4,843 research outputs found
A computational examination of directional stability for smooth and chined forebodies at high-alpha
Computational Fluid Dynamics (CFD) has been used to study aircraft forebody flowfields at low-speed, angle-of-attack conditions with sideslip. The purpose is to define forebody geometries which provide good directional stability characteristics under these conditions. The flows over the experimentally investigated F-5A forebody and chine type configuration, previously computed by the authors, were recomputed with better grid topology and resolution. The results were obtained using a modified version of CFL3D (developed at NASA Langley) to solve either the Euler equations or the Reynolds equations employing the Baldwin-Lomax turbulence model with the Degani-Schiff modification to account for massive crossflow separation. Based on the results, it is concluded that current CFD methods can be used to investigate the aerodynamic characteristics of forebodies to achieve desirable high angle-of-attack characteristics. An analytically defined generic forebody model is described, and a parametric study of various forebody shapes was then conducted to determine which shapes promote a positive contribution to directional stability at high angle-of-attack. An unconventional approach for presenting the results is used to illustrate how the positive contribution arises. Based on the results of this initial parametric study, some guidelines for aerodynamic design to promote positive directional stability are presented
Doping and Field-Induced Insulator-Metal Transitions in Half-Doped Manganites
We argue that many properties of the half-doped manganites may be understood
in terms of a new two-(eg electron)-fluid description, which is energetically
favorable at intermediate Jahn-Teller (JT) coupling. This emerges from a
competition between canting of the core spins of Mn promoting mobile carriers
and polaronic trapping of carriers by JT defects, in the presence of CE,
orbital and charge order. We show that this explains several features of the
doping and magnetic field induced insulator-metal transitions, as the
particle-hole asymmetry and the smallness of the transition fields.Comment: 4 pages, 4 figure
Instabilities and Insulator-Metal transitions in Half-Doped Manganites induced by Magnetic-Field and Doping
We discuss the phase diagram of the two-orbital model of half-doped
manganites by calculating self-consistently the Jahn-Teller (JT) distortion
patterns, charge, orbital and magnetic order at zero temperature. We analyse
the instabilities of these phases caused by electron or hole doping away from
half-doping, or by the application of a magnetic-field. For the CE insulating
phase of half-doped manganites, in the intermediate JT coupling regime, we show
that there is a competition between canting of spins (which promotes mobile
carriers) and polaronic self-trapping of carriers by JT defects. This results
in a marked particle-hole asymmetry, with canting winning only on the electron
doped side of half-doping. We also show that the CE phase undergoes a
first-order transition to a ferromagnetic metallic phase when a magnetic-field
is applied, with abrupt changes in the lattice distortion patterns. We discuss
the factors that govern the intriguingly small scale of the transition fields.
We argue that the ferromagnetic metallic phases involved have two types of
charge carriers, localised and band-like, leading to an effective two-fluid
model.Comment: 22 pages, 28 figure
Membrane mediated aggregation of curvature inducing nematogens and membrane tubulation
The shapes of cell membranes are largely regulated by membrane associated,
curvature active, proteins. We use a numerical model of the membrane with
elongated membrane inclusions, recently developed by us, which posses
spontaneous directional curvatures that could be different along and
perpendicular to its long axis. We show that, due to membrane mediated
interactions these curvature inducing membrane nematogens can oligomerize
spontaneously, even at low concentrations, and change the local shape of the
membrane. We demonstrate that for a large group of such inclusions, where the
two spontaneous curvatures have equal sign, the tubular conformation and
sometime the sheet conformation of the membrane are the common equilibrium
shapes. We elucidate the factors necessary for the formation of these {\it
protein lattices}. Furthermore, the elastic properties of the tubes, like their
compressional stiffness and persistence length are calculated. Finally, we
discuss the possible role of nematic disclination in capping and branching of
the tubular membranes.Comment: 15pages, 8 figure
Refinement and application of acoustic impulse technique to study nozzle transmission characteristics
An improved acoustic impulse technique was developed and was used to study the transmission characteristics of duct/nozzle systems. To accomplish the above objective, various problems associated with the existing spark-discharge impulse technique were first studied. These included (1) the nonlinear behavior of high intensity pulses, (2) the contamination of the signal with flow noise, (3) low signal-to-noise ratio at high exhaust velocities, and (4) the inability to control or shape the signal generated by the source, specially when multiple spark points were used as the source. The first step to resolve these problems was the replacement of the spark-discharge source with electroacoustic driver(s). These included (1) synthesizing on acoustic impulse with acoustic driver(s) to control and shape the output signal, (2) time domain signal averaging to remove flow noise from the contaminated signal, (3) signal editing to remove unwanted portions of the time history, (4) spectral averaging, and (5) numerical smoothing. The acoustic power measurement technique was improved by taking multiple induct measurements and by a modal decomposition process to account for the contribution of higher order modes in the power computation. The improved acoustic impulse technique was then validated by comparing the results derived by an impedance tube method. The mechanism of acoustic power loss, that occurs when sound is transmitted through nozzle terminations, was investigated. Finally, the refined impulse technique was applied to obtain more accurate results for the acoustic transmission characteristics of a conical nozzle and a multi-lobe multi-tube supressor nozzle
The Exotic Barium Bismuthates
We review the remarkable properties, including superconductivity,
charge-density-wave ordering, and metal-insulator transitions, of lead- and
potassium-doped barium bismuthate. We discuss some of the early theoretical
studies of these systems. Our recent theoretical work, on the negative-U\/,
extended-Hubbard model for these systems, is also described. Both the large-
and intermediate-U\/ regimes of this model are examined, using mean-field and
random-phase approximations, particularly with a view to fitting various
experimental properties of these bismuthates. On the basis of our studies, we
point out possibilities for exotic physics in these systems. We also emphasize
the different consequences of electronic and phonon-mediated mechanisms for the
negative U.\/ We show that, for an electronic mechanism, the \secin
\,\,phases of these bismuthates must be unique, with their transport properties
{\it dominated by charge Cooperon bound states}. This can explain the
observed difference between the optical and transport gaps. We propose other
experimental tests for this novel mechanism of charge transport and comment on
the effects of disorder.Comment: UUencoded LaTex file, 122 pages, figures available on request To
appear in Int. J. Mod. Phys. B as a review articl
Enrichment Procedures for Soft Clusters: A Statistical Test and its Applications
Clusters, typically mined by modeling locality of attribute spaces, are often evaluated for their ability to demonstrate ‘enrichment’ of categorical features. A cluster enrichment procedure evaluates the membership of a cluster for significant representation in pre-defined categories of interest. While classical enrichment procedures assume a hard clustering definition, in this paper we introduce a new statistical test that computes enrichments for soft clusters. We demonstrate an application of this test in refining and evaluating soft clusters for classification of remotely sensed images
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