33,252 research outputs found
New method for critical failure prediction of complex systems
Rigorous analytical technique, called criticality determination methodology /or CD technique/ determines the probability that a given complex system will successfully achieve stated objectives. The CD technique identifies critical elements of the system by a failure mode and effects analysis
Aerodynamic performance of conventional and advanced design labyrinth seals with solid-smooth abradable, and honeycomb lands
Labyrinth air seal static and dynamic performance was evaluated using solid, abradable, and honeycomb lands with standard and advanced seal designs. The effects on leakage of land surface roughness, abradable land porosity, rub grooves in abradable lands, and honeycomb land cell size and depth were studied using a standard labyrinth seal. The effects of rotation on the optimum seal knife pitch were also investigated. Selected geometric and aerodynamic parameters for an advanced seal design were evaluated to derive an optimized performance configuration. The rotational energy requirements were also measured to determine the inherent friction and pumping energy absorbed by the various seal knife and land configurations tested in order to properly assess the net seal system performance level. Results indicate that: (1) seal leakage can be significantly affected with honeycomb or abradable lands; (2) rotational energy absorption does not vary significantly with the use of a solid-smooth, an abradable, or a honeycomb land; and (3) optimization of an advanced lab seal design produced a configuration that had leakage 25% below a conventional stepped seal
Small Energy Scale for Mixed-Valent Uranium Materials
We investigate a two-channel Anderson impurity model with a magnetic
and a quadrupolar ground doublet, and a excited triplet. Using
the numerical renormalization group method, we find a crossover to a non-Fermi
liquid state below a temperature varying as the triplet-doublet
splitting to the 7/2 power. To within numerical accuracy, the non-linear
magnetic susceptibility and the contribution to the linear
susceptibility are given by universal one-parameter scaling functions. These
results may explain UBe as mixed valent with a small crossover scale
.Comment: 4 pages, 3 figures, REVTeX, to appear in Phys. Rev. Let
The incidence of mid-infrared excesses in G and K giants
Using photometric data from the 2MASS and GLIMPSE catalogues, I investigate
the incidence of mid-infrared excesses (~10 microns) of G and K stars of
luminosity class III. In order to obtain a large sample size, stars are
selected using a near-IR colour-magnitude diagram. Sources which are candidates
for showing mid-IR excess are carefully examined and modelled to determined
whether they are likely to be G/K giants. It is found that mid-IR excesses are
present at a level of (1.8 +/- 0.4) x 10^-3. While the origin of these excesses
remains uncertain, it is plausible that they arise from debris discs around
these stars. I note that the measured incidence is consistent with a scenario
in which dust lifetimes in debris discs are determined by Poynting-Robertson
drag rather than by collisions.Comment: Accepted for publication in MNRAS. 13 pages, 5 figures, 2 tables (1
landscape table
Robust pricing and hedging of double no-touch options
Double no-touch options, contracts which pay out a fixed amount provided an
underlying asset remains within a given interval, are commonly traded,
particularly in FX markets. In this work, we establish model-free bounds on the
price of these options based on the prices of more liquidly traded options
(call and digital call options). Key steps are the construction of super- and
sub-hedging strategies to establish the bounds, and the use of Skorokhod
embedding techniques to show the bounds are the best possible.
In addition to establishing rigorous bounds, we consider carefully what is
meant by arbitrage in settings where there is no {\it a priori} known
probability measure. We discuss two natural extensions of the notion of
arbitrage, weak arbitrage and weak free lunch with vanishing risk, which are
needed to establish equivalence between the lack of arbitrage and the existence
of a market model.Comment: 32 pages, 5 figure
The Hubble Constant
Considerable progress has been made in determining the Hubble constant over
the past two decades. We discuss the cosmological context and importance of an
accurate measurement of the Hubble constant, and focus on six high-precision
distance-determination methods: Cepheids, tip of the red giant branch, maser
galaxies, surface brightness fluctuations, the Tully-Fisher relation and Type
Ia supernovae. We discuss in detail known systematic errors in the measurement
of galaxy distances and how to minimize them. Our best current estimate of the
Hubble constant is 73 +/-2 (random) +/-4 (systematic) km/s/Mpc. The importance
of improved accuracy in the Hubble constant will increase over the next decade
with new missions and experiments designed to increase the precision in other
cosmological parameters. We outline the steps that will be required to deliver
a value of the Hubble constant to 2% systematic uncertainty and discuss the
constraints on other cosmological parameters that will then be possible with
such accuracy.Comment: To be published in Annual Reviews of Astronomy and Astrophysics, Vol.
