10,406 research outputs found
New microelectronic power amplifier
Integrated push-pull power amplifier fabricated on a chip of silicon has interdigitated power transistors and is hermetically encapsulated in a beryllia flat package. It provides current output greater than the nominal 10 amperes from an input current drive of 1 ampere
The influence of interpolation and station network density on the distributions and trends of climate variables in gridded daily data
We study the influence of station network density on the distributions and trends in indices of area-average daily precipitation and temperature in the E-OBS high resolution gridded dataset of daily climate over Europe, which was produced with the primary purpose of Regional Climate Model evaluation. Area averages can only be determined with reasonable accuracy from a sufficiently large number of stations within a grid-box. However, the station network on which E-OBS is based comprises only 2,316 stations, spread unevenly across approximately 18,000 0.22A degrees grid-boxes. Consequently, grid-box data in E-OBS are derived through interpolation of stations up to 500 km distant, with the distance of stations that contribute significantly to any grid-box value increasing in areas with lower station density. Since more dispersed stations have less shared variance, the resultant interpolated values are likely to be over-smoothed, and extreme daily values even more so. We perform an experiment over five E-OBS grid boxes for precipitation and temperature that have a sufficiently dense local station network to enable a reasonable estimate of the area-average. We then create a series of randomly selected station sub-networks ranging in size from four to all stations within the E-OBS interpolation search radii. For each sub-network realisation, we estimate the grid-box average applying the same interpolation methodology as used for E-OBS, and then evaluate the effect of network density on the distribution of daily values, as well as trends in extremes indices. The results show that when fewer stations have been used for the interpolation, both precipitation and temperature are over-smoothed, leading to a strong tendency for interpolated daily values to be reduced relative to the "true" area-average. The smoothing is greatest for higher percentiles, and therefore has a disproportionate effect on extremes and any derived extremes indices. For many regions of the E-OBS dataset, the station density is sufficiently low to expect this smoothing effect to be significant and this should be borne in mind by any users of the E-OBS dataset
Wideband pulse propagation: single-field and multi-field approaches to Raman interactions
We model the process of ultra broadband light generation in which a pair of
laser pulses separated by the Raman frequency drive a Raman transition. In
contrast to the usual approach using separate field envelopes for the different
frequency components, we treat the field as a single entity. This requires the
inclusion of few-cycle corrections to the pulse propagation. Our single-field
model makes fewer approximations and is mathematically (and hence
computationally) simpler, although it does require greater computational
resources to implement. The single-field theory reduces to the traditional
multi-field one using appropriate approximations.Comment: 6 pages, two 3-part figure
Numerical Investigation on Flow Separation Control of Low Reynolds Number Sinusoidal Aerofoils
The paper presents a computational analysis of the characteristics of a NACA 634-
021 aerofoil incorporated with sinusoidal leading-edge protuberances at Re = 14,000.
The protuberances are characterized by an amplitude and wavelength of 12% and 50%
of the aerofoil chord length respectively. An unsteady Reynolds Average Navier Stokes
(RANS) analysis of the full-span aerofoils was carried out using Transition SST (Shear
Stress Transport) turbulence model across five different angles-of-attack (AOA).
Comparisons with previous experimental results reported good qualitative agreements
in terms of flow separation when the aerofoils are pitched at higher AOAs. Results
presented here comprised of near-wall flow visualizations of the flow separation bubble
at the peaks and troughs of the protuberances. Additionally, results indicate that the
aerofoil with leading-edge protuberances displayed distinctive wall shear streamline and
iso-contour characteristics at different span-wise positions. This implies that even at a
low Reynolds number, implementations of these leading-edge protuberances could have
positive or adverse effects on flow separation
Rotational Instabilities and Centrifugal Hangup
One interesting class of gravitational radiation sources includes rapidly
rotating astrophysical objects that encounter dynamical instabilities. We have
carried out a set of simulations of rotationally induced instabilities in
differentially rotating polytropes. An =1.5 polytrope with the Maclaurin
rotation law will encounter the =2 bar instability at .
Our results indicate that the remnant of this instability is a persistent
bar-like structure that emits a long-lived gravitational radiation signal.
Furthermore, dynamical instability is shown to occur in =3.33 polytropes
with the -constant rotation law at . In this case, the
dominant mode of instability is =1. Such instability may allow a
centrifugally-hung core to begin collapsing to neutron star densities on a
dynamical timescale. If it occurs in a supermassive star, it may produce
gravitational radiation detectable by LISA.Comment: 13 pages (includes 11 figures) and 1 separate jpeg figure; to appear
in Astrophysical Sources of Gravitational Radiation, AIP conference
proceedings, edited by Joan M. Centrell
Studies in transplantation immunity, with special reference to the induction of specific unresponsiveness towards skin allografts in mice treated with Antigen, Bordetella Pertussis and Antilymphocyte serum
Imperial Users onl
Characterisation of the dynamical quantum state of a zero temperature Bose-Einstein condensate
We describe the quantum state of a Bose-Einstein condensate at zero
temperature. By evaluating the Q-function we show that the ground state of
Bose-Einstein condensate under the Hartree approximation is squeezed. We find
that multimode Schroedinger cat states are generated as the condensate evolves
in a ballistic expansion.Comment: 13 pages, 6 figure
Advanced rocket engine cryogenic turbopump bearing thermal model
A lumped node thermal model was developed representing the Space Shuttle Main Engine (SSME) liquid oxygen (LOX) turbopump turbine end bearings operating in a cryogenically cooled bearing tester. Bearing elements, shaft, carrier, housing, cryogen flow characteristics, friction heat, and fluid viscous energy are included in the model. Heat transfer characteristics for the regimes of forced convection boiling are modeled for liquid oxygen (LOX) and liquid nitrogen (LN2). Large temperature differences between the cryogenic fluid and baring contact surfaces require detailed nodal representation in these areas. Internal loads and friction heat are affected by temperature dependent operating clearances requiring iterations between bearing thermal and mechanical models. Analyses indicate a thermal-mechanical coupling resulting in reduced operating clearances, increased loading and heating which can contribute to premature bearing failure. Contact surfaces operate at temperatures above local saturation resulting in vapor rather than liquid in the contacts, precluding possible liquid film lubrication. Elevated temperatures can reduce lubrication, increase friction, and reduce surface hardness supporting a surface failure mode rather than subsurface fatigue
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