22,851 research outputs found
A study of the high frequency limitations of series resonant converters
A transformer induced oscillation in series resonant (SR) converters is studied. It may occur in the discontinuous current mode. The source of the oscillation is an unexpected resonant circuit formed by normal resonance components in series with the magnetizing inductance of the output transformers. The methods for achieving cyclic stability are: to use a half bridge SR converter where q0.5. Q should be as close to 1.0 as possible. If 0.5q1.0, the instability will be avoided if psi2/3q-1/3. The second objective was to investigate a power field effect transistor (FET) version of the SR converter capable of operating at frequencies above 100 KHz, to study component stress and losses at various frequencies
Was the Universe Reionized by Massive Population-III Stars?
The WMAP satellite has measured a large optical depth to electron scattering
after cosmological recombination of 0.17+-0.04, implying significant
reionization of the primordial gas only ~200 million years after the big bang.
However, the most recent overlap of intergalactic HII regions must have occured
at z<9 based on the Lyman-alpha forest constraint on the thermal history of the
intergalactic medium. Here we argue that a first generation of metal-free stars
with a heavy (rather than Salpeter) mass function is therefore required to
account for much of the inferred optical depth. This conclusion holds if
feedback regulates star formation in early dwarf galaxies as observed in
present-day dwarfs.Comment: 4 pages, 1 figure, replaced to match version accepted by ApJ Letter
Strong convergence rates of probabilistic integrators for ordinary differential equations
Probabilistic integration of a continuous dynamical system is a way of
systematically introducing model error, at scales no larger than errors
introduced by standard numerical discretisation, in order to enable thorough
exploration of possible responses of the system to inputs. It is thus a
potentially useful approach in a number of applications such as forward
uncertainty quantification, inverse problems, and data assimilation. We extend
the convergence analysis of probabilistic integrators for deterministic
ordinary differential equations, as proposed by Conrad et al.\ (\textit{Stat.\
Comput.}, 2017), to establish mean-square convergence in the uniform norm on
discrete- or continuous-time solutions under relaxed regularity assumptions on
the driving vector fields and their induced flows. Specifically, we show that
randomised high-order integrators for globally Lipschitz flows and randomised
Euler integrators for dissipative vector fields with polynomially-bounded local
Lipschitz constants all have the same mean-square convergence rate as their
deterministic counterparts, provided that the variance of the integration noise
is not of higher order than the corresponding deterministic integrator. These
and similar results are proven for probabilistic integrators where the random
perturbations may be state-dependent, non-Gaussian, or non-centred random
variables.Comment: 25 page
Well-Posedness And Accuracy Of The Ensemble Kalman Filter In Discrete And Continuous Time
The ensemble Kalman filter (EnKF) is a method for combining a dynamical model
with data in a sequential fashion. Despite its widespread use, there has been
little analysis of its theoretical properties. Many of the algorithmic
innovations associated with the filter, which are required to make a useable
algorithm in practice, are derived in an ad hoc fashion. The aim of this paper
is to initiate the development of a systematic analysis of the EnKF, in
particular to do so in the small ensemble size limit. The perspective is to
view the method as a state estimator, and not as an algorithm which
approximates the true filtering distribution. The perturbed observation version
of the algorithm is studied, without and with variance inflation. Without
variance inflation well-posedness of the filter is established; with variance
inflation accuracy of the filter, with resepct to the true signal underlying
the data, is established. The algorithm is considered in discrete time, and
also for a continuous time limit arising when observations are frequent and
subject to large noise. The underlying dynamical model, and assumptions about
it, is sufficiently general to include the Lorenz '63 and '96 models, together
with the incompressible Navier-Stokes equation on a two-dimensional torus. The
analysis is limited to the case of complete observation of the signal with
additive white noise. Numerical results are presented for the Navier-Stokes
equation on a two-dimensional torus for both complete and partial observations
of the signal with additive white noise
The buckling of thin-walled circular cylinders under axial compression and bending
Bucking of thin-walled electroplated copper and Mylar circular cylinders under axial compression and bendin
The Damping of Panels by Thick Layers of Elastic Porous Media
In this paper a technique is presented for calculating the response of a panel to a line force input when loaded by both a finiteâdepth layer of elastic porous material and a heavy fluid. Damping theories normally require that an attached damping layer be thin compared to the flexural wavelength in the base panel. Here this requirement is avoided by allowing explicitly for wave propagation within the damping layer. Specifically, the porous damping layer is modeled using a theory derived by Biot that allows for the existence of two dilatational waves and a transverse wave. Conditions required to couple the porous medium to the panel and an adjacent fluid will be discussed. A formal solution for the plate response may be obtained easily in the wavenumber domain. Although it is not possible to obtain the spatial response analytically in this instance, it will be shown that under practical circumstances the required inversion integral may be evaluated efficiently and âexactlyâ by using the fast Fourier transform algorithm. Results will be given illustrating the damping potential of thick layers of porous materials
Responding to accents after experiencing interactive or mediated speech
Very little known is about how speakers learn
about and/or respond to speech experienced
without the possibility for interaction. This paper
reports an experiment which considers the effects
of two kinds of exposure to speech (interactive or
non-interactive mediated) on Scottish English
speakersâ responses to another accent (Southern
British English), for two processing tasks,
phonological awareness and speech production.
Only marginal group effects are found according to
exposure type. The main findings show a
difference between subjects according to exposure
type before exposure, and individual shifts in
responses to speech according to exposure type
Radiation of Angular Momentum by Neutrinos from Merged Binary Neutron Stars
We study neutrino emission from the remnant of an inspiraling binary neutron
star following coalescence. The mass of the merged remnant is likely to exceed
the stability limit of a cold, rotating neutron star. However, the angular
momentum of the remnant may also approach or even exceed the Kerr limit, J/M^2
= 1, so that total collapse may not be possible unless some angular momentum is
dissipated. We find that neutrino emission is very inefficient in decreasing
the angular momentum of these merged objects and may even lead to a small
increase in J/M^2. We illustrate these findings with a post-Newtonian,
ellipsoidal model calculation. Simple arguments suggest that the remnant may
form a bar mode instability on a timescale similar to or shorter than the
neutrino emission timescale, in which case the evolution of the remnant will be
dominated by the emission of gravitational waves.Comment: 12 pages AASTeX, 2 figures, to appear in Ap
Acoustic characterization of crack damage evolution in sandstone deformed under conventional and true triaxial loading
We thank the Associate Editor, Michelle Cooke, and the reviewers, Ze'ev Reches and Yves Guéguen, for useful comments which helped to improve the manuscript. We thank J.G. Van Munster for providing access to the true triaxial apparatus at KSEPL and for technical support during the experimental program. We thank R. Pricci for assistance with technical drawings of the apparatus. This work was partly funded by NERC award NE/N002938/1 and by a NERC Doctoral Studentship, which we gratefully acknowledge. Supporting data are included in a supporting information file; any additional data may be obtained from J.B. (e-mail: [email protected]).Peer reviewedPublisher PD
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