48, 2010, consisting of 79 pages, 13 figures, 2 table
The impact of subspecialty services on healthcare delivery - a community health centre based study
Objectives. The objective was to evaluate the role of a paediatric surgical consultant at a primary health care facility. Design. Descriptive and prospective. Setting. In the process of planning and implementation of the 2010 health plan of the Provincial Government of the Western Cape, a shift occurred in the delivery of health care to children from a provincially based hospital system to a municipally based primary health care system. To contribute towards enabling this process, the Department of Paediatric Surgery at Red Cross War Memorial Children’s Hospital established a paediatric surgical day clinic at a local community health centre during 2001. Subjects. Information was obtained from patient data sheets containing details of consultations at the sub-specialist surgical clinic at Michael Mapongwana Community Health Centre. Results. Over a 58-month period 1 171 children were seen, of whom 655 were male and 427 female. Their ages ranged from 0 to 19 years, the largest group being under 1 year. Eighty per cent of patients were accompanied by their mothers. The correct diagnosis was established by the nurse practitioners in 71%. General paediatric surgical conditions predominated, followed by medical, dermatological, orthopaedic, trauma, otolaryngo-pharyngology, infectious diseases, ophthalmology, urology, neurosurgery, malignancy and maxillofacial conditions. The details are set out in the report. In total 597 patients were referred directly to an appropriate care facility and 574 patients could be managed entirely at the clinic level. Conclusions. This study demonstrated the significant public health problem of paediatric surgical disease. It emphasised the preventative and cost-effective role of a surgical clinic at primary health care level. The clinic allowed for timely surgical intervention in 65% of surgical cases, thereby decreasing inappropriate tertiary referrals. We believe that bringing specialists into the community can only strengthen the 2010 health care plan
The utility of computed tomography for recent-onset partial seizures in childhood
No Abstract. South African Medical Journal Vol. 96(9) (Part 2) 2006: 945-94
Kondo Effects and Multipolar Order in the cubic PrTr2Al20 (Tr=Ti, V)
Our single crystal study reveals that PrTr2Al20 (Tr = Ti and V) provides the
first examples of a cubic {\Gamma}3 nonmagnetic ground doublet system that
shows the Kondo effect including a -ln T dependent resistivity. The {\Gamma}3
quadrupolar moments in PrV2Al20 induce anomalous metallic behavior through
hybridization with conduction electrons, such as T^{1/2} dependent resistivity
and susceptibility below ~ 20 K down to its ordering temperature T_O = 0.6 K.
In PrTi2Al20, however, quadrupoles are well-localized and exhibit an order at
T_O = 2.0 K. Stronger Kondo coupling in PrV2Al20 than in PrTi2Al20 suppresses
quadrupolar ordering, and instead promotes hybridization between the {\Gamma}3
doublet and conduction electrons, leading to most likely the quadrupolar Kondo
effect.Comment: 12 pages, 4 figure
The Luminosity Dependence of Quasar Clustering
We investigate the luminosity dependence of quasar clustering, inspired by
numerical simulations of galaxy mergers that incorporate black hole growth.
These simulations have motivated a new interpretation of the quasar luminosity
function. In this picture, the bright end of the quasar luminosity function
consists of quasars radiating nearly at their peak luminosities, while the
faint end consists mainly of very similar sources, but at dimmer phases in
their evolution. We combine this model with the statistics of dark matter halos
that host quasar activity. We find that, since bright and faint quasars are
mostly similar sources seen in different evolutionary stages, a broad range in
quasar luminosities corresponds to only a narrow range in the masses of quasar
host halos. On average, bright and faint quasars reside in similar host halos.
Consequently, we argue that quasar clustering should depend only weakly on
luminosity. This prediction is in qualitative agreement with recent
measurements of the luminosity dependence of the quasar correlation function
(Croom et al. 2005) and the galaxy-quasar cross-correlation function
(Adelberger & Steidel 2005). Future precision clustering measurements from SDSS
and 2dF, spanning a large range in luminosity, should provide a strong test of
our model.Comment: 9 pages, 4 figures, submitted to Ap
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