1,043 research outputs found
Jet heat transfer in the vicinity of a rotating grinding wheel
Abstract: Impinging jets are known as a method of achieving high convective heat transfer coefficients. One potential application of impinging jet heat transfer is the air jet cooling of a grinding process. A grinding process generates heat that must be dissipated to avoid thermal damage. To date, this has been achieved using flood cooling with a traditional coolant such as an oil and water mixture; however, using a jet of air in its place has obvious environmental and economic benefits. For a range of grinding test configurations, results are presented of the convective heat transfer from the workpiece, along the notional plane of cut, and of the air flow velocity in a two-dimensional plane perpendicular to the workpiece. It has been shown that a boundary layer that develops around the rotating grinding wheel has the effect of displacing a peak in the distribution of the local heat transfer coefficient from the notional arc of cut. To effectively cool the grinding zone, therefore, it is necessary to penetrate this boundary layer and this can only be achieved when the jet velocity is substantially greater than the tangential velocity of the wheel
c-axis Josephson Tunnelling in Twinned and Untwinned YBCO-Pb Junctions
Within a microscopic two band model of planes and chains with a pairing
potential in the planes and off diagonal pairing between planes and chains we
find that the chains make the largest contribution to the Josephson tunnelling
current and that through them the d-wave part of the gap contributes to the
current. This is contrary to the usual assumption that for a d-wave tetragonal
superconductor the c-axis Josephson current for incoherent tunnelling into an
s-wave superconductor is zero while that of a d-wave orthorhombic
superconductor with a small s-wave component to its gap it is small but
non-zero. Nevertheless it has been argued that the effect of twins in YBCO
would lead to cancellation between pairs of twins and so the observation of a
current in c-axis YBCO-Pb experiments is evidence against a d-wave type order
parameter. We argue that both theory and experiment give evidence that the two
twin orientations are not necessarily equally abundant and that the ratio of
tunnelling currents in twinned and untwinned materials should be related to the
relative abundance of the two twin orientations.Comment: 6 pages, RevTeX 3.0, 15 PostScript figur
Orthorhombically Mixed s and d Wave Superconductivity and Josephson Tunneling
The effect of orthorhombicity on Josephson tunneling in high T
superconductors such as YBCO is studied for both single crystals and highly
twinned crystals. It is shown that experiments on highly twinned crystals
experimentally determine the symmetry of the superconducting twin boundaries
(which can be either even or odd with respect to a reflection in the twinning
plane). Conversely, Josephson experiments on highly twinned crystals can not
experimentally determine whether the superconductivity is predominantly
-wave or predominantly -wave. The direct experimental determination of
the order-parameter symmetry by Josephson tunneling in YBCO thus comes from the
relatively few experiments which have been carried out on untwinned single
crystals.Comment: 5 pages, RevTeX file, 1 figure available on request
([email protected]
Benefits of Ground-Based Photometric Follow-Up for Transiting Extrasolar Planets Discovered with Kepler and CoRoT
Currently, over forty transiting planets have been discovered by ground-based
photometric surveys, and space-based missions like Kepler and CoRoT are
expected to detect hundreds more. Follow-up photometric observations from the
ground will play an important role in constraining both orbital and physical
parameters for newly discovered planets, especially those with small radii (R_p
less than approximately 4 Earth radii) and/or intermediate to long orbital
periods (P greater than approximately 30 days). Here, we simulate transit light
curves from Kepler-like photometry and ground-based observations in the
near-infrared (NIR) to determine how jointly modeling space-based and
ground-based light curves can improve measurements of the transit duration and
planet-star radius ratio. We find that adding observations of at least one
ground-based transit to space-based observations can significantly improve the
accuracy for measuring the transit duration and planet-star radius ratio of
small planets (R_p less than approximately 4 Earth radii) in long-period (~1
year) orbits, largely thanks to the reduced effect of limb darkening in the
NIR. We also demonstrate that multiple ground-based observations are needed to
gain a substantial improvement in the measurement accuracy for small planets
with short orbital periods (~3 days). Finally, we consider the role that higher
ground-based precisions will play in constraining parameter measurements for
typical Kepler targets. Our results can help inform the priorities of transit
follow-up programs (including both primary and secondary transit of planets
discovered with Kepler and CoRoT), leading to improved constraints for transit
durations, planet sizes, and orbital eccentricities.Comment: 29 pages, including 4 tables and 5 figures; accepted for publication
in Ap
Management and Tillage Infl uence Barley Forage Productivity and Water Use in Dryland Cropping Systems
