30,569 research outputs found
Volume-reflecting dielectric heat shield
White, volume-reflecting dielectric material absorbs essentially none of the incident radiant energy, and continues to reflect even though in severe environment its surface is melted and is being vaporized. Process of overall reflectance in dielectric material, involving internal refractions and reflections, is similar to process of reflection in paints
On the soft X-ray spectrum of cooling flows
Strong evidence for cooling flows has been found in low resolution X-ray
imaging and spectra of many clusters of galaxies. However high resolution X-ray
spectra of several clusters from the Reflection Grating Spectrometer (RGS) on
XMM-Newton now show a soft X-ray spectrum inconsistent with a simple cooling
flow. The main problem is a lack of the emission lines expected from gas
cooling below 1--2 keV. Lines from gas at about 2--3 keV are observed, even in
a high temperature cluster such as A 1835, indicating that gas is cooling down
to about 2--3 keV, but is not found at lower temperatures. Here we discuss
several solutions to the problem; heating, mixing, differential absorption and
inhomogeneous metallicity. Continuous or sporadic heating creates further
problems, including the targetting of the heat at the cooler gas and also the
high total energy required. So far there is no clear observational evidence for
widespread heating, or shocks, in cluster cores, except in radio lobes which
occupy only part of the volume. The implied ages of cooling flows are short, at
about 1 Gyr. Mixing. or absorption, of the cooling gas are other possibilities.
Alternatively, if the metals in the intracluster medium are not uniformly
spread but are clumped, then little line emission is expected from the gas
cooling below 1 keV. The low metallicity part cools without line emission
whereas the strengths of the soft X-ray lines from the metal-rich gas depend on
the mass fraction of that gas and not on the abundance, since soft X-ray line
emission dominates the cooling function below 2 keV.Comment: 5 pages, with 2 figures, submitted to MNRA
The design of aircraft brake systems, employing cooling to increase brake life
A research program was initiated to determine the feasibility of using cooling to increase brake life. An air cooling scheme was proposed, constructed and tested with various designs. Straight and curved slotting of the friction material was tested. A water cooling technique, similar to the air cooling procedure, was evaluated on a curved slotted rotor. Also investigated was the possibility of using a phase-change material within the rotor to absorb heat during braking. Various phase-changing materials were tabulated and a 50%, (by weight) LiF - BeF2 mixing was chosen. It was shown that corrosion was not a problem with this mixture. A preliminary design was evaluated on an actual brake. Results showed that significant improvements in lowering the surface temperature of the brake occurred when air or water cooling was used in conjunction with curved slotted rotors
Screening for health risks: A social science perspective
Health screening promises to reduce risks to individuals via probabilistic sifting of populations for medical conditions. The categorisation and selection of 'conditions' such as cardiovascular events, dementia and depression for screening itself requires prior interpretive labour which usually remains unexamined. Screening systems can take diverse organisational forms and varying relationships to health status, as when purported disease precursors, for example 'pre-cancerous' polyps, or supposed risk factors, such as high cholesterol themselves, become targets for screening. Screening at best yields small, although not necessarily unworthwhile, net population health gains. It also creates new risks, leaving some individuals worse-off than if they had been left alone. The difficulties associated with attempting to measure small net gains through randomised controlled trials are sometimes underestimated. Despite endemic doubts about its clinical utility, bibliometric analysis of published papers shows that responses to health risks are coming to be increasingly thought about in terms of screening. This shift is superimposed on a strengthening tendency to view health through the lens of risk. It merits further scrutiny as a societal phenomenon
Stability of the Submillimeter Brightness of the Atmosphere Above Mauna Kea, Chajnantor and the South Pole
The summit of Mauna Kea in Hawaii, the area near Cerro Chajnantor in Chile,
and the South Pole are sites of large millimeter or submillimeter wavelength
telescopes. We have placed 860 GHz sky brightness monitors at all three sites
and present a comparative study of the measured submillimeter brightness due to
atmospheric thermal emission. We report the stability of that quantity at each
site.Comment: 6 figure
Strong Optomechanical Squeezing of Light
We create squeezed light by exploiting the quantum nature of the mechanical
interaction between laser light and a membrane mechanical resonator embedded in
an optical cavity. The radiation pressure shot noise (fluctuating optical force
from quantum laser amplitude noise) induces resonator motion well above that of
thermally driven motion. This motion imprints a phase shift on the laser light,
hence correlating the amplitude and phase noise, a consequence of which is
optical squeezing. We experimentally demonstrate strong and continuous
optomechanical squeezing of 1.7 +/- 0.2 dB below the shot noise level. The peak
level of squeezing measured near the mechanical resonance is well described by
a model whose parameters are independently calibrated and that includes thermal
motion of the membrane with no other classical noise sources.Comment: 12 pages, 8 figure
Principal sources and dispersal patterns of suspended particulate matter in nearshore surface waters of the northeast Pacific Ocean and the Hawaiian Islands
The author has identified the following significant results. ERTS-1 multispectral scanner imagery of the nearshore surface waters of the Northeast Pacific Ocean is proving to be a useful tool for determining source and dispersal of suspended particulate matter. The principal sources of the turbid water, seen best on the green and red bands, are river and stream effluents and actively eroding coastlines; secondary sources are waste effluents and production of planktonic organisms, but these may sometimes be masked by the very turbid plumes of suspended sediment being discharged into the nearshore zone during times of high river discharge. The configuration and distribution of the plumes of turbid water also can be used to infer near-surface current directions. Comparison of imagery of the nearshore water off the northern California coast from October 1972 and January 1973 shows a reversal of the near-surface currents, from predominantly south-setting in the fall (California Current) to north-setting in the winter (Davidson Current)
The Kelvin Formula for Thermopower
Thermoelectrics are important in physics, engineering, and material science
due to their useful applications and inherent theoretical difficulty,
especially in strongly correlated materials. Here we reexamine the framework
for calculating the thermopower, inspired by ideas of Lord Kelvin from 1854. We
find an approximate but concise expression, which we term as the Kelvin formula
for the the Seebeck coefficient. According to this formula, the Seebeck
coefficient is given as the particle number derivative of the entropy
, at constant volume and temperature ,
. This formula is shown to be competitive compared to other
approximations in various contexts including strongly correlated systems. We
finally connect to a recent thermopower calculation for non-Abelian fractional
quantum Hall states, where we point out that the Kelvin formula is exact.Comment: 6 pages, 2 figure
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