13,556 research outputs found
The Sun as a Star: 13 years of SoHO
The best known Solar oscillation-like star is the Sun. During the last 14
years, the ESA/NASA Solar and Heliospheric Observatory (SoHO) has been
continuously observing this star from the Lagrange point L1 with an enormous
success. Among the 11 instruments placed onboard, 3 of them are dedicated to
helioseismology: GOLF, VIRGO and MDI. The first two observe the Sun as a star
by integrating the velocity or intensity signal of the visible solar disk into
a single point. They are thus similar to any other observation done in
asteroseismology. During this review I will present the most important results
obtained during the mission concerning the Sun seen as a star and how this
results have evolved our thoughts of the inside of our star.Comment: Proceedings of the Stellar Pulsation Conference. Santa Fe. 6 pages, 4
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Valve recession: From experiment to predictive model
Increasing demands on engine performance and cost reductions have meant that advances made in materials and production technology are often outpaced This frequently results in wear problems occurring with engine components. Few models exist for predicting wear, and consequently each wear problem has to be investigated, the cause isolated and remedial action taken. The objective of this work was to carry out experimental studies to investigate valve and seat insert wear mechanisms and use the test results to develop a recession prediction tool to assess the potential for valve recession and solve problems that occur more quickly. Experimental apparatus has been developed that is capable of providing a valid simulation of the wear of diesel automotive inlet valves and seats. Test methodologies developed have isolated the effects of impact and sliding. A semi-empirical wear model for predicting valve recession has been developed based on data gathered during the bench testing. A software program, RECESS, was developed to run the model. Model predictions are compared with engine dynamometer tests and bench tests. The model can be used to give a quantitative prediction of the valve recession to be expected with a particular material pair or a qualitative assessment of how parameters need to be altered in order to reduce recession. The valve recession model can be integrated into an industrial environment in order to help reduce costs and timescales involved in solving valve/seat wear problems
The use of LANDSAT digital data and computer-implemented techniques for an agricultural application
Agricultural applications procedures are described for use of LANDSAT digital data and other digitalized data (e.g., soils). The results of having followed these procedures are shown in production estimates for cotton and soybeans in Washington County, Mississippi. Examples of output products in both line printer and map formats are included, and a product adequacy assessment is made
Supersymmetry of Noncompact MQCD-like Membrane Instantons and Heat Kernel Asymptotics
We perform a heat kernel asymptotics analysis of the nonperturbative
superpotential obtained from wrapping of an M2-brane around a supersymmetric
noncompact three-fold embedded in a (noncompact) G_2-manifold as obtained in
[1], the three-fold being the one relevant to domain walls in Witten's MQCD
[2], in the limit of small "zeta", a complex constant that appears in the
Riemann surfaces relevant to defining the boundary conditions for the domain
wall in MQCD. The MQCD-like configuration is interpretable, for small but
non-zero zeta as a noncompact/"large" open membrane instanton, and for
vanishing zeta, as the type IIA D0-brane (for vanishing M-theory cicle radius).
We find that the eta-function Seeley de-Witt coefficients vanish, and we get a
perfect match between the zeta-function Seeley de-Witt coefficients (up to
terms quadratic in zeta) between the Dirac-type operator and one of the two
Laplace-type operators figuring in the superpotential. This is an extremely
strong signature of residual supersymmetry for the nonperturbative
configurations in M-theory considered in this work.Comment: 21 pages, LaTeX; v3: several clarifying remarks added, to appear in
JHE
Integrating Dynamics and Wear Modelling to Predict Railway Wheel Profile Evolution
The aim of the work described was to predict wheel
profile evolution by integrating multi-body dynamics
simulations of a wheelset with a wear model.
The wear modelling approach is based on a wear
index commonly used in rail wear predictions. This
assumes wear is proportional to Tγ, where T is tractive
force and γ is slip at the wheel/rail interface. Twin disc
testing of rail and wheel materials was carried out to
generate wear coefficients for use in the model.
The modelling code is interfaced with
ADAMS/Rail, which produces multi-body dynamics
simulations of a railway wheelset and contact conditions
at the wheel/rail interface. Simplified theory of rolling
contact is used to discretise the contact patches
produced by ADAMS/Rail and calculate traction and
slip within each.