Annual cereal forages are resilient in water use (WU), water use efficiency (WUE), and weed control compared with grain crops in dryland systems. The combined influence of tillage and management systems on annual cereal forage productivity and WU is not well documented. We conducted a field study for the effects of tillage (no-till and tilled) and management (ecological and conventional) systems on WU and performance of forage barley (Hordeum vulgare L.) and weed biomass in two crop rotations (wheat [Triticum aestivum L.]–forage barley–pea [Pisum sativum L.] and wheat–forage barley–corn [Zea mays L.] –pea) from 2004 to 2010 in eastern Montana. Conventional management included recommended seeding rates, broadcast N fertilization, and short stubble height of wheat. Ecological management included 33% greater seeding rates, banded N fertilization at planting, and taller wheat stubble. Forage barley in ecological management had 28 more plants m–2, 2 cm greater height, 65 more tillers m–2, 606 kg ha–1 greater crop biomass, 3.5 kg ha–1 mm–1greater WUE, and 47% reduction in weed biomass at harvest than in conventional management. Pre-plant and post-harvest soil water contents were similar among tillage and management systems, but barley WU was 13 mm greater in 4-yr than 3-yr rotation. Tillage had little effect on barley performance and WU. Dryland forage barley with higher seeding rate and banded N fertilization in more diversified rotation produced more yield and used water more efficiently than that with conventional seeding rate, broadcast N fertilization, and less diversified rotation in the semiarid northern Great Plains
Asymmetric magnetic interference patterns in 0-pi Josephson junctions
We examine the magnetic interference patterns of Josephson junctions with a
region of 0- and of pi-phase shift. Such junctions have recently been realized
as c-axis YBCO-Pb junctions with a single twin boundary in YBCO. We show that
in general the junction generates self-fields which introduces an asymmetry in
the critical current under reversal of the magnetic field. Numerical
calculations of these asymmetries indicate they account well for the
unexplained features observed in single twin boundary junctions.Comment: 4 pages, 3 figure
A New Spectroscopic and Photometric Analysis of the Transiting Planet Systems TrES-3 and TrES-4
We report new spectroscopic and photometric observations of the parent stars
of the recently discovered transiting planets TrES-3 and TrES-4. A detailed
abundance analysis based on high-resolution spectra yields [Fe/H] , K, and for TrES-3,
and [Fe/H] , K, and for TrES-4. The accuracy of the effective temperatures is supported
by a number of independent consistency checks. The spectroscopic orbital
solution for TrES-3 is improved with our new radial-velocity measurements of
that system, as are the light-curve parameters for both systems based on newly
acquired photometry for TrES-3 and a reanalysis of existing photometry for
TrES-4. We have redetermined the stellar parameters taking advantage of the
strong constraint provided by the light curves in the form of the normalized
separation (related to the stellar density) in conjunction with our
new temperatures and metallicities. The masses and radii we derive are
M_\star=0.928_{-0.048}^{+0.028} M_{\sun},R_\star = 0.829_{-0.022}^{+0.015}
R_{\sun}, and M_\star = 1.404_{-0.134}^{+0.066} M_{\sun},
R_\star=1.846_{-0.087}^{+0.096} R_{\sun} for TrES-3 and TrES-4, respectively.
With these revised stellar parameters we obtain improved values for the
planetary masses and radii. We find , for TrES-3, and
, for TrES-4. We confirm TrES-4 as the planet with the largest
radius among the currently known transiting hot Jupiters.Comment: 42 pages, 10 tables, 8 figures. Accepted for publication in the
Astrophysical Journa
Characterizing Transiting Extrasolar Planets with Narrow-Band Photometry and GTC/OSIRIS
We report the first extrasolar planet observations from the 10.4-m Gran
Telescopio Canarias (GTC), currently the world's largest, fully steerable,
single-aperture optical telescope. We used the OSIRIS tunable filter imager on
the GTC to acquire high-precision, narrow-band photometry of the transits of
the giant exoplanets, TrES-2b and TrES-3b. We obtained near-simultaneous
observations in two near-infrared (NIR) wavebands (790.2 and 794.4 +/- 2.0 nm)
specifically chosen to avoid water vapor absorption and skyglow so as to
minimize the atmospheric effects that often limit the precision of ground-based
photometry. Our results demonstrate a very-high photometric precision with
minimal atmospheric contamination despite relatively poor atmospheric
conditions and some technical problems with the telescope. We find the
photometric precision for the TrES-2 observations to be 0.343 and 0.412 mmag
for the 790.2 and 794.4 nm light curves, and the precision of the TrES-3
observations was found to be 0.470 and 0.424 mmag for the 790.2 and 794.4 nm
light curves. We also discuss how future follow-up observations of transiting
planets with this novel technique can contribute to the characterization of
Neptune- and super-Earth-size planets to be discovered by space-based missions
like CoRoT and Kepler, as well as measure atmospheric properties of giant
planets, such as the strength of atmospheric absorption features.Comment: 9 pages, including 3 figures and 2 tables; accepted for publication
in MNRA
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