15,036 research outputs found
Shock waves and drag in the numerical calculation of isentropic transonic flow
Properties of the shock relations for steady, irrotational, transonic flow are discussed and compared for the full and approximate governing potential in common use. Results from numerical experiments are presented to show that the use of proper finite difference schemes provide realistic solutions and do not introduce spurious shock waves. Analysis also shows that realistic drags can be computed from shock waves that occur in isentropic flow. In analogy to the Oswatitsch drag equation, which relates the drag to entropy production in shock waves, a formula is derived for isentropic flow that relates drag to the momentum gain through an isentropic shock. A more accurate formula for drag, based on entropy production, is also derived, and examples of wave drag evaluation based on these formulas are given and comparisons are made with experimental results
Testing the Impact of Higher Achievement's Year-Round Out-of-School-Time Program on Academic Outcomes
Presents findings from a multiyear evaluation of an intensive long-term OST program's effect on low-income middle school students' academic performance, attitudes, and behaviors. Outlines implications for financially strapped districts
Summer Snapshot: Exploring the Impact of Higher Achievement's Year-Round Out-of-School-Time Program on Summer Learning
Assesses the impact of a multiyear, intensive, academically focused OST program for motivated but underserved middle school students on test scores, summer program participation, and summer learning loss. Examines contributing factors and implications
Spectrophotometry of 2 complete samples of flat radio spectrum quasars
Spectrophotometry of two complete samples of flat-spectrum radio quasars show that for these objects there is a strong correlation between the equivalent width of the CIV wavelength 1550 emission line and the luminosity of the underlying continuum. Assuming Friedmann cosmologies, the scatter in this correlation is a minimum for q (sub o) is approximately 1. Alternatively, luminosity evolution can be invoked to give compact distributions for q (sub o) is approximately 0 models. A sample of Seyfert galaxies observed with IUE shows that despite some dispersion the average equivalent width of CIV wavelength 1550 in Seyfert galaxies is independent of the underlying continuum luminosity. New redshifts for 4 quasars are given
Economic and demographic issues related to deployment of the Satellite Power System (SPS)
Growth in energy consumption stimulated interest in exploitation of renewable sources of electric energy. One technology that was proposed is the Satellite Power System (SPS). Before committing the U.S. to such a large program, the Department of Energy and the National Aeronautics and Space Administration are jointly participating in an SPS Concept Development and Evaluation Program. This white paper on industrial and population relocation is part of the FY 78 preliminary evaluation of related socio-economic issues. Results of four preliminary assessment activities are documented
A site-specific standard for comparing dynamic solar ultraviolet protection characteristics of established tree canopies
A standardised procedure for making fair and comparable assessments of the ultraviolet protection of an established tree canopy that takes into account canopy movement and the changing position of the sun is presented for use by government, planning, and environmental health authorities. The technique utilises video image capture and replaces the need for measurement by ultraviolet radiometers for surveying shade quality characteristics of trees growing in public parks, playgrounds and urban settings. The technique improves upon tree shade assessments that may be based upon single measurements of the ultraviolet irradiance observed from a fixed point of view. The presented technique demonstrates how intelligent shade audits can be conducted without the need for specialist equipment, enabling the calculation of the Shade Protection Index (SPI) and Ultraviolet Protection Factor (UPF) for any discreet time interval and over a full calendar year
Symbolic computation of exact solutions expressible in hyperbolic and elliptic functions for nonlinear PDEs
Algorithms are presented for the tanh- and sech-methods, which lead to
closed-form solutions of nonlinear ordinary and partial differential equations
(ODEs and PDEs). New algorithms are given to find exact polynomial solutions of
ODEs and PDEs in terms of Jacobi's elliptic functions.
For systems with parameters, the algorithms determine the conditions on the
parameters so that the differential equations admit polynomial solutions in
tanh, sech, combinations thereof, Jacobi's sn or cn functions. Examples
illustrate key steps of the algorithms.
The new algorithms are implemented in Mathematica. The package
DDESpecialSolutions.m can be used to automatically compute new special
solutions of nonlinear PDEs. Use of the package, implementation issues, scope,
limitations, and future extensions of the software are addressed.
A survey is given of related algorithms and symbolic software to compute
exact solutions of nonlinear differential equations.Comment: 39 pages. Software available from Willy Hereman's home page at
http://www.mines.edu/fs_home/whereman
Multiquantum well structure with an average electron mobility of 4.0×10^6 cm^2/V s
We report a modulation-doped multiquantum well structure which suppresses the usual ambient light effect associated with modulation doping. Ten GaAs quantum wells 300-Å wide are symmetrically modulation doped using Si δ doping at the center of 3600-Å-wide Al0.1Ga0.9As barriers. The low field mobility of each well is 4.0×10^6 cm/V s at a density of 6.4×10^10 cm^−2 measured at 0.3 K either in the dark, or during, or after, exposure to light. This mobility is an order of magnitude improvement over previous work on multiwells
Self-Consistent Response of a Galactic Disk to an Elliptical Perturbation Halo Potential
We calculate the self-consistent response of an axisymmetric galactic disk
perturbed by an elliptical halo potential of harmonic number m = 2, and obtain
the net disk ellipticity. Such a potential is commonly expected to arise due to
a galactic tidal encounter and also during the galaxy formation process. The
self-gravitational potential corresponding to the self-consistent,
non-axisymmetric density response of the disk is obtained by inversion of
Poisson equation for a thin disk. This response potential is shown to oppose
the perturbation potential, because physically the disk self-gravity resists
the imposed potential. This results in a reduction in the net ellipticity of
the perturbation halo potential in the disk plane. The reduction factor
denoting this decrease is independent of the strength of the perturbation
potential, and has a typical minimum value of 0.75 - 0.9 for a wide range of
galaxy parameters. The reduction is negligible at all radii for higher
harmonics (m > or = 3) of the halo potential. (abridged).Comment: 26 pages (LaTex- aastex style), 3 .eps figures. To appear in the
Astrophysical Journal, Vol. 542, Oct. 20, 200
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