2,373 research outputs found
Transonic rotor tip design using numerical optimization
The aerodynamic design procedure for a new blade tip suitable for operation at transonic speeds is illustrated. For the first time, 3 dimensional numerical optimization was applied to rotor tip design, using the recent derivative of the ROT22 code, program R22OPT. Program R22OPT utilized an efficient quasi-Newton optimization algorithm. Multiple design objectives were specified. The delocalization of the shock wave was to be eliminated in forward flight for an advance ratio of 0.41 and a tip Mach number of 0.92 at psi = 90 deg. Simultaneously, it was sought to reduce torque requirements while maintaining effective restoring pitching moments. Only the outer 10 percent of the blade span was modified; the blade area was not to be reduced by more than 3 percent. The goal was to combine the advantages of both sweptback and sweptforward blade tips. A planform that featured inboard sweepback was combined with a sweptforward tip and a taper ratio of 0.5. Initially, the ROT22 code was used to find by trial and error a planform geometry which met the design goals. This configuration had an inboard section with a leading edge sweep of 20 deg and a tip section swept forward at 25 deg; in addition, the airfoils were modified
Vortex Washboard Voltage Noise in Type-II Superconductors
In order to characterize flux flow through disordered type-II
superconductors, we investigate the effects of columnar and point defects on
the vortex velocity / voltage power spectrum in the driven non-equilibrium
steady state. We employ three-dimensional Metropolis Monte Carlo simulations to
measure relevant physical observables including the force-velocity /
current-voltage (I-V) characteristics, vortex spatial arrangement and structure
factor, and mean flux line radius of gyration. Our simulation results compare
well to earlier findings and physical intuition. We focus specifically on the
voltage noise power spectra in conjunction with the vortex structure factor in
the presence of weak columnar and point pinning centers. We investigate the
vortex washboard noise peak and associated higher harmonics, and show that the
intensity ratios of the washboard harmonics are determined by the strength of
the material defects rather than the type of pins present. Through varying
columnar defect lengths and pinning strengths as well as magnetic flux density
we further explore the effect of the material defects on vortex transport. It
is demonstrated that the radius of gyration displays quantitatively unique
features that depend characteristically on the type of material defects present
in the sample.Comment: Latex, 17 pages, 14 figure
Moisture, a Vital but Neglected Factor in the Seasonal Ecology of Insects: Hypotheses and Tests of Mechanisms
Of the major physical factors that influence insect seasonal ecology, moisture is least understood and least appreciated. It is our premise that experimental probing of insects from diverse zones and various habitats would reveal general patterns of insect responses to moisture that are as striking as those for photoperiod and temperature. Using the paradigms of photoperiod and temperature as ecophysiological determinants of insect seasonality, we hypothesize that moisture influences insect life cycles via one or more of three mechanisms—as a token stimulus for diapause, modulator of developmental or reproductive rates, or behavioral cue for vital seasonal events. For heuristic purposes, we offer each of these hypotheses in close association with approaches for testing their validity in insects that undergo dry-season dormancy. The approaches appear appropriate for examining the role of moisture in the life histories of terrestrial invertebrates other than insects, as well as plants and microbes that have a seasonal resting stage. Elucidating moisture's role in insect seasonal cycles is critical to the development of comprehensive phenological models, improved insect management systems, and identification of novel evolutionary mechanisms for adaptation to wet-dry seasons, especially in tropical, subtropical, and Mediterranean region
FRW Universe Models in Conformally Flat Spacetime Coordinates. I: General Formalism
The 3-space of a universe model is defined at a certain simultaneity. Hence
space depends on which time is used. We find a general formula generating all
known and also some new transformations to conformally flat spacetime
coordinates. A general formula for the recession velocity is deduced.Comment: 12 page
The angular power spectrum of radio emission at 2.3 GHz
We have analysed the Rhodes/HartRAO survey at 2326 MHz and derived the global
angular power spectrum of Galactic continuum emission. In order to measure the
