3,901 research outputs found
Excursions of the integral of the Brownian motion
The integrated Brownian motion is sometimes known as the Langevin process.
Lachal studied several excursion laws induced by the latter. Here we follow a
different point of view developed by Pitman for general stationary processes.
We first construct a stationary Langevin process and then determine explicitly
its stationary excursion measure. This is then used to provide new descriptions
of Ito's excursion measure of the Langevin process reflected at a completely
inelastic boundary, which has been introduced recently by Bertoin.Comment: In this second version, some consequent changes of notations and
presentation. The space we work on for Proposition 2 and Theorem 2 changed a
bit (the proofs are unchanged
Kinetic decomposition for periodic homogenization problems
We develop an analytical tool which is adept for detecting shapes of
oscillatory functions, is useful in decomposing homogenization problems into
limit-problems for kinetic equations, and provides an efficient framework for
the validation of multi-scale asymptotic expansions. We apply it first to a
hyperbolic homogenization problem and transform it to a hyperbolic limit
problem for a kinetic equation. We establish conditions determining an
effective equation and counterexamples for the case that such conditions fail.
Second, when the kinetic decomposition is applied to the problem of enhanced
diffusion, it leads to a diffusive limit problem for a kinetic equation that in
turn yields the effective equation of enhanced diffusion
Control of plume interference effects on axisymmetric afterbodies
Plume interference effects on the axisymmetric flowfields around powered missiles are investigated using computational techniques. The study is mainly to understand the physics of the plume-induced shock and separation particularly at high plume to exit pressure ratios with and without shock-turbulent boundary layer control methods
Large-eddy simulation of the flow in a lid-driven cubical cavity
Large-eddy simulations of the turbulent flow in a lid-driven cubical cavity
have been carried out at a Reynolds number of 12000 using spectral element
methods. Two distinct subgrid-scales models, namely a dynamic Smagorinsky model
and a dynamic mixed model, have been both implemented and used to perform
long-lasting simulations required by the relevant time scales of the flow. All
filtering levels make use of explicit filters applied in the physical space (on
an element-by-element approach) and spectral (modal) spaces. The two
subgrid-scales models are validated and compared to available experimental and
numerical reference results, showing very good agreement. Specific features of
lid-driven cavity flow in the turbulent regime, such as inhomogeneity of
turbulence, turbulence production near the downstream corner eddy, small-scales
localization and helical properties are investigated and discussed in the
large-eddy simulation framework. Time histories of quantities such as the total
energy, total turbulent kinetic energy or helicity exhibit different evolutions
but only after a relatively long transient period. However, the average values
remain extremely close
The effect of in-plane magnetic field on the spin Hall effect in Rashba-Dresselhaus system
In a two-dimensional electron gas with Rashba and Dresselhaus spin-orbit
couplings, there are two spin-split energy surfaces connected with a degenerate
point. Both the energy surfaces and the topology of the Fermi surfaces can be
varied by an in-plane magnetic field. We find that, if the chemical potential
falls between the bottom of the upper band and the degenerate point, then
simply by changing the direction of the magnetic field, the magnitude of the
spin Hall conductivity can be varied by about 100 percent. Once the chemical
potential is above the degenerate point, the spin Hall conductivity becomes the
constant , independent of the magnitude and direction of the magnetic
field. In addition, we find that the in-plane magnetic field exerts no
influence on the charge Hall conductivity.Comment: 11 pages, 3 figures, to be published on Phys. Rev.
Implementation challenges of maternal health care in Ghana: the case of health care providers in the Tamale Metropolis
Background: Achieving the Millennium Development Goal (MDG) of improving maternal health has become a focus in recent times for the majority of countries in sub-Saharan Africa. Ghana’s maternal mortality is still high indicating that
there are challenges in the provision of quality maternal health care at the facility level. This study examined the
implementation challenges of maternal health care services in the Tamale Metropolis of Ghana.
Methods: Purposive sampling was used to select study participants and qualitative strategies, including in-depth
interviews, focus group discussions and review of documents employed for data collection. The study participants
included midwives (24) and health managers (4) at the facility level.
Results: The study revealed inadequate in-service training, limited knowledge of health policies by midwives, increased
workload, risks of infection, low motivation, inadequate labour wards, problems with transportation, and difficulties in following the procurement act, among others as some of the challenges confronting the successful implementation of
the MDGs targeting maternal and child health in the Tamale Metropolis.
Conclusions: Implementation of maternal health interventions should take into consideration the environment or the
context under which the interventions are implemented by health care providers to ensure they are successful. The
study recommends involving midwives in the health policy development process to secure their support and commitment towards successful implementation of maternal health interventions
Proton decay matrix elements with domain-wall fermions
Hadronic matrix elements of operators relevant to nucleon decay in grand
unified theories are calculated numerically using lattice QCD. In this context,
the domain-wall fermion formulation, combined with non-perturbative
renormalization, is used for the first time. These techniques bring reduction
of a large fraction of the systematic error from the finite lattice spacing.
Our main effort is devoted to a calculation performed in the quenched
approximation, where the direct calculation of the nucleon to pseudoscalar
matrix elements, as well as the indirect estimate of them from the nucleon to
vacuum matrix elements, are performed. First results, using two flavors of
dynamical domain-wall quarks for the nucleon to vacuum matrix elements are also
presented to address the systematic error of quenching, which appears to be
small compared to the other errors. Our results suggest that the representative
value for the low energy constants from the nucleon to vacuum matrix elements
are given as |alpha| simeq |beta| simeq 0.01 GeV^3. For a more reliable
estimate of the physical low energy matrix elements, it is better to use the
relevant form factors calculated in the direct method. The direct method tends
to give smaller value of the form factors, compared to the indirect one, thus
enhancing the proton life-time; indeed for the pi^0 final state the difference
between the two methods is quite appreciable.Comment: 56 pages, 17 figures, a comment and two references added in the
introduction, typo corrected in Eq.1
Exact asymptotic expansions for the cylindrical Poisson-Boltzmann equation
The mathematical theory of integrable Painleve/Toda type systems sheds new
light on the behavior of solutions to the Poisson-Boltzmann equation for the
potential due to a long rod-like macroion. We investigate here the case of
symmetric electrolytes together with that of 1:2 and 2:1 salts. Short and large
scale features are analyzed, with a particular emphasis on the low salinity
regime. Analytical expansions are derived for several quantities relevant for
polyelectrolytes theory, such as the Manning radius. In addition, accurate and
practical expressions are worked out for the electrostatic potential, which
improve upon previous work and cover the full range of radial distances
Theory of Spin Hall conductivity in n-doped GaAs
We develop a theory of extrinsic spin currents in semiconductors, resulting
from spin-orbit coupling at charged scatterers, which leads to skew scattering
and side jump contributions to the spin Hall conductance. Applying the theory
to bulk n-GaAs, without any free parameters, we find spin currents that are in
reasonable agreement with recent experiments by Kato et al. [Science 306, 1910
(2004)].Comment: 5 pages, 1 figur
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