11,787 research outputs found
Metamodelling of multivariable engine models for real-time flight simulation.
Sophisticated real-time distributed flight simulation environments may be constructed from a wide range of modelling and simulation tools. In this way accuracy, detail and model flexibility may be incorporated into the simulator. Distributed components may be constructed by a wide range of methods, from high level environments such as Matlab, through coded environments such as C or Fortran to hardware-in-the- loop. In this paper the Response Surface Methodology is combined with a hyper-heuristic (evolutionary algorithm) and applied to the representation of computationally intensive non-linear multivariable engine modelling. The paper investigates the potential for metamodelling (models of models) dynamic models which were previously too slow to be included in multi-component, high resolution real-time simulation environments. A multi-dimensional gas turbine model with five primary control inputs, six environmental inputs and eleven outputs is considered. An investigation has been conducted to ascertain to what extent these systems can be approximated by response surfaces with experiments which have been designed by hyper-heuristics as a first step towards automatic modelling methodology
Perfect separation of intraband and interband excitations in PdCoO
The temperature dependence of the optical properties of the delafossite
PdCoO has been measured in the a-b planes over a wide frequency range. The
optical conductivity due to the free-carrier (intraband) response falls well
below the interband transitions, allowing the plasma frequency to be determined
from the -sum rule. Drude-Lorentz fits to the complex optical conductivity
yield estimates for the free-carrier plasma frequency and scattering rate. The
in-plane plasma frequency has also been calculated using density functional
theory. The experimentally-determined and calculated values for the plasma
frequencies are all in good agreement; however, at low temperature the
optically-determined scattering rate is much larger than the estimate for the
transport scattering rate, indicating a strong frequency-dependent
renormalization of the optical scattering rate. In addition to the expected
in-plane infrared-active modes, two very strong features are observed that are
attributed to the coupling of the in-plane carriers to the out-of-plane
longitudinal optic modes.Comment: 7 pages with five figures and three tables; 4 pages of supplementary
materia
Lattice QCD Production on Commodity Clusters at Fermilab
We describe the construction and results to date of Fermilab's three
Myrinet-networked lattice QCD production clusters (an 80-node dual Pentium III
cluster, a 48-node dual Xeon cluster, and a 128-node dual Xeon cluster). We
examine a number of aspects of performance of the MILC lattice QCD code running
on these clusters.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 6 pages, LaTeX, 8 eps figures. PSN
TUIT00
Nematic Fermi Fluids in Condensed Matter Physics
Correlated electron fluids can exhibit a startling array of complex phases,
among which one of the more surprising is the electron nematic, a
translationally invariant metallic phase with a spontaneously generated spatial
anisotropy. Classical nematics generally occur in liquids of rod-like
molecules; given that electrons are point-like, the initial theoretical
motivation for contemplating electron nematics came from thinking of the
electron fluid as a quantum melted electron crystal, rather than a strongly
interacting descendent of a Fermi gas. That such phases exist in nature was
established by dramatic transport experiments in ultra-clean quantum Hall
systems in 1999 and in Sr3Ru2O7 in a strong magnetic field in 2007.
In this paper, we briefly review the theoretical considerations governing
nematic order, summarize the quantum Hall and Sr3Ru2O7 experiments that
unambiguously establish the existence of this phase, and survey some of the
current evidence for such a phase in the cuprate and Fe-based high temperature
superconductors.Comment: 30 pages, 7 figures (some in color); to appear in Annual Reviews of
Condensed Matter Physics. Edited version
Stable Infrastructure-based Routing for Intelligent Transportation Systems
Intelligent Transportation Systems (ITSs) have been instrumental
in reshaping transportation towards safer roads, seamless
logistics, and digital business-oriented services under the umbrella of
smart city platforms. Undoubtedly, ITS applications will demand
stable routing protocols that not only focus on Inter-Vehicle Communications
but also on providing a fast, reliable and secure interface to
the infrastructure. In this paper, we propose a novel stable infrastructure-
based routing protocol for urban VANETs. It enables vehicles
proactively to maintain fresh routes towards Road-Side Units
(RSUs) while reactively discovering routes to nearby vehicles. It
builds routes from highly stable connected intersections using a selection
policy which uses a new intersection stability metric. Simulation
experiments performed with accurate mobility and propagation
models have confirmed the efficiency of the new protocol and its
adaptability to continuously changing network status in the urban
environment
Photoemission Quasi-Particle Spectra of SrRuO
Multi-band quasi-particle calculations based on perturbation theory and
dynamical mean field methods show that the creation of a photoemission hole
state in SrRuO is associated with a highly anisotropic self-energy.
Since the narrow Ru-derived bands are more strongly distorted by
Coulomb correlations than the wide band, charge is partially
transferred from to , thereby shifting the van
Hove singularity close to the Fermi level.Comment: 4 pages, to be published in PRB Rapid Com
Upper critical field in layered superconductors
The theoretical statements about a restoration of a superconductivity at
magnetic fields higher than the quasiclassical upper critical field and a
reentrance of superconductivity at temperatures in the
superconductors with open Fermi surfaces are reinvestigated taking into account
a scattering of quasiparticles on the impurities.
The system of integral equations for determination of the upper critical
field parallel to the conducting planes in a layered conventional and
unconventional superconductors with impurities are derived. The
values for the "clean" case in the Ginzburg-Landau regime and at any
temperature in the "dirty" case are found analytically. The upper limit of the
superconductor purity when the upper critical field definately has a finite
value is established.Comment: 10 page
Detailed Topography of the Fermi Surface of Sr2RuO4
We apply a novel analysis of the field and angle dependence of the
quantum-oscillatory amplitudes in the unconventional superconductor Sr2RuO4 to
map its Fermi surface in unprecedented detail, and to obtain previously
inaccessible information on the band dispersion. The three quasi-2D Fermi
surface sheets not only exhibit very diverse magnitudes of warping, but also
entirely different dominant warping symmetries. We use the data to reassess
recent results on c-axis transport phenomena.Comment: REVTeX, 4 page
Adjusting for bias introduced by instrumental variable estimation in the Cox Proportional Hazards Model
Instrumental variable (IV) methods are widely used for estimating average
treatment effects in the presence of unmeasured confounders. However, the
capability of existing IV procedures, and most notably the two-stage residual
inclusion (2SRI) procedure recommended for use in nonlinear contexts, to
account for unmeasured confounders in the Cox proportional hazard model is
unclear. We show that instrumenting an endogenous treatment induces an
unmeasured covariate, referred to as an individual frailty in survival analysis
parlance, which if not accounted for leads to bias. We propose a new procedure
that augments 2SRI with an individual frailty and prove that it is consistent
under certain conditions. The finite sample-size behavior is studied across a
broad set of conditions via Monte Carlo simulations. Finally, the proposed
methodology is used to estimate the average effect of carotid endarterectomy
versus carotid artery stenting on the mortality of patients suffering from
carotid artery disease. Results suggest that the 2SRI-frailty estimator
generally reduces the bias of both point and interval estimators compared to
traditional 2SRI.Comment: 27 pages, 8 figures, 4 table
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