5,471 research outputs found
Cosmology With A Dark Refraction Index
We review Gordon's optical metric and the transport equations for the
amplitude and polarization of a geometrical optics wave traveling in a gravity
field. We apply the theory to the FLRW cosmologies by associating a refraction
index with the cosmic fluid. We then derive an expression for the accumulated
effect of a refraction index on the distance redshift relations and fit the
Hubble curve of current supernova observations with a non-accelerating
cosmological model. We also show that some observational effects caused by
inhomogeneities, e.g. the Sachs-Wolfe effect, can be interpreted as being
caused by an effective index of refraction, and hence this theory could extend
to other speed of light communications such as gravitational radiation and
neutrino fluxes.Comment: 21 pages, 3 figure
Noncommutative Einstein-Maxwell pp-waves
The field equations coupling a Seiberg-Witten electromagnetic field to
noncommutative gravity, as described by a formal power series in the
noncommutativity parameters , is investigated. A large
family of solutions, up to order one in , describing
Einstein-Maxwell null pp-waves is obtained. The order-one contributions can be
viewed as providing noncommutative corrections to pp-waves. In our solutions,
noncommutativity enters the spacetime metric through a conformal factor and is
responsible for dilating/contracting the separation between points in the same
null surface. The noncommutative corrections to the electromagnetic waves,
while preserving the wave null character, include constant polarization, higher
harmonic generation and inhomogeneous susceptibility. As compared to pure
noncommutative gravity, the novelty is that nonzero corrections to the metric
already occur at order one in .Comment: 19 revtex pages. One refrence suppressed, two references added. Minor
wording changes in the abstract, introduction and conclusio
The practical application of a finite difference method for analyzing transonic flow over oscillating airfoils and wings
Separating the velocity potential into steady and unsteady parts and linearizing the resulting unsteady equations for small disturbances was performed. The steady velocity potential was obtained first from the well known nonlinear equation for steady transonic flow. The unsteady velocity potential was then obtained from a linear differential equation in complex form with spatially varying coefficients. Since sinusoidal motion is assumed, the unsteady equation is independent of time. The results of an investigation into the relaxation-solution-instability problem was discussed. Concepts examined include variations in outer boundary conditions, a coordinate transformation so that the boundary condition at infinity may be applied to the outer boundaries of the finite difference region, and overlapping subregions. The general conclusion was that only a full direct solution in which all unknowns are obtained at the same time will avoid the solution instabilities of relaxation. An analysis of the one-dimensional form of the unsteady transonic equation was studied to evaluate errors between exact and finite difference solutions. Pressure distributions were presented for a low-aspect-ratio clipped delta wing at Mach number of 0.9 and for a moderate-aspect-ratio rectangular wing at a Mach number of 0.875
Shuttle on-orbit contamination and environmental effects
Ensuring the compatibility of the space shuttle system with payloads and payload measurements is discussed. An extensive set of quantitative requirements and goals was developed and implemented by the space shuttle program management. The performance of the Shuttle system as measured by these requirements and goals was assessed partly through the use of the induced environment contamination monitor on Shuttle flights 2, 3, and 4. Contamination levels are low and generally within the requirements and goals established. Additional data from near-term payloads and already planned contamination measurements will complete the environment definition and allow for the development of contamination avoidance procedures as necessary for any payload
Development of learning objectives for neurology in a veterinary curriculum: part I: undergraduates
Background
With an increasing caseload of veterinary neurology patients in first opinion practice, there is a requirement to establish relevant learning objectives for veterinary neurology encompassing knowledge, skills and attitudes for veterinary undergraduate students in Europe. With help of experts in veterinary neurology from the European College of Veterinary Neurology (ECVN) and the European Society of Veterinary Neurology (ESVN) a survey of veterinary neurologic learning objectives using a modified Delphi method was conducted. The first phase comprised the development of a draft job description and learning objectives by a working group established by the ECVN. In the second phase, a quantitative questionnaire (multiple choice, Likert scale and free text) covering 140 learning objectives and subdivided into 8 categories was sent to 341 ESVN and ECVN members and a return rate of 62% (n = 213/341) was achieved.
