14,335 research outputs found
Oblique and parallel modes of vortex shedding in the wake of a circular cylinder at low Reynolds numbers
Two fundamental characteristics of the low-Reynolds-number cylinder wake, which have involved considerable debate, are first the existence of discontinuities in the Strouhal-Reynolds number relationship, and secondly the phenomenon of oblique vortex shedding. The present paper shows that both of these characteristics of the wake are directly related to each other, and that both are influenced by the boundary conditions at the ends of the cylinder, even for spans of hundreds of diameters in length. It is found that a Strouhal discontinuity exists, which is not due to any of the previously proposed mechanisms, but instead is caused by a transition from one oblique shedding mode to another oblique mode. This transition is explained by a change from one mode where the central flow over the span matches the end boundary conditions to one where the central flow is unable to match the end conditions. In the latter case, quasi-periodic spectra of the velocity fluctuations appear; these are due to the presence of spanwise cells of different frequency. During periods when vortices in neighbouring cells move out of phase with each other, ‘vortex dislocations’ are observed, and are associated with rather complex vortex linking between the cells. However, by manipulating the end boundary conditions, parallel shedding can be induced, which then results in a completely continuous Strouhal curve. It is also universal in the sense that the oblique-shedding Strouhal data (S_θ) can be collapsed onto the parallel-shedding Strouhal curve (S_0) by the transformation, S_0 = S_θ/cosθ, where θ is the angle of oblique shedding. Close agreement between measurements in two distinctly different facilities confirms the continuous and universal nature of this Strouhal curve. It is believed that the case of parallel shedding represents truly two-dimensional shedding, and a comparison of Strouhal frequency data is made with several two-dimensional numerical simulations, yielding a large disparity which is not clearly understood. The oblique and parallel modes of vortex shedding are both intrinsic to the flow over a cylinder, and are simply solutions to different problems, because the boundary conditions are different in each case
Oblique and parallel modes of vortex shedding in the wake of a circular cylinder at low Reynolds numbers
Consideration of permanent tidal deformation in the orbit determination and data analysis for the Topex/Poseidon mission
The effects of the permanent tidal effects of the Sun and Moon with specific applications to satellite altimeter data reduction are reviewed in the context of a consistent definition of geoid undulations. Three situations are applicable not only for altimeter reduction and geoid definition, but also for the second degree zonal harmonic of the geopotential and the equatorial radius. A recommendation is made that sea surface heights and geoid undulations placed on the Topex/Poseidon geophysical data record should be referred to the mean Earth case (i.e., with the permanent effects of the Sun and Moon included). Numerical constants for a number of parameters, including a flattening and geoid geopotential, are included
A Political Economy of Privatization Contracts : The Case of Water and Sanitation in Ghana and Argentina
This document is the Accepted Manuscript version of the following article: Hulya Dagdeviren, Simon A. Robertson, 'A Political Economy of Privatization Contracts: The Case of Water and Sanitation in Ghana and Argentina', Competition & Change, Vol. 18 (2): 150-163, April 2014. The final, published version is available online at DOI: https://doi.org/10.1179/1024529414Z.00000000053. Published by SAGE.In general, the process and outcomes of privatization have been studied from the point of view of efficiency. In this article, we consider issues in the course of contract design, implementation, management and enforcement in privatized public services and utilities. The study is based on two case studies, involving several water concessions in Argentina and a management contract in the urban water sector in Ghana. Three key arguments are presented on the basis of these case studies. The first is that an individualistic analytical framework is often utilized by the mainstream economic perspectives, but these are inadequate for a comparative assessment of private versus public provision in public services where there are distinct collective or group interests and hence a wider socio-economic context and representation of different interests becomes highly important. Instead, the article proposes a political economy perspective, which pays due attention to distributional issues, group interests, ideology of states and power relations for the assessment of privatization contracts. Second, the administrative capacity of states and their resources play a key role for the outcomes of privatization. Finally, while some contractual issues could be resolved through resourcing and experience over time, others are inherent to the contractual relations with little prospect of remedy.Peer reviewe
A rapid review of communication strategies for physical activity guidelines and physical activity promotion: A review of worldwide strategies
BACKGROUND: To support the strategy development for communication of the updated physical activity (PA) guidelines, the UK Chief Medical Officers’ Expert Panel for Communication was created.
METHODS: To help inform this process a rapid review was performed to identify and describe how other nations are communicating their PA guidelines and PA generally. Elements of the Health enhancing physical activity (HEPA) policy audit tool (PAT) created by the World Health Organization (WHO) were used to investigate all 195 countries.