angular power spectrum of the diffuse component, point sources were removed
from the map by median filtering. A least-square fit to the angular power
spectrum of the entire survey with a power law spectrum C_l proportional to
l^{-alpha}, gives alpha = 2.43 +/- 0.01 for l = 2-100. The angular power
spectrum of radio emission appears to steepen at high Galactic latitudes and
for observed regions with |b| > 20 deg, the fitted spectral index is alpha =
2.92 +/- 0.07. We have extrapolated this result to 30 GHz (the lowest frequency
channel of Planck) and estimate that no significant contribution to the sky
temperature fluctuation is likely to come from synchrotron at degree-angular
scalesComment: 10 pages, 10 figures, accepted for publication by Astronomy &
Astrophysic
FRW Universe Models in Conformally Flat Spacetime Coordinates. II: Universe models with negative and vanishing spatial curvature
We deduce general expressions for the line element of universe models with
negative and vanishing spatial curvature described by conformally flat
spacetime coordinates. The empty Milne universe model and models with dust,
radiation and vacuum energy are exhibited. Discussing the existence of particle
horizons we show that there is continual creation of space, matter and energy
when conformal time is used in Friedmann-Robertson-Walker models with negative
spatial curvature.Comment: 25 pages, 12 figure
Analysis of CMB foregrounds using a database for Planck
Within the scope of the Planck IDIS (Integrated Data Information System)
project we have started to develop the data model for time-ordered data and
full-sky maps. The data model is part of the Data Management Component (DMC), a
software system designed according to a three-tier architecture which allows
complete separation between data storage and processing. The DMC is already
being used for simulation activities and the modeling of some foreground
components. We have ingested several Galactic surveys into the database and
used the science data-access interface to process the data. The data structure
for full-sky maps utilises the HEALPix tessellation of the sphere. We have been
able to obtain consistent measures of the angular power spectrum of the
Galactic radio continuum emission between 408 MHz and 2417 MHz.Comment: 7 pages, 6 figures. To appear in the Proceedings of the MPA/ESO/MPE
Joint Astronomy Conference "Mining The Sky
Planck-LFI: Design and Performance of the 4 Kelvin Reference Load Unit
The LFI radiometers use a pseudo-correlation design where the signal from the
sky is continuously compared with a stable reference signal, provided by a
cryogenic reference load system. The reference unit is composed by small
pyramidal horns, one for each radiometer, 22 in total, facing small absorbing
targets, made of a commercial resin ECCOSORB CR (TM), cooled to approximately
4.5 K. Horns and targets are separated by a small gap to allow thermal
decoupling. Target and horn design is optimized for each of the LFI bands,
centered at 70, 44 and 30 GHz. Pyramidal horns are either machined inside the
radiometer 20K module or connected via external electro-formed bended
waveguides. The requirement of high stability of the reference signal imposed a
careful design for the radiometric and thermal properties of the loads.
Materials used for the manufacturing have been characterized for thermal, RF
and mechanical properties. We describe in this paper the design and the
performance of the reference system.Comment: This is an author-created, un-copyedited version of an article
accepted for publication in JINST. IOP Publishing Ltd is not responsible for
any errors or omissions in this version of the manuscript or any version
derived from it. The definitive publisher authenticated version is available
online at [10.1088/1748-0221/4/12/T12006]. 14 pages, 34 figure
Correlations, fluctuations and stability of a finite-size network of coupled oscillators
The incoherent state of the Kuramoto model of coupled oscillators exhibits
marginal modes in mean field theory. We demonstrate that corrections due to
finite size effects render these modes stable in the subcritical case, i.e.
when the population is not synchronous. This demonstration is facilitated by
the construction of a non-equilibrium statistical field theoretic formulation
of a generic model of coupled oscillators. This theory is consistent with
previous results. In the all-to-all case, the fluctuations in this theory are
due completely to finite size corrections, which can be calculated in an
expansion in 1/N, where N is the number of oscillators. The N -> infinity limit
of this theory is what is traditionally called mean field theory for the
Kuramoto model.Comment: 25 pages (2 column), 12 figures, modifications for resubmissio
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