8,787 research outputs found
A numerical simulation of the NFAC (National Full-scale Aerodynamics Complex) open-return wind tunnel inlet flow
The flow into an open return wind tunnel inlet was simulated using Euler equations. An explicit predictor-corrector method was employed to solve the system. The calculation is time-accurate and was performed to achieve a steady-state solution. The predictions are in reasonable agreement with the experimental data. Wall pressures are accurately predicted except in a region of recirculating flow. Flow-field surveys agree qualitatively with laser velocimeter measurements. The method can be used in the design process for open return wind tunnels
Domain wall mobility in nanowires: transverse versus vortex walls
The motion of domain walls in ferromagnetic, cylindrical nanowires is
investigated numerically by solving the Landau-Lifshitz-Gilbert equation for a
classical spin model in which energy contributions from exchange, crystalline
anisotropy, dipole-dipole interaction, and a driving magnetic field are
considered. Depending on the diameter, either transverse domain walls or vortex
walls are found. The transverse domain wall is observed for diameters smaller
than the exchange length of the given material. Here, the system behaves
effectively one-dimensional and the domain wall mobility agrees with a result
derived for a one-dimensional wall by Slonczewski. For low damping the domain
wall mobility decreases with decreasing damping constant. With increasing
diameter, a crossover to a vortex wall sets in which enhances the domain wall
mobility drastically. For a vortex wall the domain wall mobility is described
by the Walker-formula, with a domain wall width depending on the diameter of
the wire. The main difference is the dependence on damping: for a vortex wall
the domain wall mobility can be drastically increased for small values of the
damping constant up to a factor of .Comment: 5 pages, 6 figure
A Component Based Heuristic Search Method with Evolutionary Eliminations
Nurse rostering is a complex scheduling problem that affects hospital
personnel on a daily basis all over the world. This paper presents a new
component-based approach with evolutionary eliminations, for a nurse scheduling
problem arising at a major UK hospital. The main idea behind this technique is
to decompose a schedule into its components (i.e. the allocated shift pattern
of each nurse), and then to implement two evolutionary elimination strategies
mimicking natural selection and natural mutation process on these components
respectively to iteratively deliver better schedules. The worthiness of all
components in the schedule has to be continuously demonstrated in order for
them to remain there. This demonstration employs an evaluation function which
evaluates how well each component contributes towards the final objective. Two
elimination steps are then applied: the first elimination eliminates a number
of components that are deemed not worthy to stay in the current schedule; the
second elimination may also throw out, with a low level of probability, some
worthy components. The eliminated components are replenished with new ones
using a set of constructive heuristics using local optimality criteria.
Computational results using 52 data instances demonstrate the applicability of
the proposed approach in solving real-world problems.Comment: 27 pages, 4 figure
A Note On R-Parity Violation and Fermion Masses
We consider a class of supersymmetric SU(3)\times SU(2)\times U(1) multihiggs
models in which R-parity is violated through bilinear Higgs-lepton
interactions. The required, due to R-parity violation, higgs-lepton rotations
introduce an alternative way to generate the phenomenologically desirable
fermion mass matrix structures independently of the equality of Yukawas,
possibly imposed by superstring or other unification.Comment: 8 pages, uses LaTeX2
Flammability limits, ignition energy, and flame speeds in H₂–CH₄–NH₃–N₂O–O₂–N₂ mixtures
