2,008 research outputs found
The mineral and lipid composition of the arteries of white and bantu children
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Experimental investigation of circumferentially non-uniform heat flux on the heat transfer coefficient in a smooth horizontal tube with buoyancy driven secondary flow
In this experimental investigation the influence of non-uniform heat flux distributions on the internal heat transfer coefficient in a horizontal circular tube was studied for liquid water. The tube had an inner diameter of 27.8 mm and a length to diameter ratio of 72. Different outer wall heat flux conditions were studied for Reynolds numbers ranging from 650 to 2600 at a Prandtl number of approximately 6.5. Heat flux distributions included fully uniform heating (which had a circumferential angle span of 360°) and different partial uniform heat flux distributions with angle spans of 180° or 90° at different circumferential positions. Depending on the angle span, local heat flux intensities ranging from 1658 W/m2 to 6631 W/m2 were tested. Results indicate that the average steady state Nusselt number is greatly influenced by the applied heat flux position and intensity. Highest average heat transfer coefficients were achieved for cases where the applied heat flux was positioned on the lower half (in terms of gravity) of the tube circumference, while the lowest heat transfer coefficients were achieved when the heating was applied to the upper half of the tube. Smaller angle spans produced lower heat transfer coefficients. The relative thermal performance of the different heating scenarios where characterised and described by means of newly developed heat transfer coefficient correlations for angle spans of 180° and 90° which correlated 92% and 96% of the data respectively within 3% of the measured Nusselt number.The National Research Foundation ( NRF ), Eskom Tertiary Education Support Programme ( TESP ), University of Stellenbosch/University of Pretoria , South African National Energy Research Institute ( SANERI )/South African National Energy Development Institute ( SANEDI ), Council for Scientific and Industrial Research ( CSIR ), Energy Efficiency and Demand Side Management ( EEDSM ) Hub and NAC.http://www.elsevier.com/locate/etfs2019-11-01hj2019Mechanical and Aeronautical Engineerin
Sample-to-sample torque fluctuations in a system of coaxial randomly charged surfaces
Polarizable randomly charged dielectric objects have been recently shown to
exhibit long-range lateral and normal interaction forces even when they are
effectively net neutral. These forces stem from an interplay between the
quenched statistics of random charges and the induced dielectric image charges.
This type of interaction has recently been evoked to interpret measurements of
Casimir forces in vacuo, where a precise analysis of such disorder-induced
effects appears to be necessary. Here we consider the torque acting on a
randomly charged dielectric surface (or a sphere) mounted on a central axle
next to another randomly charged surface and show that although the resultant
mean torque is zero, its sample-to-sample fluctuation exhibits a long-range
behavior with the separation distance between the juxtaposed surfaces and that,
in particular, its root-mean-square value scales with the total area of the
surfaces. Therefore, the disorder-induced torque between two randomly charged
surfaces is expected to be much more pronounced than the disorder-induced
lateral force and may provide an effective way to determine possible disorder
effects in experiments, in a manner that is independent of the usual normal
force measurement.Comment: 7 pages, 3 fig
Statistical Theory of Spin Relaxation and Diffusion in Solids
A comprehensive theoretical description is given for the spin relaxation and
diffusion in solids. The formulation is made in a general
statistical-mechanical way. The method of the nonequilibrium statistical
operator (NSO) developed by D. N. Zubarev is employed to analyze a relaxation
dynamics of a spin subsystem. Perturbation of this subsystem in solids may
produce a nonequilibrium state which is then relaxed to an equilibrium state
due to the interaction between the particles or with a thermal bath (lattice).
