aqr and H1-36 Arae have
revealed a 22-GHz water maser in the former and 1612-MHz hydroxyl and weak
22-GHz water maser emission from the latter. H1-36 has thus become the first
known symbiotic OH/IR star. We have also detected weak OH line emission from
the vicinity of R Aqr, but we note that there are small discrepencies between
the OH- and H2O-line velocities and positions. These detections demonstrate
unequivocally that dust can shield some circumstellar hydroxyl and water
molecules from dissociation, even in systems which possess intense local
sources of UV. Finally, we discuss some of the implications of these
observations. The narrow profile of the water maser in R Aqr means that there
may finally be an opportunity to determine the system's orbital parameters. We
also point out that high resolution synthesis observations may trace the
distribution of dust in H1-36 and R Aqr, possibly throwing light on the
mass-loss process in symbiotic Miras and placing constraints on the amount of
collimation experienced by UV radiation from their hot, compact companions.Comment: 7 pages; no figures attached; LaTex (MN style); postscript figures
via anonymous ftp in /users/ers on astro.caltech.edu; University of Toronto
pre-print; ERSRJI
Biomass production and nitrogen dynamics in an integrated aquaculture/agriculture system
A combined aquaculture/agriculture system that brings together the three major components of a Controlled Ecological Life Support System (CELSS) - biomass production, biomass processing, and waste recycling - was developed to evaluate ecological processes and hardware requirements necessary to assess the feasibility of and define design criteria for integration into the Kennedy Space Center (KSC) Breadboard Project. The system consists of a 1 square meter plant growth area, a 500 liter fish culture tank, and computerized monitoring and control hardware. Nutrients in the hydrophonic solution were derived from fish metabolites and fish food leachate. In five months of continuous operation, 27.0 kg of lettuce tops, 39.9 kg of roots and biofilm, and 6.6 kg of fish (wet weights) were produced with 12.7 kg of fish food input. Based on dry weights, a biomass conversion index of 0.52 was achieved. A nitrogen budget was derived to determine partitioning of nitrogen within various compartments of the system. Accumulating nitrogen in the hypoponic solution indicated a need to enlarge the plant growth area, potentially increasing the biomass production and improving the biomass conversion index
JointZone: users' view of an adaptive online learning resource for rheumatology
This paper describes an online learning resource for rheumatology that was designed for a wide constituency of users including primarily undergraduate medical students and health professionals. Although the online resources afford an informal learning environment, the site was pedagogically designed to comply with the general recommendations of the Standing Committee on Training and Education of EULAR (European League Against Rheumatism) for a rheumatology core curriculum. Any Internet user may freely browse the site content with optional registration providing access to adaptive features that personalize the user’s view, for example, providing a reading history and targeted support based on scores from completed case studies. The site has now been available since early 2003, and an online survey of site registrants indicates that well structured pedagogical materials that reflect a learners’ dominant ‘community of practice’ appear to be a successful aid to informal learning
On the Theory of Killing Orbits in Space-Time
This paper gives a theoretical discussion of the orbits and isotropies which
arise in a space-time which admits a Lie algebra of Killing vector fields. The
submanifold structure of the orbits is explored together with their induced
Killing vector structure. A general decomposition of a space-time in terms of
the nature and dimension of its orbits is given and the concept of stability
and instability for orbits introduced. A general relation is shown linking the
dimensions of the Killing algebra, the orbits and the isotropies. The
well-behaved nature of "stable" orbits and the possible miss-behaviour of the
"unstable" ones is pointed out and, in particular, the fact that independent
Killing vector fields in space-time may not induce independent such vector
fields on unstable orbits. Several examples are presented to exhibit these
features. Finally, an appendix is given which revisits and attempts to clarify
the well-known theorem of Fubini on the dimension of Killing orbits.Comment: Latex, 19 pages, no figur
Condensate splitting in an asymmetric double well for atom chip based sensors
We report on the adiabatic splitting of a BEC of Rb atoms by an
asymmetric double-well potential located above the edge of a perpendicularly
magnetized TbGdFeCo film atom chip. By controlling the barrier height and
double-well asymmetry the sensitivity of the axial splitting process is
investigated through observation of the fractional atom distribution between
the left and right wells. This process constitutes a novel sensor for which we
infer a single shot sensitivity to gravity fields of . From a simple analytic model we propose improvements
to chip-based gravity detectors using this demonstrated methodology.Comment: 4 pages, 5 figure
Mission analysis of solar powered aircraft
The effect of a real mission scenario on a solar powered airplane configuration which had been developed in previous work were assessed. The mission used was surveillance of crop conditions over a route from Phoenix to Tucson to Tombstone, Arizona. Appendices are attached which address the applicability of existing platforms and payloads to do this mission
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