Experiments on flammability limits, ignition energies, and flame speeds were carried out in a 11.25- and a 400-liter combustion vessel at initial pressures and temperatures of 100 kPa and 295 K, respectively. Flammability maps of hydrogen–nitrous oxide–nitrogen, methane–nitrous oxide–nitrogen, ammonia–nitrous oxide–nitrogen, and ammonia–nitrous oxide–air, as well as lean flammability limits of various hydrogen–methane–ammonia–nitrous oxide–oxygen–nitrogen mixtures were determined. Ignition energy bounds of methane–nitrous oxide, ammonia–nitrous oxide, and ammonia–nitrous oxide–nitrogen mixtures have been determined and the influence of small amounts of oxygen on the flammability of methane–nitrous oxide–nitrogen mixtures has been investigated. Flame speeds have been measured and laminar burning velocities have been determined for ammonia–air–nitrous oxide and various hydrogen–methane–ammonia–nitrous oxide–oxygen–nitrogen mixtures. Lower and upper flammability limits (mixing fan on, turbulent conditions) for ignition energies of 8 J are: H₂–N₂O: 4.5 ∼ 5.0% H₂(LFL), 76 ∼ 80% H₂(UFL); CH₄–N₂O: 2.5 ∼ 3.0% CH₄(LFL), 43 ∼ 50% CH₄(UFL); NH₃–N₂O: 5.0 ∼ 5.2% NH₃(LFL), 67.5 ∼ 68% NH₃(UFL). Inerting concentrations are: H₂–N₂O–N₂: 76% N₂; CH₄–N₂O–N₂: 70.5% N₂; NH₃–N₂O–N₂: 61% N₂; NH₃–N₂O–air: 85% air. Flammability limits of methane–nitrous oxide–nitrogen mixtures show no pronounced dependence on small amounts of oxygen (<5%). Generally speaking, flammable gases with large initial amounts of nitrous oxide or ammonia show a strong dependence of flammability limits on ignition energy
Flammability limits, ignition energy, and flame speeds in H₂–CH₄–NH₃–N₂O–O₂–N₂ mixtures
Experiments on flammability limits, ignition energies, and flame speeds were carried out in a 11.25- and a 400-liter combustion vessel at initial pressures and temperatures of 100 kPa and 295 K, respectively. Flammability maps of hydrogen–nitrous oxide–nitrogen, methane–nitrous oxide–nitrogen, ammonia–nitrous oxide–nitrogen, and ammonia–nitrous oxide–air, as well as lean flammability limits of various hydrogen–methane–ammonia–nitrous oxide–oxygen–nitrogen mixtures were determined. Ignition energy bounds of methane–nitrous oxide, ammonia–nitrous oxide, and ammonia–nitrous oxide–nitrogen mixtures have been determined and the influence of small amounts of oxygen on the flammability of methane–nitrous oxide–nitrogen mixtures has been investigated. Flame speeds have been measured and laminar burning velocities have been determined for ammonia–air–nitrous oxide and various hydrogen–methane–ammonia–nitrous oxide–oxygen–nitrogen mixtures. Lower and upper flammability limits (mixing fan on, turbulent conditions) for ignition energies of 8 J are: H₂–N₂O: 4.5 ∼ 5.0% H₂(LFL), 76 ∼ 80% H₂(UFL); CH₄–N₂O: 2.5 ∼ 3.0% CH₄(LFL), 43 ∼ 50% CH₄(UFL); NH₃–N₂O: 5.0 ∼ 5.2% NH₃(LFL), 67.5 ∼ 68% NH₃(UFL). Inerting concentrations are: H₂–N₂O–N₂: 76% N₂; CH₄–N₂O–N₂: 70.5% N₂; NH₃–N₂O–N₂: 61% N₂; NH₃–N₂O–air: 85% air. Flammability limits of methane–nitrous oxide–nitrogen mixtures show no pronounced dependence on small amounts of oxygen (<5%). Generally speaking, flammable gases with large initial amounts of nitrous oxide or ammonia show a strong dependence of flammability limits on ignition energy
From D3-Branes to Lifshitz Space-Times
We present a simple embedding of a z=2 Lifshitz space-time into type IIB
supergravity. This is obtained by considering a stack of D3-branes in type IIB
supergravity and deforming the world-volume by a plane wave. The plane wave is
sourced by the type IIB axion. The superposition of the plane wave and the
D3-branes is 1/4 BPS. The near horizon geometry of this configuration is a
5-dimensional z=0 Schroedinger space-time times a 5-sphere. This geometry is
also 1/4 BPS. Upon compactification along the direction in which the wave is
traveling the 5-dimensional z=0 Schroedinger space-time reduces to a
4-dimensional z=2 Lifshitz space-time. The compactification is such that the
circle is small for weakly coupled type IIB string theory. This reduction
breaks the supersymmetries. Further, we propose a general method to construct
analytic z=2 Lifshitz black brane solutions. The method is based on deforming
5-dimensional AdS black strings by an axion wave and reducing to 4-dimensions.
We illustrate this method with an example.Comment: version 3: version published in Classical and Quantum Gravit
The changing nature of risk and risk management: the challenge of borders, uncertainty and resilience
No abstract available
The Price of WMAP Inflation in Supergravity
The three-year data from WMAP are in stunning agreement with the simplest
possible quadratic potential for chaotic inflation, as well as with new or
symmetry-breaking inflation. We investigate the possibilities for incorporating
these potentials within supergravity, particularly of the no-scale type that is
motivated by string theory. Models with inflation driven by the matter sector
may be constructed in no-scale supergravity, if the moduli are assumed to be
stabilised by some higher-scale dynamics and at the expense of some
fine-tuning. We discuss specific scenarios for stabilising the moduli via
either D- or F-terms in the effective potential, and survey possible
inflationary models in the presence of D-term stabilisation.Comment: 15 pages, 6 figures, plain Late
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