The wear model combines the simplified theory of
rolling contact, ADAMS/Rail output and the wear
coefficients to predict the wear and hence the change of
wheel profile for given track layouts
Evidence of increasing acoustic emissivity at high frequency with solar cycle 23 in Sun-as-a-star observations
We used long high-quality unresolved (Sun-as-a-star observations) data
collected by GOLF and VIRGO instruments on board the ESA/NASA SOHO satellite to
investigate the amplitude variation with solar cycle 23 in the high-frequency
band (5.7 < nu< 6.3 mHz). We found an enhancement of acoustic emissivity over
the ascending phase of about 18+-3 in velocity observations and a slight
enhancement of 3+-2 in intensity. Mode conversion from fast acoustic to fast
magneto-acoustic waves could explain the enhancement in velocity observations.
These findings open up the possibility to apply the same technique to stellar
intensity data, in order to investigate stellar-magnetic activity.Comment: Proceedings of the Stellar Pulsation. Santa Fe, USA. 3 pages, 5
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The influence of atmosphere on the performance of pure-phase WZ and ZB InAs nanowire transistors
We compare the characteristics of phase-pure MOCVD grown ZB and WZ InAs
nanowire transistors in several atmospheres: air, dry pure N and O, and
N bubbled through liquid HO and alcohols to identify whether
phase-related structural/surface differences affect their response. Both WZ and
ZB give poor gate characteristics in dry state. Adsorption of polar species
reduces off-current by 2-3 orders of magnitude, increases on-off ratio and
significantly reduces sub-threshold slope. The key difference is the greater
sensitivity of WZ to low adsorbate level. We attribute this to facet structure
and its influence on the separation between conduction electrons and surface
adsorption sites. We highlight the important role adsorbed species play in
nanowire device characterisation. WZ is commonly thought superior to ZB in InAs
nanowire transistors. We show this is an artefact of the moderate humidity
found in ambient laboratory conditions: WZ and ZB perform equally poorly in the
dry gas limit yet equally well in the wet gas limit. We also highlight the
vital role density-lowering disorder has in improving gate characteristics, be
it stacking faults in mixed-phase WZ or surface adsorbates in pure-phase
nanowires.Comment: Accepted for publication in Nanotechnolog
Hydrographic data from R/V endeavor cruise #90
The final cruise of the NSF sponsored Warm Core Rings Program studied a Warm Core Ring (WCR) in the Fall of 1982 as it formed from a large northward meander of the Gulf Stream. This ring, known as 82-H or the eighth ring identified in 1982, formed over the New England Seamounts near 39.5 deg N, 65 deg W. Surveys using Expendable Bathythermographs, Conductivity-Temperature-Depth-Oxygen stations and Doppler Current Profiling provide a look at the genesis of a WCR. These measurements reveal that WCR 82-H separated from the Gulf Stream sometime between October 2-5. This ring was a typical WCR with a diameter of about 200 km and speeds in the high velocity core of the 175 cm/sec. Satellite imagery of 82-H following the cruise showed that it drifted WSW in the Slope Water region at almost 9 km/day, had at least one interaction with the Gulf Stream and was last observed on February 8, 1983 at 39 deg N, 72 deg W
Infrared ground-based astronomy with the Hughes 256 X 256 PtSi array
It is shown that large format PtSi Schottky diode infrared arrays, the Hughes 256 X 256 hybrid Schottky array in particular, are competitive alternatives to the smaller format photovoltaic arrays for ground-based astronomy. The modest quantum efficiency of the PtSi compared to the photovoltaic devices is more than compensated for by the larger format. The use of hybrid technology yields effective fill factors of nearly 100 percent, and the low dark current, noise, excellent imaging characteristics, cost, and solid nitrogen operating temperature add to the effectiveness of this array for ground-based imaging. In addition to discussing the characteristics of this array, researchers present laboratory test data and astronomical results achieved at Kitt Peak
A Note on Mirror Symmetry for Manifolds with Spin(7) Holonomy
Starting from the superconformal algebras associated with manifolds, I
extend the algebra to the manifolds with spin(7) holonomy. I show how the
mirror symmetry in manifolds with spin(7) holonomy arises as the automorphism
in the extended sperconformal algebra. The automorphism is realized as 14 kinds
of T-dualities on the supersymmetric toroidal fibrations. One class of
Joyce's orbifolds are pairwise identified under the symmetry.Comment: 12 pages, harvmac bi
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