29,271 research outputs found
Project Management in NASA: The system and the men
An analytical description of the NASA project management system is presented with emphasis on the human element. The NASA concept of project management, program managers, and the problems and strengths of the NASA system are discussed
Simulating `Complex' Problems with Quantum Monte Carlo
We present a new quantum Monte Carlo algorithm suitable for generically
complex problems, such as systems coupled to external magnetic fields or anyons
in two spatial dimensions. We find that the choice of gauge plays a nontrivial
role, and can be used to reduce statistical noise in the simulation.
Furthermore, it is found that noise can be greatly reduced by approximate
cancellations between the phases of the (gauge dependent) statistical flux and
the external magnetic flux.Comment: Revtex, 11 pages. 3 postscript files for figures attache
Intermittency and the passive nature of the magnitude of the magnetic field
It is shown that the statistical properties of the magnitude of the magnetic
field in turbulent electrically conducting media resemble, in the inertial
range, those of passive scalars in fully developed three-dimensional fluid
turbulence. This conclusion, suggested by the data from Advanced Composition
Explorer, is supported by a brief analysis of the appropriate
magnetohydrodynamic equations
Dynamically-Driven Star Formation In Models Of NGC 7252
We present new dynamical models of the merger remnant NGC 7252 which include
star formation simulated according to various phenomenological rules. By using
interactive software to match our model with the observed morphology and gas
velocity field, we obtain a consistent dynamical model for NGC 7252. In our
models, this proto-elliptical galaxy formed by the merger of two similar
gas-rich disk galaxies which fell together with an initial pericentric
separation of ~2 disk scale lengths approximately 620 Myr ago. Results from two
different star formation rules--- density-dependent and shock-induced--- show
significant differences in star formation during and after the first passage.
Shock-induced star formation yields a prompt and wide-spread starburst at the
time of first passage, while density-dependent star formation predicts a more
slowly rising and centrally concentrated starburst. A comparison of the
distributions and ages of observed clusters with results of our simulations
favors shock-induced mechanism of star formation in NGC 7252. We also present
simulated color images of our model of NGC 7252, constructed by incorporating
population synthesis with radiative transfer and dust attenuation. Overall the
predicted magnitudes and colors of the models are consistent with observations,
although the simulated tails are fainter and redder than observed. We suggest
that a lack of star formation in the tails, reflected by the redder colors, is
due to an incomplete description of star formation in our models rather than
insufficient gas in the tails.Comment: 11 pages, 9 figures, to be published in MNRA
Non-linear rheology of active particle suspensions: Insights from an analytical approach
We consider active suspensions in the isotropic phase subjected to a shear
flow. Using a set of extended hydrodynamic equations we derive a variety of
{\em analytical} expressions for rheological quantities such as shear viscosity
and normal stress differences. In agreement to full-blown numerical
calculations and experiments we find a shear thickening or -thinning behaviour
depending on whether the particles are contractile or extensile. Moreover, our
analytical approach predicts that the normal stress differences can change
their sign in contrast to passive suspensions.Comment: 11 pages, 10 figures, appear in PR
Novel highly conductive and transparent graphene based conductors
Future wearable electronics, displays and photovoltaic devices rely on highly
conductive, transparent and yet mechanically flexible materials. Nowadays
indium tin oxide (ITO) is the most wide spread transparent conductor in
optoelectronic applications, however the mechanical rigidity of this material
limits its use for future flexible devices. Here we report novel transparent
conductors based on few layer graphene (FLG) intercalated with ferric chloride
(FeCl3) with an outstandingly high electrical conductivity and optical
transparency. We show that upon intercalation a record low sheet resistance of
8.8 Ohm/square is attained together with an optical transmittance higher than
84% in the visible range. These parameters outperform the best values of ITO
and of other carbon-based materials, making these novel transparent conductors
the best candidates for future flexible optoelectronics
Determining the terahertz optical properties of subwavelength films using semiconductor surface plasmons
Copyright © 2008 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 93 (2008) and may be found at http://link.aip.org/link/?APPLAB/93/241115/1By employing a combination of time-domain measurements and numerical calculations, we demonstrate that the semiconductor InSb supports a strongly confined surface plasmon (SP) in the terahertz frequency range. We show that these SPs can be used to enhance the light-matter interaction with dielectric layers above the semiconductor surface, thereby allowing us to detect the presence of polystyrene layers around 1000 times thinner than the free space wavelength of the terahertz light. Finally we discuss the viability of using semiconductor SPs for the purposes of terahertz sensing and spectroscopy
Light outcoupling efficiency of top-emitting organic light-emitting diodes
Copyright © 2004 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters 84 (2004) and may be found at http://link.aip.org/link/?APPLAB/84/2986/1We report results obtained from modeling the light outcoupling efficiency of top–emitting organic light-emitting diode (OLED) structures and compare them with results from conventional substrate-emitting structures. We investigate two types of emissive material, small molecule and conjugated polymers, and study three different cathode materials; aluminum, silver, and calcium. We show that top-emitting OLEDs may have outcoupling efficiencies comparable to their substrate-emitting counterparts, and that the choice of cathode material is critical to the optical performance of the device
SeaWiFS technical report series. Volume 10: Modeling of the SeaWiFS solar and lunar observations
Post-launch stability monitoring of the Sea-viewing Wide Field-of-view Sensor (SeaWifs) will include periodic sweeps of both an onboard solar diffuser plate and the moon. The diffuser views will provide short-term checks and the lunar views will monitor long-term trends in the instrument's radiometric stability. Models of the expected sensor response to these observations were created on the SeaWiFS computer at the National Aeronautics and Space Administration's (NASA) Goddard Space Flight Center (GSFC) using the Interactive Data Language (IDL) utility with a graphical user interface (GUI). The solar model uses the area of intersecting circles to simulate the ramping of sensor response while viewing the diffuser. This model is compared with preflight laboratory scans of the solar diffuser. The lunar model reads a high-resolution lunar image as input. The observations of the moon are simulated with a bright target recovery algorithm that includes ramping and ringing functions. Tests using the lunar model indicate that the integrated radiance of the entire lunar surface provides a more stable quantity than the mean of radiances from centralized pixels. The lunar model is compared to ground-based scans by the SeaWiFS instrument of a full moon in December 1992. Quality assurance and trend analyses routines for calibration and for telemetry data are also discussed
- …