23,019,941 research outputs found
Simulation of wind turbine wake interaction using the vorticity transport model
The aerodynamic interactions that can occur within a wind farm can result in the constituent turbines generating a lower power output than would be possible if each of the turbines were operated in isolation. Tightening of the constraints on the siting of wind farms is likely to increase the scale of the problem in the future. The aerodynamic performance of turbine rotors and the mechanisms that couple the fluid dynamics of multiple rotors can be most readily understood by simplifying the problem and considering the interaction between only two rotors. The aerodynamic interaction between two rotors in both co-axial and offset configurations has been simulated using the Vorticity Transport Model. The aerodynamic interaction is a function of the tip speed ratio, and both the streamwise and crosswind separation between the rotors. The simulations show that the momentum deficit at a turbine operating within the wake developed by the rotor of a second turbine is governed by the development of instabilities within the wake of the upwind rotor, and the ensuing structure of the wake as it impinges on the downwind rotor. If the wind farm configuration or wind conditions are such that a turbine rotor is subject to partial impingement by the wake produced by an upstream turbine, then significant unsteadiness in the aerodynamic loading on the rotor blades of the downwind turbine can result, and this unsteadiness can have considerable implications for the fatigue life of the blade structure and rotor hub
Exactly solvable model of dissipative vortex tunneling
I consider the problem of vortex tunneling in a two-dimensional
superconductor. The vortex dynamics is governed by the Magnus force and the
Ohmic friction force. Under-barrier motion in the vicinity of the saddle point
of the pinning potential leads to a model with quadratic Hamiltonian which can
be analytically diagonalized. I find the dependence of the tunneling
probability on the normal state quasiparticle relaxation time with a
minimum at , where is the level spacing of the
quasiparticle bound states inside the vortex core. The results agree
qualitatively with the available experimental data.Comment: RevTeX, 6 pages, 2 figures. Published versio
Simulating the aerodynamic performance and wake dynamics of a vertical-axis wind turbine
The accurate prediction of the aerodynamics and performance of vertical-axis wind turbines is essential if their design is to be improved but poses a signifi cant challenge to numerical simulation tools. The cyclic motion of the blades induces large variations in the angle of attack of the blades that can manifest as dynamic stall. In addition, predicting the interaction between the blades and the wake developed by the rotor requires a high-fi delity representation of the vortical structures within the fl ow fi eld in which the turbine operates. The aerodynamic performance and wake dynamics of a Darrieus-type vertical-axis wind turbine consisting of two straight blades is simulated using Brown’s Vorticity Transport Model. The predicted variation with azimuth of the normal and tangential force on the turbine blades compares well with experimental measurements. The interaction between the blades and the vortices that are shed and trailed in previous revolutions of the turbine is shown to have a signifi cant effect on the distribution of aerodynamic loading on the blades. Furthermore, it is suggested that the disagreement between experimental and numerical data that has been presented in previous studies arises because the blade–vortex interactions on the rotor were not modelled with sufficient fidelity
Phase diagram for diblock copolymer melts under cylindrical confinement
We extensively study the phase diagram of a diblock copolymer melt confined
in a cylindrical nanopore using real-space self-consistent mean-field theory.
