17,653 research outputs found
SCAR arrow-wing active flutter suppression system
The potential performance and direct operating cost benefits of an active flutter suppression system (FSS) for the NASA arrow-wing supersonic cruise configuration were determined. A FSS designed to increase the flutter speed of the baseline airplane 20 percent. A comparison was made of the performance and direct operating cost between the FSS equipped aircraft and a previously defined configuration with structural modifications to provide the same flutter speed. Control system synthesis and evaluation indicated that a FSS could provide the increase in flutter speed without degrading airplane reliability, safety, handling qualities, or ride quality, and without increasing repeated loads or hydraulic and electrical power capacity requirements
Hybrid propulsion technology program. Volume 1: Conceptional design package
A concept design study was performed to configure two sizes of hybrid boosters; one which duplicates the advanced shuttle rocket motor vacuum thrust time curve and a smaller, quarter thrust level booster. Two sizes of hybrid boosters were configured for either pump-fed or pressure-fed oxygen feed systems. Performance analyses show improved payload capability relative to a solid propellant booster. Size optimization and fuel safety considerations resulted in a 4.57 m (180 inch) diameter large booster with an inert hydrocarbon fuel. The preferred diameter for the quarter thrust level booster is 2.53 m (96 inches). As part of the design study critical technology issues were identified and a technology acquisition and demonstration plan was formulated
Effects of thermal motion on electromagnetically induced absorption
We describe the effect of thermal motion and buffer-gas collisions on a
four-level closed N system interacting with strong pump(s) and a weak probe.
This is the simplest system that experiences electromagnetically induced
absorption (EIA) due to transfer of coherence via spontaneous emission from the
excited to ground state. We investigate the influence of Doppler broadening,
velocity-changing collisions (VCC), and phase-changing collisions (PCC) with a
buffer gas on the EIA spectrum of optically active atoms. In addition to exact
expressions, we present an approximate solution for the probe absorption
spectrum, which provides physical insight into the behavior of the EIA peak due
to VCC, PCC, and wave-vector difference between the pump and probe beams. VCC
are shown to produce a wide pedestal at the base of the EIA peak, which is
scarcely affected by the pump-probe angular deviation, whereas the sharp
central EIA peak becomes weaker and broader due to the residual Doppler-Dicke
effect. Using diffusion-like equations for the atomic coherences and
populations, we construct a spatial-frequency filter for a spatially structured
probe beam and show that Ramsey narrowing of the EIA peak is obtained for beams
of finite width
Liver transplantation before 1 year of age
Since 1981, 20 infants younger than 1 year of age received 26 orthotopic liver transplants. Immunosuppression was with cyclosporine and corticosteroids. Thirteen (65%) of the reciplents were discharged from the hospital. To date, 12 (60%) of the 20 reciplents are surviving, with follow-up of 1 to 56 months (average 14 months). The 5-year acluarial survival is 53.8%. The allograft liver function in the majority of surviving infants is excellent. The predominant causes of mortality were primary nonfunction of the allograft (three patients) and sepsis (three). Major morbidity was caused by hepatic artery thrombosis (five patients), gastrointestinal complications (six), biliary tract complications (five), and bacterial and viral infections (13). Six patients underwent retransplantation; three of these six survived. Results could be improved by prevention of hepatic artery thrombosis, by decreasing the incidence of sepsis, and by procurement of more and better suited pediatric donors. © 1987 The C. V. Mosby Company
Fermionic solution of the Andrews-Baxter-Forrester model II: proof of Melzer's polynomial identities
We compute the one-dimensional configuration sums of the ABF model using the
fermionic technique introduced in part I of this paper. Combined with the
results of Andrews, Baxter and Forrester, we find proof of polynomial
identities for finitizations of the Virasoro characters
as conjectured by Melzer. In the thermodynamic limit
these identities reproduce Rogers--Ramanujan type identities for the unitary
minimal Virasoro characters, conjectured by the Stony Brook group. We also
present a list of additional Virasoro character identities which follow from
our proof of Melzer's identities and application of Bailey's lemma.Comment: 28 pages, Latex, 7 Postscript figure
Constant Size Molecular Descriptors For Use With Machine Learning
A set of molecular descriptors whose length is independent of molecular size
