1,748 research outputs found
Further tests of a model-based scheme for predicting pilot opinion ratings for large commercial transports
A methodology was demonstrated for assessing longitudinal-axis handling qualities of transport aircraft on the basis of closed-loop criteria. Six longitudinal-axis approach configurations were studied covering a range of handling quality problems that included the presence of flexible aircraft modes. Using closed-loop performance requirements derived from task analyses and pilot interviews, predictions of performance/workload tradeoffs were obtained using an analytical pilot/vehicle model. A subsequent manned simulation study yielded objective performance measures and Cooper-Harper pilot ratings that were largely consistent with each other and with analytic predictions
Analytical and simulator study of advanced transport
An analytic methodology, based on the optimal-control pilot model, was demonstrated for assessing longitidunal-axis handling qualities of transport aircraft in final approach. Calibration of the methodology is largely in terms of closed-loop performance requirements, rather than specific vehicle response characteristics, and is based on a combination of published criteria, pilot preferences, physical limitations, and engineering judgment. Six longitudinal-axis approach configurations were studied covering a range of handling qualities problems, including the presence of flexible aircraft modes. The analytical procedure was used to obtain predictions of Cooper-Harper ratings, a solar quadratic performance index, and rms excursions of important system variables
A covariant formalism of spin precession with respect to a reference congruence
We derive an effectively three-dimensional relativistic spin precession
formalism. The formalism is applicable to any spacetime where an arbitrary
timelike reference congruence of worldlines is specified. We employ what we
call a stopped spin vector which is the spin vector that we would get if we
momentarily make a pure boost of the spin vector to stop it relative to the
congruence. Starting from the Fermi transport equation for the standard spin
vector we derive a corresponding transport equation for the stopped spin
vector. Employing a spacetime transport equation for a vector along a
worldline, corresponding to spatial parallel transport with respect to the
congruence, we can write down a precession formula for a gyroscope relative to
the local spatial geometry defined by the congruence. This general approach has
already been pursued by Jantzen et. al. (see e.g. Jantzen, Carini and Bini,
Ann. Phys. 215 (1997) 1), but the algebraic form of our respective expressions
differ. We are also applying the formalism to a novel type of spatial parallel
transport introduced in Jonsson (Class. Quantum Grav. 23 (2006) 1), as well as
verifying the validity of the intuitive approach of a forthcoming paper
(Jonsson, Am. Journ. Phys. 75 (2007) 463) where gyroscope precession is
explained entirely as a double Thomas type of effect. We also present the
resulting formalism in explicit three-dimensional form (using the boldface
vector notation), and give examples of applications.Comment: 27 pages, 8 figure
Tilting mutation of weakly symmetric algebras and stable equivalence
We consider tilting mutations of a weakly symmetric algebra at a subset of
simple modules, as recently introduced by T. Aihara. These mutations are
defined as the endomorphism rings of certain tilting complexes of length 1.
Starting from a weakly symmetric algebra A, presented by a quiver with
relations, we give a detailed description of the quiver and relations of the
algebra obtained by mutating at a single loopless vertex of the quiver of A. In
this form the mutation procedure appears similar to, although significantly
more complicated than, the mutation procedure of Derksen, Weyman and Zelevinsky
for quivers with potentials. By definition, weakly symmetric algebras connected
by a sequence of tilting mutations are derived equivalent, and hence stably
equivalent. The second aim of this article is to study these stable
equivalences via a result of Okuyama describing the images of the simple
modules. As an application we answer a question of Asashiba on the derived
Picard groups of a class of self-injective algebras of finite representation
type. We conclude by introducing a mutation procedure for maximal systems of
orthogonal bricks in a triangulated category, which is motivated by the effect
that a tilting mutation has on the set of simple modules in the stable
category.Comment: Description and proof of mutated algebra made more rigorous (Prop.
3.1 and 4.2). Okuyama's Lemma incorporated: Theorem 4.1 is now Corollary 5.1,
and proof is omitted. To appear in Algebras and Representation Theor
Crossing Borders, Crossing Genres: Utilizing Genres to Explore Literary Themes Through Genre Fiction
Genre fiction can be used to explore literary themes found in marginalized literature such as Alicia Gaspar de Alba’s Desert Blood: The Juárez Murders, Emma Pérez’s Forgetting the Alamo or Blood Money, and Octavia Butler’s Kindred. Each author uses the respective genres of hard-boiled detective fiction, American Western literature, and science fiction to explore the elements of borderland literature and the neo-slave narrative. These elements include hybrid identities, the clash between two cultures, disjunctive localities, and the marginalization of both ethnic groups and women. This thesis will show how each genre’s elements are used to further explore the elements of borderland fiction and the neo-slave narrative and will argue that the conventions of genre and the political concerns of borderland literature and neo-slave narratives are mutually constitutive. This thesis will demonstrate that the conventions of genre, rather than detracting from the important political work of the novels, actually heightens it effectively, highlighting the radical work that genre can do
Magnetic Flux Braiding: Force-Free Equilibria and Current Sheets
We use a numerical nonlinear multigrid magnetic relaxation technique to
investigate the generation of current sheets in three-dimensional magnetic flux
braiding experiments. We are able to catalogue the relaxed nonlinear force-free
equilibria resulting from the application of deformations to an initially
undisturbed region of plasma containing a uniform, vertical magnetic field. The
deformations are manifested by imposing motions on the bounding planes to which
the magnetic field is anchored. Once imposed the new distribution of magnetic
footpoints are then taken to be fixed, so that the rest of the plasma must then
relax to a new equilibrium configuration. For the class of footpoint motions we
have examined, we find that singular and nonsingular equilibria can be
generated. By singular we mean that within the limits imposed by numerical
resolution we find that there is no convergence to a well-defined equilibrium
as the number of grid points in the numerical domain is increased. These
singular equilibria contain current "sheets" of ever-increasing current
intensity and decreasing width; they occur when the footpoint motions exceed a
certain threshold, and must include both twist and shear to be effective. On
the basis of these results we contend that flux braiding will indeed result in
significant current generation. We discuss the implications of our results for
coronal heating.Comment: 13 pages, 12 figure
The homotopy theory of dg-categories and derived Morita theory
The main purpose of this work is the study of the homotopy theory of
dg-categories up to quasi-equivalences. Our main result provides a natural
description of the mapping spaces between two dg-categories and in
terms of the nerve of a certain category of -bimodules. We also prove
that the homotopy category is cartesian closed (i.e. possesses
internal Hom's relative to the tensor product). We use these two results in
order to prove a derived version of Morita theory, describing the morphisms
between dg-categories of modules over two dg-categories and as the
dg-category of -bi-modules. Finally, we give three applications of our
results. The first one expresses Hochschild cohomology as endomorphisms of the
identity functor, as well as higher homotopy groups of the \emph{classifying
space of dg-categories} (i.e. the nerve of the category of dg-categories and
quasi-equivalences between them). The second application is the existence of a
good theory of localization for dg-categories, defined in terms of a natural
universal property. Our last application states that the dg-category of
(continuous) morphisms between the dg-categories of quasi-coherent (resp.
