3,796 research outputs found
Stress redistribution due to creep in nimonic 90 ministry of aviation contract no. PD/28/021 report for the period June, 1965 to July, 1966: part 2
Part I of this report, dated September 1965, deals mainly with
the design and construction of the special apparatus to simulate the
stress redistribution occurring during the creep of a cooled turbine
blade. It reported some preliminary experiments using aluminium alloy
test specimens.
During the past year, the period covered by this report, a series
of experiments have been completed using specimens machined from nimonic
90 alloy.
Experience gained in operating the apparatus, have resulted in
several modifications both to the apparatus and the associated instrumentation.
These are detailed within this report
The Ups and Downs of Cyclic Universes
We investigate homogeneous and isotropic oscillating cosmologies with
multiple fluid components. Transfer of energy between these fluids is included
in order to model the effects of non-equilibrium behavior on closed universes.
We find exact solutions which display a range of new behaviors for the
expansion scale factor. Detailed examples are studied for the exchange of
energy from dust or scalar field into radiation. We show that, contrary to
expectation, it is unlikely that such models can offer a physically viable
solution to the flatness problem.Comment: 15 pages, 4 figure
Stress redistribution due to creep in nimonic 90 ministry of aviation contract no. PD/28/021 report for the period January 1964 - June 1965: part 1
The period covered by this report has been devoted to the design,
construction development and calibration of a special apparatus to simulate
the stress redistribution conditions occurring during the creep of a cooled
turbine blade. The experimental assembly consists of two creep machines,
each operating at a different temperature, so controlled that a load is
shared between them. maintaining equal creep strains (and in consequence
equal creep rates) in each specimen. The stress in each specimen and the
creep strain of the pair are automatically measured and recorded by a
specially developed unit. Some preliminary results on an aluminium alloy
are presented
Decaying Gravity
We consider the possibility of energy being exchanged between the scalar and
matter fields in scalar-tensor theories of gravity. Such an exchange provides a
new mechanism which can drive variations in the gravitational 'constant' G. We
find exact solutions for the evolution of spatially flat
Friedman-Roberston-Walker cosmologies in this scenario and discuss their
behaviour at both early and late times. We also consider the physical
consequences and observational constraints on these models.Comment: 13 page
The Existence of Godel, Einstein and de Sitter Universes
We determine the general conditions for the existence of Godel, Einstein
static, and de Sitter universes in gravity theories derived from a Lagrangian
that is an arbitrary function of the scalar curvature and Ricci and Riemann
curvature invariants. Explicit expressions for the solutions are found in terms
of the parameters defining the Lagrangian. We also determine the conditions on
the Lagrangian of the theory under which time-travel is allowed in the Godel
universes.Comment: 8 pages, no figure
Acoustic and aerodynamic study of a pusher-propeller aircraft model
An aerodynamic and acoustic study was made of a pusher-propeller aircraft model in the NASA-Ames 7 x 10 ft Wind Tunnel. The test section was changed to operate as an open jet. The 591 mm diameter unswept propeller was operated alone and in the wake of three empennages: an I tail, Y tail, and a V tail. The radiated noise and detailed wake properties were measured. Results indicate that the unsteady blade loading caused by the blade interactions with the wake mean velocity distribution had a strong effect on the harmonics of blade passage noise. The blade passage harmonics above the first were substantially increased in all horizontal directions by the empennage/propeller interaction. Directivity in the plane of the propeller was maximum perpendicular to the blade surface. Increasing the tail loading caused the propeller harmonics to increase 3 to 5 dB for an empennage/propeller spacing of 0.38 mean empennage chords. The interaction noise became weak as empennage propeller spacing was increased beyond 1.0 mean empennage chord lengths. Unlike the mean wake deficit, the wake turbulence had only a small effect on the propeller noise, that effect being a small increase in the broadband noise
Flow-field Survey of an Empennage Wake Interacting with a Pusher Propeller