The generalized kinetic equations were derived previously for a system weakly
coupled to a thermal bath to elucidate the nature of transport and relaxation
processes. In this paper, these results are used to describe the relaxation and
diffusion of nuclear spins in solids. The aim is to formulate a successive and
coherent microscopic description of the nuclear magnetic relaxation and
diffusion in solids. The nuclear spin-lattice relaxation is considered and the
Gorter relation is derived. As an example, a theory of spin diffusion of the
nuclear magnetic moment in dilute alloys (like Cu-Mn) is developed. It is shown
that due to the dipolar interaction between host nuclear spins and impurity
spins, a nonuniform distribution in the host nuclear spin system will occur and
consequently the macroscopic relaxation time will be strongly determined by the
spin diffusion. The explicit expressions for the relaxation time in certain
physically relevant cases are given.Comment: 41 pages, 119 Refs. Corrected typos, added reference
Interatomic potentials for atomistic simulations of the Ti-Al system
Semi-empirical interatomic potentials have been developed for Al, alpha-Ti,
and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large
database of experimental as well as ab-initio data. The ab-initio calculations
were performed by the linear augmented plane wave (LAPW) method within the
density functional theory to obtain the equations of state for a number of
crystal structures of the Ti-Al system. Some of the calculated LAPW energies
were used for fitting the potentials while others for examining their quality.
The potentials correctly predict the equilibrium crystal structures of the
phases and accurately reproduce their basic lattice properties. The potentials
are applied to calculate the energies of point defects, surfaces, planar faults
in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al
system, the proposed potentials provide reasonable description of the lattice
thermal expansion, demonstrating their usefulness in the molecular dynamics or
Monte Carlo studies at high temperatures. The energy along the tetragonal
deformation path (Bain transformation) in gamma-TiAl calculated with the EAM
potential is in a fairly good agreement with LAPW calculations. Equilibrium
point defect concentrations in gamma-TiAl are studied using the EAM potential.
It is found that antisite defects strongly dominate over vacancies at all
compositions around stoichiometry, indicating that gamm-TiAl is an antisite
disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press
Dynamic Evolution Model of Isothermal Voids and Shocks
We explore self-similar hydrodynamic evolution of central voids embedded in
an isothermal gas of spherical symmetry under the self-gravity. More
specifically, we study voids expanding at constant radial speeds in an
isothermal gas and construct all types of possible void solutions without or
with shocks in surrounding envelopes. We examine properties of void boundaries
and outer envelopes. Voids without shocks are all bounded by overdense shells
and either inflows or outflows in the outer envelope may occur. These
solutions, referred to as type void solutions, are further
divided into subtypes and
according to their characteristic behaviours across the sonic critical line
(SCL). Void solutions with shocks in envelopes are referred to as type
voids and can have both dense and quasi-smooth edges.
Asymptotically, outflows, breezes, inflows, accretions and static outer
envelopes may all surround such type voids. Both cases of
constant and varying temperatures across isothermal shock fronts are analyzed;
they are referred to as types and
void shock solutions. We apply the `phase net matching procedure' to construct
various self-similar void solutions. We also present analysis on void
generation mechanisms and describe several astrophysical applications. By
including self-gravity, gas pressure and shocks, our isothermal self-similar
void (ISSV) model is adaptable to various astrophysical systems such as
planetary nebulae, hot bubbles and superbubbles in the interstellar medium as
well as supernova remnants.Comment: 24 pages, 13 figuers, accepted by ApS
A theory of Plasma Membrane Calcium Pump stimulation and activity
The ATP-driven Plasma Membrane Calcium pump or Ca(2+)-ATPase (PMCA) is
characterized by a high affinity to calcium and a low transport rate compared
to other transmembrane calcium transport proteins. It plays a crucial role for
calcium extrusion from cells. Calmodulin is an intracellular calcium buffering
protein which is capable in its Ca(2+) liganded form of stimulating the PMCA by
increasing both the affinity to calcium and the maximum calcium transport rate.
