1,617 research outputs found
Model-following control for an oblique-wing aircraft
A variable-skew oblique wing offers a substantial aerodynamic performance advantage for aircraft missions that require both high efficiency in subsonic flight and supersonic dash or cruise. The most obvious characteristic of the oblique-wing concept is the asymmetry associated with wing-skew angle which results in significant aerodynamic and inertial cross-coupling between the aircraft longitudinal and lateral-directional axes. A technique for synthesizing a decoupling controller while providing the desired stability augmentation. The proposed synthesis procedure uses the cncept of explicit model following. Linear quadratic optimization techniques are used to design the linear feedback system. The effectiveness of the control laws developed in achieving the desired decoupling is illustrated for a given flight condition by application to linearized equations of motion, and also to the nonlinear equations of six degrees of freedom of motion with nonlinear aerodynamic data
Model of the meniscus of an ionic liquid ion source.
A simple model of the transfer of charge and ion evaporation in the meniscus of an ionic-liquid ion source working in the purely ionic regime is proposed on the basis of order-of-magnitude estimates which show that, in this regime, _i_ the flow in the meniscus is dominated by the viscosity of the liquid and is affected very little by the mass flux accompanying ion evaporation, and _ii_ the effect of the space charge around the evaporating surface is negligible and the evaporation current is controlled by the finite electrical conductivity of the liquid. The model predicts that a stationary meniscus of a very polar liquid undergoing ion evaporation is nearly hydrostatic and can exist only below a certain value of the applied electric field, at which the meniscus attains its maximum elongation but stays smooth. The electric current vs applied electric field characteristic displays a frozen regime of negligible ion evaporation at low fields and a conduction-controlled regime at higher fields, with a sharp transition between the two regimes owing to the high sensitivity of the ion evaporation rate to the electric field. A simplified treatment of the flow in the capillary or liquid layer through which liquid is delivered to the meniscus shows that the size of the meniscus decreases and the maximum attainable current increases when the feeding pressure is decreased, and that appropriate combinations of feeding pressure and pressure drop may lead to high maximum currents
Field-Dependent Tilt and Birefringence of Electroclinic Liquid Crystals: Theory and Experiment
An unresolved issue in the theory of liquid crystals is the molecular basis
of the electroclinic effect in the smectic-A phase. Recent x-ray scattering
experiments suggest that, in a class of siloxane-containing liquid crystals, an
electric field changes a state of disordered molecular tilt in random
directions into a state of ordered tilt in one direction. To investigate this
issue, we measure the optical tilt and birefringence of these liquid crystals
as functions of field and temperature, and we develop a theory for the
distribution of molecular orientations under a field. Comparison of theory and
experiment confirms that these materials have a disordered distribution of
molecular tilt directions that is aligned by an electric field, giving a large
electroclinic effect. It also shows that the net dipole moment of a correlated
volume of molecules, a key parameter in the theory, scales as a power law near
the smectic-A--smectic-C transition.Comment: 18 pages, including 9 postscript figures, uses REVTeX 3.0 and
epsf.st
Enfermedad meningocócica invasiva en paciente presumiblemente inmunocompetente
La enfermedad meningocócica invasiva es causada por la invasión de Neisseira meningitidis en el torrente
sanguíneo la cual puede resultar en meningococcemia y shock séptico, púrpura fulminante, meningitis
meningocócica, neumonía, artritis y pericarditis. Por los desafíos diagnósticos que presentó y la alta
mortalidad describimos la presentación de un caso de meningococcemia y meningitis. Varón de 46 años,
sin antecedentes conocidos, que presentó enfermedad meningocócica invasiva por Neisseira Meningitidis
serogrupo W que en el curso de la internación se le diagnostica mieloma múltiple tipo IgG. Aunque, la
infección por meningococo en pacientes con mieloma múltiple es baja, este caso nos planteó la necesidad
de sospecharla, en pacientes presumiblemente inmucompetentes, con meningitis bacteriana, en presencia
de hallazgos clínicos no explicados por la infección y compatibles con daño a órgano blanco por dicha
enfermedad
Particle and light fragment emission in peripheral heavy ion collisions at Fermi energies
A systematic investigation of the average multiplicities of light charged
particles and intermediate mass fragments emitted in peripheral and
semiperipheral collisions is presented as a function of the beam energy,
violence of the collision and mass of the system.
