6,498 research outputs found
A H2 PEM fuel cell and high energy dense battery hybrid energy source for an urban electric vehicle
Electric vehicles are set to play a prominent role in addressing the energy and environmental impact of an increasing road transport population by offering a more energy efficient and less polluting drive-train alternative to conventional internal combustion engine (ICE) vehicles. Given the energy (and hence range) and performance limitations of electro-chemical battery storage systems, hybrid systems combining energy and power dense storage technologies have been proposed for vehicle applications. The paper discusses the application of a hydrogen fuel cell as a range extender for an urban electric vehicle for which the primary energy source is provided by a high energy dense battery. A review of fuel cell systems and automotive drive-train application issues are discussed, together with an overview of the battery technology. The prototype fuel cell and battery component simulation models are presented and their performance as a combined energy/power source assessed for typical urban and sub-urban driving scenario
Constructing smooth potentials of mean force, radial, distribution functions and probability densities from sampled data
In this paper a method of obtaining smooth analytical estimates of
probability densities, radial distribution functions and potentials of mean
force from sampled data in a statistically controlled fashion is presented. The
approach is general and can be applied to any density of a single random
variable. The method outlined here avoids the use of histograms, which require
the specification of a physical parameter (bin size) and tend to give noisy
results. The technique is an extension of the Berg-Harris method [B.A. Berg and
R.C. Harris, Comp. Phys. Comm. 179, 443 (2008)], which is typically inaccurate
for radial distribution functions and potentials of mean force due to a
non-uniform Jacobian factor. In addition, the standard method often requires a
large number of Fourier modes to represent radial distribution functions, which
tends to lead to oscillatory fits. It is shown that the issues of poor sampling
due to a Jacobian factor can be resolved using a biased resampling scheme,
while the requirement of a large number of Fourier modes is mitigated through
an automated piecewise construction approach. The method is demonstrated by
analyzing the radial distribution functions in an energy-discretized water
model. In addition, the fitting procedure is illustrated on three more
applications for which the original Berg-Harris method is not suitable, namely,
a random variable with a discontinuous probability density, a density with long
tails, and the distribution of the first arrival times of a diffusing particle
to a sphere, which has both long tails and short-time structure. In all cases,
the resampled, piecewise analytical fit outperforms the histogram and the
original Berg-Harris method.Comment: 14 pages, 15 figures. To appear in J. Chem. Phy
Efficient algorithms for rigid body integration using optimized splitting methods and exact free rotational motion
Hamiltonian splitting methods are an established technique to derive stable
and accurate integration schemes in molecular dynamics, in which additional
accuracy can be gained using force gradients. For rigid bodies, a tradition
exists in the literature to further split up the kinetic part of the
Hamiltonian, which lowers the accuracy. The goal of this note is to comment on
the best combination of optimized splitting and gradient methods that avoids
splitting the kinetic energy. These schemes are generally applicable, but the
optimal scheme depends on the desired level of accuracy. For simulations of
liquid water it is found that the velocity Verlet scheme is only optimal for
crude simulations with accuracies larger than 1.5%, while surprisingly a
modified Verlet scheme (HOA) is optimal up to accuracies of 0.4% and a fourth
order gradient scheme (GIER4) is optimal for even higher accuracies.Comment: 2 pages, 1 figure. Added clarifying comments. Accepted for
publication in the Journal of Chemical Physic
Semi-invariants of symmetric quivers of tame type
A symmetric quiver is a finite quiver without oriented cycles
equipped with a contravariant involution on . The involution allows us to define a nondegenerate bilinear form on
a representation $V$ of $Q$. We shall say that $V$ is orthogonal if is
symmetric and symplectic if is skew-symmetric. Moreover, we define an
action of products of classical groups on the space of orthogonal
representations and on the space of symplectic representations. So we prove
that if is a symmetric quiver of tame type then the rings of
semi-invariants for this action are spanned by the semi-invariants of
determinantal type and, when matrix defining is skew-symmetric, by
the Pfaffians . To prove it, moreover, we describe the symplectic and
orthogonal generic decomposition of a symmetric dimension vector
Quantum free energy differences from non-equilibrium path integrals: I. Methods and numerical application
The imaginary-time path integral representation of the canonical partition
function of a quantum system and non-equilibrium work fluctuation relations are
combined to yield methods for computing free energy differences in quantum
systems using non-equilibrium processes. The path integral representation is
isomorphic to the configurational partition function of a classical field
theory, to which a natural but fictitious Hamiltonian dynamics is associated.
