30,978 research outputs found
On the attenuation of sound by three-dimensionally segmented acoustic liners in a rectangular duct
Axial segmentation of acoustically absorbing liners in rectangular, circular or annual duct configurations is a very useful concept for obtaining higher noise attenuation with respect to the bandwidth of absorption as well as the maximum attenuation. As a consequence, advanced liner concepts are proposed which induce a modal energy transfer in both cross-sectional directions to further reduce the noise radiated from turbofan engines. However, these advanced liner concepts require three-dimensional geometries which are difficult to treat theoretically. A very simple three-dimensional problem is investigated analytically. The results show a strong dependence on the positioning of the liner for some incident source modes while the effect of three-dimensional segmentation appears to be negligible over the frequency range considered
GRAVSAT/GEOPAUSE covariance analysis including geopotential aliasing
A conventional covariance analysis for the GRAVSAT/GEOPAUSE mission is described in which the uncertainties of approximately 200 parameters, including the geopotential coefficients to degree and order 12, are estimated over three different tracking intervals. The estimated orbital uncertainties for both GRAVSAT and GEOPAUSE reach levels more accurate than presently available. The adjusted measurement bias errors approach the mission goal. Survey errors in the low centimeter range are achieved after ten days of tracking. The ability of the mission to obtain accuracies of geopotential terms to (12, 12) one to two orders of magnitude superior to present accuracy levels is clearly shown. A unique feature of this report is that the aliasing structure of this (12, 12) field is examined. It is shown that uncertainties for unadjusted terms to (12, 12) still exert a degrading effect upon the adjusted error of an arbitrarily selected term of lower degree and order. Finally, the distribution of the aliasing from the unestimated uncertainty of a particular high degree and order geopotential term upon the errors of all remaining adjusted terms is listed in detail
Exciton-Population Inversion and Terahertz Gain in Resonantly Excited Semiconductors
The build-up of exciton populations in resonantly laser excited
semiconductors is studied microscopically. For excitation at the -exciton
resonance, it is shown that polarization with a strict -type radial symmetry
can be efficiently converted into an incoherent -type population. As a
consequence, inversion between the and exciton states can be obtained
leading to the appearance of significant terahertz gain.Comment: 4 pages, 3 figure
Off-Shell Electromagnetic Form Fators of the Nucleon in Chiral Perturbation Theory
We study the electromagnetic form factors of a nucleon in next-to-leading
order chiral perturbation theory (CPT) in the case where one of the nucleons is
off its mass shell. We calculate the leading nonanalytic contributions to
relevant measures for the off-shell dependence in the limited kinematical range
allowed.Comment: 3 pages LaTeX with worldsci.sty (available by mailing
[email protected] and typing "get worldsci.sty" in the subject line),
invited talk given at the International Symposium on Medium Energy Physics,
Beijing, August 199
Self-interaction errors in nuclear energy density functionals
When applied to a single nucleon, nuclear energy density functionals may
yield a non-vanishing internal energy thus implying that the nucleon is
interacting with itself. It is shown how to avoid this unphysical feature for
semi-local phenomenological functionals containing all possible bilinear
combinations of local densities and currents up to second order in the
derivatives. The method outlined in this Rapid Communication could be easily
extended to functionals containing higher order terms, and could serve as a
guide for constraining the time-odd part of the functional
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