The data have been collected with the "Fiasco" setup in the reactions
93Nb+93Nb at 17, 23, 30, 38AMeV and 116Sn+116Sn at 30, 38AMeV.
The midvelocity emission has been separated from the emission of the
projectile-like fragment. This last component appears to be compatible with an
evaporation from an equilibrated source at normal density, as described by the
statistical code Gemini at the appropriate excitation energy.
On the contrary, the midvelocity emission presents remarkable differences for
what concerns both the dependence of the multiplicities on the energy deposited
in the midvelocity region and the isotopic composition of the emitted light
charged particles.Comment: 18 pages, 17 figures, Revtex
Problematising international placements as a site of intercultural learning
This paper theorises some of the learning outcomes of a three-year project concerning student learning in international social work placements in Malaysia. The problematic issue of promoting cultural and intercultural competence through such placements is examined, where overlapping hegemonies are discussed in terms of isomorphism of social work models, that of the nation state, together with those relating to professional values and knowledge, and the tyrannies of received ideas. A critical discussion of cultural competence as the rationale for international placements is discussed in terms of the development of the graduating social worker as a self-reflexive practitioner. The development of sustainable international partnerships able to support student placement and the issue of non-symmetrical reciprocation, typical of wide socio-economic differentials across global regions, is additionally discussed
Observation and Spectral Measurements of the Crab Nebula with Milagro
The Crab Nebula was detected with the Milagro experiment at a statistical
significance of 17 standard deviations over the lifetime of the experiment. The
experiment was sensitive to approximately 100 GeV - 100 TeV gamma ray air
showers by observing the particle footprint reaching the ground. The fraction
of detectors recording signals from photons at the ground is a suitable proxy
for the energy of the primary particle and has been used to measure the photon
energy spectrum of the Crab Nebula between ~1 and ~100 TeV. The TeV emission is
believed to be caused by inverse-Compton up-scattering scattering of ambient
photons by an energetic electron population. The location of a TeV steepening
or cutoff in the energy spectrum reveals important details about the underlying
electron population. We describe the experiment and the technique for
distinguishing gamma-ray events from the much more-abundant hadronic events. We
describe the calculation of the significance of the excess from the Crab and
how the energy spectrum is fit. The fit is consistent with values measured by
IACTs between 1 and 20 TeV. Fixing the spectral index to values that have been
measured below 1 TeV by IACT experiments (2.4 to 2.6), the fit to the Milagro
data suggests that Crab exhibits a spectral steepening or cutoff between about
20 to 40 TeV.Comment: Submitted to Astrophysical Journa
Lower Rate of Restenosis and Reinterventions With Covered vs Bare Metal Stents Following Innominate Artery Stenting
PURPOSE: To determine any difference between bare metal stents (BMS) and balloon-expandable covered stents in the treatment of innominate artery atheromatous lesions.
MATERIALS AND METHODS: A multicenter retrospective study involving 13 university hospitals in France collected 93 patients (mean age 63.2±11.1 years; 57 men) treated over a 10-year period. All patients had systolic blood pressure asymmetry >15 mm Hg and were either asymptomatic (39, 42%) or had carotid (20, 22%), vertebrobasilar (24, 26%), and/or brachial (20, 22%) symptoms. Innominate artery stenosis ranged from 50% to 70% in 4 (4%) symptomatic cases and between 70% and 90% in 52 (56%) cases; 28 (30%) lesions were preocclusive and 8 (9%) were occluded. One (1%) severely symptomatic patient had a <50% stenosis. Demographic characteristics, operative indications, and procedure details were compared between the covered (36, 39%) and BMS (57, 61%) groups. Multivariate analysis was performed to determine relative risks of restenosis and reinterventions [reported with 95% confidence intervals (CI)].
RESULTS: The endovascular procedures were performed mainly via retrograde carotid access (75, 81%). Perioperative strokes occurred in 4 (4.3%) patients. During the mean 34.5±31.2-month follow-up, 30 (32%) restenoses were detected and 13 (20%) reinterventions were performed. Relative risks were 6.9 (95% CI 2.2 to 22.2, p=0.001) for restenosis and 14.6 (95% CI 1.8 to 120.8, p=0.004) for reinterventions between BMS and covered stents. The severity of the treated lesions had no influence on the results.
CONCLUSION: Patients treated with BMS for innominate artery stenosis have more frequent restenoses and reinterventions than patients treated with covered stents
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