We introduce a new model of this stimulation process and derive analytical
expressions for experimental observables in order to determine the model
parameters on the basis of specific experiments. We furthermore develop a model
for the pumping activity. The pumping description resolves the seeming
contradiction of the Ca(2+):ATP stoichiometry of 1:1 during a translocation
step and the observation that the pump binds two calcium ions at the
intracellular site. The combination of the calcium pumping and the stimulation
model correctly describes PMCA function. We find that the processes of
calmodulin-calcium complex attachment to the pump and of stimulation have to be
separated. Other PMCA properties are discussed in the framework of the model.
The presented model can serve as a tool for calcium dynamics simulations and
provides the possibility to characterize different pump isoforms by different
type-specific parameter sets.Comment: 24 pages, 6 figure
ASTERIX and 2.0 Knowledge Management : exploring the appropriation of 2.0 KMS via the Myth of the Gaulish village
International audienceKnowledge Management Systems (KMS) in companies have profoundly changed in recent years. They have become KMS 2.0 that aim to transform the firm and are driven by a new relationship to knowledge in line with 2.0 organisations. These 2.0 KMS have implemented modes of organisation that disrupt those that previously guided firms’ performance. This can sometimes lead to paradoxical organizational dysfunctions as witnessed by the difficulties faced by some traditionally hierarchical French companies. Through a case study of Constructor and a theoretical background on IS appropriation in organizations and myths in management, we show how the Asterix myth contributes to understanding how 2.0. KMS are appropriated in such companies. We find evidence of similarities regarding knowledge and Knowledge Management between the Asterix’ myth and the behaviours and practices concerning knowledge management within Constructor. As a result, the Asterix’ myth may be a relevant perspective for understanding the obstacles, advantages and appropriations of 2.0. KMS within French organizations
Nonlinear Integer Programming
Research efforts of the past fifty years have led to a development of linear
integer programming as a mature discipline of mathematical optimization. Such a
level of maturity has not been reached when one considers nonlinear systems
subject to integrality requirements for the variables. This chapter is
dedicated to this topic.
The primary goal is a study of a simple version of general nonlinear integer
problems, where all constraints are still linear. Our focus is on the
computational complexity of the problem, which varies significantly with the
type of nonlinear objective function in combination with the underlying
combinatorial structure. Numerous boundary cases of complexity emerge, which
sometimes surprisingly lead even to polynomial time algorithms.
We also cover recent successful approaches for more general classes of
problems. Though no positive theoretical efficiency results are available, nor
are they likely to ever be available, these seem to be the currently most
successful and interesting approaches for solving practical problems.
It is our belief that the study of algorithms motivated by theoretical
considerations and those motivated by our desire to solve practical instances
should and do inform one another. So it is with this viewpoint that we present
the subject, and it is in this direction that we hope to spark further
research.Comment: 57 pages. To appear in: M. J\"unger, T. Liebling, D. Naddef, G.
Nemhauser, W. Pulleyblank, G. Reinelt, G. Rinaldi, and L. Wolsey (eds.), 50
Years of Integer Programming 1958--2008: The Early Years and State-of-the-Art
Surveys, Springer-Verlag, 2009, ISBN 354068274
Design of Experiments for Screening
The aim of this paper is to review methods of designing screening
experiments, ranging from designs originally developed for physical experiments
to those especially tailored to experiments on numerical models. The strengths
and weaknesses of the various designs for screening variables in numerical
models are discussed. First, classes of factorial designs for experiments to
estimate main effects and interactions through a linear statistical model are
described, specifically regular and nonregular fractional factorial designs,
supersaturated designs and systematic fractional replicate designs. Generic
issues of aliasing, bias and cancellation of factorial effects are discussed.
Second, group screening experiments are considered including factorial group
screening and sequential bifurcation. Third, random sampling plans are
discussed including Latin hypercube sampling and sampling plans to estimate
elementary effects. Fourth, a variety of modelling methods commonly employed
with screening designs are briefly described. Finally, a novel study
demonstrates six screening methods on two frequently-used exemplars, and their
performances are compared
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