It is shown that if this system is prepared in an equilibrium state, after
which a control parameter in the fictitious Hamiltonian is changed in a finite
time, then formally the Jarzynski non-equilibrium work relation and the Crooks
fluctuation relation are shown to hold, where work is defined as the change in
the energy as given by the fictitious Hamiltonian. Since the energy diverges
for the classical field theory in canonical equilibrium, two regularization
methods are introduced which limit the number of degrees of freedom to be
finite. The numerical applicability of the methods is demonstrated for a
quartic double-well potential with varying asymmetry. A general parameter-free
smoothing procedure for the work distribution functions is useful in this
context.Comment: 20 pages, 4 figures. Added clarifying remarks and fixed typo
How should we interpret the two transport relaxation times in the cuprates ?
We observe that the appearance of two transport relaxation times in the
various transport coefficients of cuprate metals may be understood in terms of
scattering processes that discriminate between currents that are even, or odd
under the charge conjugation operator. We develop a transport equation that
illustrates these ideas and discuss its experimental and theoretical
consequences.Comment: 19 pages, RevTeX with 8 postscript figures included. To appear in
``Non Fermi Liquid Physics'', J. Phys:Cond. Matt. (1997
The effect of curvature and topology on membrane hydrodynamics
We study the mobility of extended objects (rods) on a spherical liquid-liquid
interface to show how this quantity is modified in a striking manner by both
the curvature and the topology of the interface. We present theoretical
calculations and experimental measurements of the interfacial fluid velocity
field around a moving rod bound to the crowded interface of a water-in-oil
droplet. By using different droplet sizes, membrane viscosities, and rod
lengths, we show that the viscosity mismatch between the interior and exterior
fluids leads to a suppression of the fluid flow on small droplets that cannot
be captured by the flat interface predictions.Comment: 4 pages, 3 figure
Analysis of the Dynamics of Liquid Aluminium: Recurrent Relation Approach
By use of the recurrent relation approach (RRA) we study the microscopic
dynamics of liquid aluminium at T=973 K and develop a theoretical model which
satisfies all the corresponding sum rules. The investigation covers the
inelastic features as well as the crossover of our theory into the
hydrodynamical and the free-particle regimes. A comparison between our
theoretical results with those following from a generalized hydrodynamical
approach is also presented. In addition to this we report the results of our
molecular dynamics simulations for liquid aluminium, which are also discussed
and compared to experimental data. The received results reveal that (i) the
microscopical dynamics of density fluctuations is defined mainly by the first
four even frequency moments of the dynamic structure factor, and (ii) the
inherent relation of the high-frequency collective excitations observed in
experimental spectra of dynamic structure factor with the two-,
three- and four-particle correlations.Comment: 11 pages, 4 figure
Measuring commissionersâ willingness-to-pay for community based childhood obesity prevention programmes using a discrete choice experiment
Background:
In the UK, rates of childhood obesity remain high. Community based programmes for child obesity prevention are available to be commissioned by local authorities. However, there is a lack of evidence regarding how programmes are commissioned and which attributes of programmes are valued most by commissioners. The aim of this study was to determine the factors that decision-makers prioritise when commissioning programmes that target childhood obesity prevention.
Methods:
An online discrete choice experiment (DCE) was used to survey commissioners and decision makers in the UK to assess their willingness-to-pay for childhood obesity programmes.
Results:
A total of 64 commissioners and other decision makers completed the DCE. The impact of programmes on behavioural outcomes was prioritised, with participants willing to pay an extra ÂŁ16,600/year if average daily fruit and vegetable intake increased for each child by one additional portion. Participants also prioritised programmes that had greater number of parents fully completing them, and were willing to pay an extra ÂŁ4810/year for every additional parent completing a programme. The number of parents enrolling in a programme (holding the number completing fixed) and hours of staff time required did not significantly influence choices.
Conclusions:
Emphasis on high programme completion rates and success increasing childrenâs fruit and vegetable intake has potential to increase commissioning of community based obesity prevention programmes
Scanning Transmission Ion Microscopy as it Complements Particle Induced X-Ray Emission Microanalysis
The early uses of Scanning Transmission Ion Microscopy (STIM) with MeV ions are reviewed. The transformation of STIM energy-loss images into maps of areal density is discussed, and is illustrated with images of a fruit fly head (Drosophila melanogaster). Freeze-dried male heads are transparent to 4-MeV protons in the dorsal and frontal directions, but in the sagittal direction the brain is opaque. STIM with molecular ions is shown to be useful for increasing contrast in low density areas. For recording registered STIM and PIXE images without changing accelerator parameters, apertures are used to accomplish the required change in beam intensity (a factor of 105). Molecular ions are used to assess contamination of the microbeam by scattered ions. Pixel by pixel ratios of x-ray intensity to areal density are taken to obtain maps of element concentrations. Calcium and iron maps are shown. Inner parts of the fly head are clearly seen in the concentration maps. The PIXE exposure caused differential displacements of inner parts of the head by 15 ÎŒm or less. Weight loss during the PIXE exposure was measured to be 3%
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