is developed for machine learning models that target thermodynamic and
electronic properties of molecules. These features are evaluated by monitoring
performance of kernel ridge regression models on well-studied data sets of
small organic molecules. The features include connectivity counts, which
require only the bonding pattern of the molecule, and encoded distances, which
summarize distances between both bonded and non-bonded atoms and so require the
full molecular geometry. In addition to having constant size, these features
summarize information regarding the local environment of atoms and bonds, such
that models can take advantage of similarities resulting from the presence of
similar chemical fragments across molecules. Combining these two types of
features leads to models whose performance is comparable to or better than the
current state of the art. The features introduced here have the advantage of
leading to models that may be trained on smaller molecules and then used
successfully on larger molecules.Comment: 18 pages, 5 figure
A kilobit hidden SNFS discrete logarithm computation
We perform a special number field sieve discrete logarithm computation in a
1024-bit prime field. To our knowledge, this is the first kilobit-sized
discrete logarithm computation ever reported for prime fields. This computation
took a little over two months of calendar time on an academic cluster using the
open-source CADO-NFS software. Our chosen prime looks random, and
has a 160-bit prime factor, in line with recommended parameters for the Digital
Signature Algorithm. However, our p has been trapdoored in such a way that the
special number field sieve can be used to compute discrete logarithms in
, yet detecting that p has this trapdoor seems out of reach.
Twenty-five years ago, there was considerable controversy around the
possibility of back-doored parameters for DSA. Our computations show that
trapdoored primes are entirely feasible with current computing technology. We
also describe special number field sieve discrete log computations carried out
for multiple weak primes found in use in the wild. As can be expected from a
trapdoor mechanism which we say is hard to detect, our research did not reveal
any trapdoored prime in wide use. The only way for a user to defend against a
hypothetical trapdoor of this kind is to require verifiably random primes
Chandra Evidence for a Flattened, Triaxial Dark Matter Halo in the Elliptical Galaxy NGC 720
(Abridged) We present an analysis of a Chandra ACIS-S observation of the
elliptical galaxy NGC 720 to verify the existence of a dark matter (DM) halo
and to measure its ellipticity. The ACIS-S3 image reveals over 60 point
sources. For semi-major axes a<~150" (18.2h_{70}^{-1} kpc) the ellipticity of
the diffuse emission is ex ~0.15, which is less than the values 0.2-0.3
obtained from ROSAT because the point sources contaminated the ROSAT values.
The Chandra data confirm the ~20 deg position angle (PA) twist discovered by
ROSAT, but the Chandra twist is more gradual also because of the point sources
contaminating the ROSAT values. Overall the ex and PA values for a<~150" can be
explained by the triaxial model of NGC 720 published by Romanowsky & Kochanek.
Since the optical image displays no substantial isophote twisting, the X-ray PA
twist requires a massive DM halo if the hot gas is in hydrostatic equilibrium.
The mass-follows-light hypothesis is also inconsistent with the Chandra
ellipticities at the 96% (98%) level for oblate (prolate) symmetry. Thus, both
the PA twist and the ellipticities of the Chandra image imply a DM halo
independent of the gas T profile -- evidence that cannot be obviated by
alternative gravity theories such as MOND. The DM density model, rho ~a^{-2},
provides the best fit and gives ellipticities of 0.37 +/- 0.03 (0.36 +/- 0.02)
for oblate (prolate) models. These moderate ellipticities for the DM halo are
inconsistent with both the nearly spherical halos predicted if the DM is
self-interacting and with the highly flattened halos predicted if the DM is
cold molecular gas. These ellipticities may also be too large to be explained
by warm DM, but are consistent with galaxy-sized halos formed in the Lambda-CDM
paradigm.Comment: 14 pages, 4 figures, ApJ, in pres
Prolactin, cortisol and thyroxine levels and the premature infant
The relationship of prolactin, cortisol and thyroxine values in cord and maternal plasma to fetal age and weight and to the incidence of hyaline membrane disease (HMD) was investigated in 80 neonates of whom 40 were born at more than 37 weeks' gestation. Of the 40 born at less than 36 weeks 11 developed HMD. Serum cortisol has been shown to be a differentiating factor for HMD, but cord thyroxine and prolactin levels seem to be related more to age and weight than to the occurrence of HMD
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