The flow field between a model empennage and a 591-mm-diameter pusher propeller was studied in the Ames 7- by 10-Foot Wind Tunnel with directional pressure probes and hot-wire anemometers. The region probed was bounded by the empennage trailing edge and downstream propeller. The wake properties, including effects of propeller operation on the empennage wake, were investigated for two empennage geometries: one, a vertical tail fin, the other, a Y-tail with a 34 deg dihedral. Results showed that the effect of the propeller on the empennage wake upstream of the propeller was not strong. The flow upstream of the propeller was accelerated in the streamwise direction by the propeller, but the empennage wake width and velocity defect were relatively unaffected by the presence of the propeller. The peak turbulence in the wake near the propeller tip station, 0.66 diameter behind the vertical tail fin, was approximately 3 percent of the free-stream velocity. The velocity field data can be used in predictions of the acoustic field due to propeller-wake interaction
On the absence of the usual weak-field limit, and the impossibility of embedding some known solutions for isolated masses in cosmologies with f(R) dark energy
This version deposited at arxiv 02-10-12 arXiv:1210.0730v1. Subsequently published in Physical Review D as Phys. Rev. D 87, 063517 (2013) http://link.aps.org/doi/10.1103/PhysRevD.87.063517. Copyright American Physical Society (APS).11 pages11 pages11 pages11 pagesThe problem of matching different regions of spacetime in order to construct inhomogeneous cosmological models is investigated in the context of Lagrangian theories of gravity constructed from general analytic functions f(R), and from non-analytic theories with f(R)=R^n. In all of the cases studied, we find that it is impossible to satisfy the required junction conditions without the large-scale behaviour reducing to that expected from Einstein's equations with a cosmological constant. For theories with analytic f(R) this suggests that the usual treatment of weak-field systems may not be compatible with late-time acceleration driven by anything other than a constant term of the form f(0), which acts like a cosmological constant. For theories with f(R)=R^n we find that no known spherically symmetric vacuum solutions can be matched to an expanding FLRW background. This includes the absence of any Einstein-Straus-like embeddings of the Schwarzschild exterior solution in FLRW spacetimes
Exact Friedmann Solutions in Higher-Order Gravity Theories
24 pages, 6 figures24 pages, 6 figures24 pages, 6 figuresWe find the general behaviour of homogeneous and isotropic cosmological models in some fourth-order theories of gravity. Explicit, exact, general solutions are given for both empty universes and those filled with a perfect fluid. For the vacuum case, solutions are found with closed, open and flat geometries, whilst the perfect fluid solutions are all spatially flat. Both early and late-time limits are studied, and attractor behaviour towards simple power-law expansion is identified. Multiple solutions to the same theories, with the same matter content and topology are found. It is shown that these solutions exhibit great variety in their evolution
Radiation from collapsing shells, semiclassical backreaction and black hole formation
We provide a detailed analysis of quantum field theory around a collapsing
shell and discuss several conceptual issues related to the emission of
radiation flux and formation of black holes. Explicit calculations are
performed using a model for a collapsing shell which turns out to be
analytically solvable. We use the insights gained in this model to draw
reliable conclusions regarding more realistic models. We first show that any
shell of mass which collapses to a radius close to will emit
approximately thermal radiation for a period of time. In particular, a shell
which collapses from some initial radius to a final radius
(where ) without forming a black hole,
will emit thermal radiation during the period . Later on (), the flux from such a
shell will decay to zero exponentially. We next study the effect of
backreaction computed using the vacuum expectation value of the stress tensor
on the collapse. We find that, in any realistic collapse scenario, the
backreaction effects do \emph{not} prevent the formation of the event horizon.
The time at which the event horizon is formed is, of course, delayed due to the
radiated flux -- which decreases the mass of the shell -- but this effect is
not sufficient to prevent horizon formation. We also clarify several conceptual
issues and provide pedagogical details of the calculations in the Appendices to
the paper.Comment: 26 pages, 6 figures, revtex4; v2 -- minor reformatting, some typos
fixed, one reference added, to appear in PR
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