perfect) complexes on two schemes (resp. smooth and proper schemes) is
quasi-equivalent to the dg-category of quasi-coherent complexes (resp. perfect)
on their product.Comment: 50 pages. Few mistakes corrected, and some references added. Thm.
8.15 is new. Minor corrections. Final version, to appear in Inventione
Recommended from our members
Land restoration after strip mining for coal
Recent legislation requires that lands surface mined for coal be returned to approximate original topography and vegetative cover be restored. Spoils provide poor rooting habitat because of extreme stoniness or excessive slope steepness which provide few niches for seeds to become lodged and also spoil may provide poor mineral nutrition, poor water retention and sometimes the spoil may even have chemical properties detrimental to plant growth (acidity, alkalinity or even unusually large amounts of toxic mineral elements i.e., copper, sodium). To provide a substrate better suited for plant growth, recommendations for restoration call for deep burial of unfavorable substrate components i.e., rocks and materials of unusual chemistry and the dressing of reshaped spoil with topsoil i.e., material with the most favorable properties for plant growth. Even though all the substrate requirements for healthy plant growth may be met, such as adding a form of available nitrogen as fertilizer, plants will not grow if weather conditions are extreme. For example, in very dry (desert) climates precipitation may be too scanty or too erratic to permit the successful establishment of many kinds of plants. Even under the most favorable conditions plant productivity averaged over a period of years is low. Also in very cold climates the growing season may be limited to only a few weeks in summer e.g., arctic and alpine tundra regions. This shortens the time available for photosynthesis and keeps plant productivity low
Bilinear forms on Grothendieck groups of triangulated categories
We extend the theory of bilinear forms on the Green ring of a finite group
developed by Benson and Parker to the context of the Grothendieck group of a
triangulated category with Auslander-Reiten triangles, taking only relations
given by direct sum decompositions. We examine the non-degeneracy of the
bilinear form given by dimensions of homomorphisms, and show that the form may
be modified to give a Hermitian form for which the standard basis given by
indecomposable objects has a dual basis given by Auslander-Reiten triangles. An
application is given to the homotopy category of perfect complexes over a
symmetric algebra, with a consequence analogous to a result of Erdmann and
Kerner.Comment: arXiv admin note: substantial text overlap with arXiv:1301.470
H3+ in Diffuse Interstellar Clouds: a Tracer for the Cosmic-Ray Ionization Rate
Using high resolution infrared spectroscopy we have surveyed twenty
sightlines for H3+ absorption. H3+ is detected in eight diffuse cloud
sightlines with column densities varying from 0.6x10^14 cm^-2 to 3.9x10^14
cm^-2. This brings to fourteen the total number of diffuse cloud sightlines
where H3+ has been detected. These detections are mostly along sightlines
concentrated in the Galactic plane, but well dispersed in Galactic longitude.
The results imply that abundant H3+ is common in the diffuse interstellar
medium. Because of the simple chemistry associated with H3+ production and
destruction, these column density measurements can be used in concert with
various other data to infer the primary cosmic-ray ionization rate, zeta_p.
Values range from 0.5x10^-16 s^-1 to 3x10^-16 s^-1 with an average of 2x10^-16
s^-1. Where H3+ is not detected the upper limits on the ionization rate are
consistent with this range. The average value of zeta_p is about an order of
magnitude larger than both the canonical rate and rates previously reported by
other groups using measurements of OH and HD. The discrepancy is most likely
due to inaccurate measurements of rate constants and the omission of effects
which were unknown when those studies were performed. We believe that the
observed column density of H3+ is the most direct tracer for the cosmic-ray
ionization rate due to its simple chemistry. Recent models of diffuse cloud
chemistry require cosmic-ray ionization rates on the order of 10^-16 s^-1 to
reproduce observed abundances of various atomic and molecular species, in rough
accord with our observational findings.Comment: Accepted to ApJ, 35 pages, 5 figures, 5 table
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