34,433 research outputs found
Engineering handbook on the atmospheric environmental guidelines for use in wind turbine generator development
The guidelines are given in the form of design criteria relative to wind speed, wind shear, turbulence, wind direction, ice and snow loading, and other climatological parameters which include rain, hail, thermal effects, abrasive and corrosive effects, and humidity. This report is a presentation of design criteria in an engineering format which can be directly input to wind turbine generator design computations. Guidelines are also provided for developing specialized wind turbine generators or for designing wind turbine generators which are to be used in a special region of the United States
Effect of time delay on feedback control of a flashing ratchet
It was recently shown that the use of feedback control can improve the
performance of a flashing ratchet. We investigate the effect of a time delay in
the implementation of feedback control in a closed-loop collective flashing
ratchet, using Langevin dynamics simulations. Surprisingly, for a large
ensemble, a well-chosen delay time improves the ratchet performance by allowing
the system to synchronize into a quasi-periodic stable mode of oscillation that
reproduces the optimal average velocity for a periodically flashing ratchet.
For a small ensemble, on the other hand, finite delay times significantly
reduce the benefit of feedback control for the time-averaged velocity, because
the relevance of information decays on a time scale set by the diffusion time
of the particles. Based on these results, we establish that experimental use of
feedback control is realistic.Comment: 6 pages, 6 figures, to appear in Europhysics Letter
Dynamic magnetic response of infinite arrays of ferromagnetic particles
Recently developed techniques to find the eigenmodes of a ferromagnetic
particle of arbitrary shape, as well as the absorption in the presence of an
inhomogeneous radio-frequency field, are extended to treat infinite lattices of
such particles. The method is applied to analyze the results of recent FMR
experiments, and yields substantially good agreement between theory and
experiment
The interpretation of the field angle dependence of the critical current in defect-engineered superconductors
We apply the vortex path model of critical currents to a comprehensive
analysis of contemporary data on defect-engineered superconductors, showing
that it provides a consistent and detailed interpretation of the experimental
data for a diverse range of materials. We address the question of whether
electron mass anisotropy plays a role of any consequence in determining the
form of this data and conclude that it does not. By abandoning this false
interpretation of the data, we are able to make significant progress in
understanding the real origin of the observed behavior. In particular, we are
able to explain a number of common features in the data including shoulders at
intermediate angles, a uniform response over a wide angular range and the
greater discrimination between individual defect populations at higher fields.
We also correct several misconceptions including the idea that a peak in the
angular dependence of the critical current is a necessary signature of strong
correlated pinning, and conversely that the existence of such a peak implies
the existence of correlated pinning aligned to the particular direction. The
consistency of the vortex path model with the principle of maximum entropy is
introduced.Comment: 14 pages, 7 figure
Constraining New Forces in the Casimir Regime Using the Isoelectronic Technique
We report the first isoelectronic differential force measurements between a
Au-coated probe and two Au-coated films, made out of Au and Ge. These
measurements, performed at submicron separations using soft
microelectromechanical torsional oscillators, eliminate the need for a detailed
understanding of the probe-film Casimir interaction. The observed differential
signal is directly converted into limits on the parameters and
which characterize Yukawa-like deviations from Newtonian gravity. We
find \alpha \lsim 10^{12} for nm, an improvement of
10 over previous limits.Comment: 10 pages, 4 figure
Catching VY Sculptoris in a low state
Context. In the context of a large campaign to determine the system
parameters of high mass transfer cataclysmic variables, we found VY Scl in a
low state in 2008. Aims. Making use of this low state, we study the stellar
components of the binary with little influence of the normally dominating
accretion disc. Methods. Time-resolved spectroscopy and photometry of VY Scl
taken during the low state are presented. We analysed the light-curve and
radial velocity curve and use time-resolved spectroscopy to calculate Doppler
maps of the dominant emission lines. Results. The spectra show narrow emission
lines of Halpha, Hbeta, HeI, NaID, and FeII, as well as faint TiO absorption
bands that trace the motion of the irradiated secondary star, and Halpha and
HeI emission line wings that trace the motion of the white dwarf. From these
radial velocities, we find an orbital period of 3.84 h, and put constraints on
binary parameters such as the mass ratio M2/M1 of 0.43 and the inclination of
15 deg. With a secondary's mass between 0.3 and 0.35 Msol, we derive the mass
for the white dwarf as M1 = 0.6-0.1 Msol.Comment: 8 pages, 9 figures, accepted for publication in A&
Tide Predictions Using Satellite Constituents
Conventional harmonic tide predictions for the last century have used/factors to modify the amplitudes of lunar constituents and u's to correct the constituent equilibrium phases (V0) as a means of approximating for a given period (one year or less) the effect of the 18.61 year cycle of the revolution of the moon’s node. Historically, there was little choice; friction in geared mechanical tidepredicting machines imposed finite limits on the number of constituents used. Doodson (1921) clearly identified and evaluated satellite constituents; his study was updated using the latest astronomical constants by Cartwright and Tayler (1971) and by Cartwright and Edden (1973). Nevertheless, satellite constituents, now readily usable on modern computers, have not been used for tide predictions. As a result, predictions have really been quasi-harmonic, requiring modifying amplitudes and phases periodically, at present every year for U.S. predictions, every two months for Canadian, and every 30 days for U.K. predictions. With satellite constituents, nineteen years of hourly tide predictions for Seattle (1921-1939) were computed from initial settings for 1 January 1921. It was not to be expected that the accuracy of harmonic tide predictions would be improved significantly by the new procedure; comparisons of annual residual variances for predictions by U.S. and Canadian procedures indicate that any improvements are small. Nevertheless, this new method removes the need for rather contrived (however clever) procedures, in particular that of constituents modifying M1 and L2 by cycles per 8.85 years (revolution of lunar perigee) in the f and u corrections for these constituents
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