We discover a rich variety of new two-dimensional equilibrium structures that
have no analog in the unconfined system. These include non-hexagonally
coordinated cylinder phases and structures intermediate between lamellae and
cylinders. We map the stability regions and phase boundaries for all the
structures we find. As the pore radius is decreased, the pore accommodates
fewer cylindrical domains and structural transitions occur as cylinders are
eliminated. Our results are consistent with experiments, but we also predict
phases yet to be observed.Comment: 12 pages, 3 figures. submitted to Physical Review Letter
Inelastic current-voltage characteristics of atomic and molecular junctions
We report first-principles calculations of the inelastic current-voltage
(I-V) characteristics of a gold point contact and a molecular junction in the
nonresonant regime. Discontinuities in the I-V curves appear in correspondence
to the normal modes of the structures. Due to the quasi-one-dimensional nature
of these systems, specific modes with large longitudinal component dominate the
inelastic I-V curves. In the case of the gold point contact, our results are in
good agreement with recent experimental data. For the molecular junction, we
find that the inelastic I-V curves are quite sensitive to the structure of the
contact between the molecule and the electrodes thus providing a powerful tool
to extract the bonding geometry in molecular wires.Comment: 4 pages, 3 figure
Modelling the aerodynamics of vertical-axis wind turbines in unsteady wind conditions
Most numerical and experimental studies of the performance of vertical-axis wind turbines have been conducted with the rotors in steady, and thus somewhat artificial, wind conditions - with the result that turbine aerodynamics, under varying wind conditions, are still poorly understood. The Vorticity Transport Model has been used to investigate the aerodynamic performance and wake dynamics, both in steady and unsteady wind conditions, of three different vertical-axis wind turbines: one with a straight-bladed configuration, another with a curved-bladed configuration and another with a helically twisted configuration. The turbines with non-twisted blades are shown to be somewhat less efficient than the turbine with helically twisted blades when the rotors are operated at constant rotational speed in unsteady wind conditions. In steady wind conditions, the power coefficients that are produced by both the straight- and the curved-bladed turbines vary considerably within one rotor revolution because of the continuously varying angle of attack on the blades and, thus, the inherent unsteadiness in the blade aerodynamic loading. These variations are much larger, and thus far more significant, than those that are induced by the unsteadiness in the wind conditions
Ambipolar charge injection and transport in a single pentacene monolayer island
Electrons and holes are locally injected in a single pentacene monolayer
island. The two-dimensional distribution and concentration of the injected
carriers are measured by electrical force microscopy. In crystalline monolayer
islands, both carriers are delocalized over the whole island. On disordered
monolayer, carriers stay localized at their injection point. These results
provide insight into the electronic properties, at the nanometer scale, of
organic monolayers governing performances of organic transistors and molecular
devices.Comment: To be published in Nano Letter
Spare Me the Complements: An Immoderate Proposal for Eliminating the "We/They" Category Boundary
A speculative hypothesis about how to eliminate the "we/they" distinction by rearing children (during early critical years) in "aggregates in flux" instead of in kinship-based families: A category cannot be formed from positive examples only: one must be able to sample both what is and what is not in a category in order to recognise the category at all. The basis for the distinction is the features shared by the members (invariants), and absent from the non-members. In "aggregates in flux," the individual members would be constantly varying (and unrelated, genetically). The only invariant would be that they are all human
Opportunité pour l’État du Valais d’implémenter la norme ISO 20022 pour le trafic des paiements
Ce travail de Bachelor consiste en l’étude de l’introduction de la norme ISO 20022 pour l’uniformisation financière auprès de l’État du Valais. Effectivement, à compter de la mi-2018, l’utilisation de cette norme pour le trafic des paiements domestiques en Suisse sera rendue obligatoire. Le mandat de ce travail a été émis par le Centre de Compétences SAP – Finance (CC SAP – Finance) qui cherche à connaître la norme et à étudier la possibilité de l’intégrer dans son système SAP. Pour répondre à ces questions, plusieurs points théoriques liés à la norme ont été développés, tels que le format eXtensible Mark-up Language (XML), les processus concernés et son introduction auprès des institutions financières. Ensuite, la situation actuelle dans le système SAP a été étudiée et la possibilité d’adaptation à la norme a été vérifiée par des tests. Finalement, une synthèse des résultats a permis de proposer des recommandations. L’État du Valais est prêt à utiliser la norme ISO 20022 après configuration et paramétrage du système SAP, nous recommandons cependant de ne pas le faire. Ces recommandations sont principalement justifiées par le fait que le principal partenaire financier de l’État du Valais dans le trafic des paiements, la Banque Cantonale du Valais (BCVs), ne prévoit pas d’adapter son infrastructure avant la fin de l’année 2014 et le début de l’année 2015
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