Results
Of these 140 learning objectives ECVN Diplomates and ESVN members considered 42 (30%) objectives as not necessary for standard clinical veterinary neurology training, 94 (67%) were graded to be learned at a beginner level and 4 (3%) at an advanced level. The following objectives were interpreted as the most important day one skills: interpret laboratory tests, perform a neurological examination and establish a neuroanatomical localization. In this survey the three most important diseases of the central nervous system included epilepsy, intervertebral disc disease and inflammatory diseases. The three most important diseases of the peripheral nervous system included polyradiculoneuritis, myasthenia gravis and toxic neuropathies.
Conclusions
The results of this study should help to reform the veterinary curriculum regarding neurology and may reduce the phenomenon of “Neurophobia”.</p
Computation of the transonic perturbation flow fields around two- and three-dimensional oscillating wings
Analytical and empirical studies of a finite difference method for the solution of the transonic flow about an harmonically oscillating wing are presented along with a discussion of the development of a pilot program for three-dimensional flow. In addition, some two- and three-dimensional examples are presented
Killing vectors and anisotropy
We consider an action that can generate fluids with three unequal stresses
for metrics with a spacelike Killing vector. The parameters in the action are
directly related to the stress anisotropies. The field equations following from
the action are applied to an anisotropic cosmological expansion and an
extension of the Gott-Hiscock cosmic string
A Characterisation of the Weylian Structure of Space-Time by Means of Low Velocity Tests
The compatibility axiom in Ehlers, Pirani and Schild's (EPS) constructive
axiomatics of the space-time geometry that uses light rays and freely falling
particles with high velocity, is replaced by several constructions with low
velocity particles only. For that purpose we describe in a space-time with a
conformal structure and an arbitrary path structure the radial acceleration, a
Coriolis acceleration and the zig-zag construction. Each of these quantities
give effects whose requirement to vanish can be taken as alternative version of
the compatibility axiom of EPS. The procedural advantage lies in the fact, that
one can make null-experiments and that one only needs low velocity particles to
test the compatibility axiom. We show in addition that Perlick's standard clock
can exist in a Weyl space only.Comment: to appear in Gen.Rel.Gra
Accelerated black holes in an anti-de Sitter universe
The C-metric is one of few known exact solutions of Einstein's field
equations which describes the gravitational field of moving sources. For a
vanishing or positive cosmological constant, the C-metric represents two
accelerated black holes in asymptotically flat or de Sitter spacetime. For a
negative cosmological constant the structure of the spacetime is more
complicated. Depending on the value of the acceleration, it can represent one
black hole or a sequence of pairs of accelerated black holes in the spacetime
with an anti-de Sitter-like infinity. The global structure of this spacetime is
analyzed and compared with an empty anti-de Sitter universe. It is illustrated
by 3D conformal-like diagrams.Comment: 14 pages, 17 figures [see
http://utf.mff.cuni.cz/~krtous/physics/CADS/ for the version with the high
quality figures and for related animations and interactive 3D diagrams
Further investigation of a finite difference procedure for analyzing the transonic flow about harmonically oscillating airfoils and wings
Analytical and empirical studies of a finite difference method for the solution of the transonic flow about harmonically oscillating wings and airfoils are presented. The procedure is based on separating the velocity potential into steady and unsteady parts and linearizing the resulting unsteady equations for small disturbances. The steady velocity potential is obtained first from the well-known nonlinear equation for steady transonic flow. The unsteady velocity potential is then obtained from a linear differential equation in complex form with spatially varying coefficients. Since sinusoidal motion is assumed, the unsteady equation is independent of time. An out-of-core direct solution procedure was developed and applied to two-dimensional sections. Results are presented for a section of vanishing thickness in subsonic flow and an NACA 64A006 airfoil in supersonic flow. Good correlation is obtained in the first case at values of Mach number and reduced frequency of direct interest in flutter analyses. Reasonable results are obtained in the second case. Comparisons of two-dimensional finite difference solutions with exact analytic solutions indicate that the accuracy of the difference solution is dependent on the boundary conditions used on the outer boundaries. Homogeneous boundary conditions on the mesh edges that yield complex eigenvalues give the most accurate finite difference solutions. The plane outgoing wave boundary conditions meet these requirements
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