RESULTS: Seventy-seven countries had their own guidelines, 53 used the WHO guidelines, and for 65 countries no guidelines could be found. For the communication, 27 countries used infographics, 56 had government policies/documents, and 11 used a mass media campaign. Only 6 of these had been evaluated. Although many countries used infographics, there were no associated evaluations. As such any future communication strategies should incorporate an evaluation. Mass media campaigns had the strongest evidence base, proving to be an effective strategy, particularly when incorporating aspects of social marketing.
CONCLUSION: This review provides an insight into strategies countries worldwide have taken to communicate PA guidelines and PA promotion. These should be carefully considered when deciding how best to communicate and promote PA
guidelines
Static Properties of Trapped Bose-Fermi Mixed Condensate of Alkali Atoms
Static properties of a bose-fermi mixture of trapped potassium atoms are
studied in terms of coupled Gross-Pitaevskii and Thomas-Fermi equations for
both repulsive and attractive bose-fermi interatomic potentials. Qualitative
estimates are given for solutions of the coupled equations, and the parameter
regions are obtained analytically for the boson-density profile change and for
the boson/fermion phase separation. Especially, the parameter ratio
is found that discriminates the region of the large boson-profile change. These
estimates are applied for numerical results for the potassium atoms and checked
their consistency. It is suggested that a small fraction of fermions could be
trapped without an external potential for the system with an attractive
boson-fermion interaction.Comment: 8 pages,5 figure
Monte Carlo energy and variance minimization techniques for optimizing many-body wave functions
We investigate Monte Carlo energy and variance minimization techniques for
optimizing many-body wave functions. Several variants of the basic techniques
are studied, including limiting the variations in the weighting factors which
arise in correlated sampling estimations of the energy and its variance. We
investigate the numerical stability of the techniques and identify two reasons
why variance minimization exhibits superior numerical stability to energy
minimization. The characteristics of each method are studied using a
non-interacting 64-electron model of crystalline silicon. While our main
interest is in solid state systems, the issues investigated are relevant to
Monte Carlo studies of atoms, molecules and solids. We identify a robust and
efficient variance minimization scheme for optimizing wave functions for large
systems.Comment: 14 pages, including 7 figures. To appear in Phys. Rev. B. For related
publications see http://www.tcm.phy.cam.ac.uk/Publications/many_body.htm
Theoretical interpretation of the experimental electronic structure of lens shaped, self-assembled InAs/GaAs quantum dots
We adopt an atomistic pseudopotential description of the electronic structure
of self-assembled, lens shaped InAs quantum dots within the ``linear
combination of bulk bands'' method. We present a detailed comparison with
experiment, including quantites such as the single particle electron and hole
energy level spacings, the excitonic band gap, the electron-electron, hole-hole
and electron hole Coulomb energies and the optical polarization anisotropy. We
find a generally good agreement, which is improved even further for a dot
composition where some Ga has diffused into the dots.Comment: 16 pages, 5 figures. Submitted to Physical Review
Vanishing Twist near Focus-Focus Points
We show that near a focus-focus point in a Liouville integrable Hamiltonian
system with two degrees of freedom lines of locally constant rotation number in
the image of the energy-momentum map are spirals determined by the eigenvalue
of the equilibrium. From this representation of the rotation number we derive
that the twist condition for the isoenergetic KAM condition vanishes on a curve
in the image of the energy-momentum map that is transversal to the line of
constant energy. In contrast to this we also show that the frequency map is
non-degenerate for every point in a neighborhood of a focus-focus point.Comment: 13 page
Mode-Wise Entanglement of Gaussian States
We address the decomposition of a multi-mode pure Gaussian state with respect
to a bi-partite division of the modes. For any such division the state can
always be expressed as a product state involving entangled two-mode squeezed
states and single mode local states at each side. The character of entanglement
of the state can therefore be understood modewise; that is, a given mode on one
side is entangled with only one corresponding mode of the other, and therefore
the total bi-partite entanglement is the sum of the modewise entanglement. This
decomposition is generally not applicable to all mixed Gaussian states.
However, the result can be extended to a special family of "isotropic" states,
characterized by a phase space covariance matrix with a completely degenerate
symplectic spectrum.Comment: 4 pages, RevTex4. Replaced with revised version with reference added
to a previous related paper. Minor typographical errors correcte
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