11,649 research outputs found
An experimental and analytical investigation of proprotor whirl flutter
The results of an experimental parametric investigation of whirl flutter are presented for a model consisting of a windmilling propeller-rotor, or proprotor, having blades with offset flapping hinges mounted on a rigid pylon with flexibility in pitch and yaw. The investigation was motivated by the need to establish a large data base from which to assess the predictability of whirl flutter for a proprotor since some question has been raised as to whether flutter in the forward whirl mode could be predicted with confidence. To provide the necessary data base, the parametric study included variation in the pylon pitch and yaw stiffnesses, flapping hinge offset, and blade kinematic pitch-flap coupling over a large range of advance ratios. Cases of forward whirl flutter and of backward whirl flutter are documented. Measured whirl flutter characteristics were shown to be in good agreement with predictions from two different linear stability analyses which employed simple, two dimensional, quasi-steady aerodynamics for the blade loading. On the basis of these results, it appears that proprotor whirl flutter, both forward and backward, can be predicted
Edge Electron Gas
The uniform electron gas, the traditional starting point for density-based
many-body theories of inhomogeneous systems, is inappropriate near electronic
edges. In its place we put forward the appropriate concept of the edge electron
gas.Comment: 4 pages RevTex with 7 ps-figures included. Minor changes in
title,text and figure
Spectrometer for Hard X-Ray Free Electron Laser Based on Diffraction Focusing
X-ray free electron lasers (XFELs) generate sequences of ultra-short,
spatially coherent pulses of x-ray radiation. We propose the diffraction
focusing spectrometer (DFS), which is able to measure the whole energy spectrum
of the radiation of a single XFEL pulse with an energy resolution of . This is much better than for most modern x-ray
spectrometers. Such resolution allows one to resolve the fine spectral
structure of the XFEL pulse. The effect of diffraction focusing occurs in a
single crystal plate due to dynamical scattering, and is similar to focusing in
a Pendry lens made from the metamaterial with a negative refraction index. Such
a spectrometer is easier to operate than those based on bent crystals. We show
that the DFS can be used in a wide energy range from 5 keV to 20 keV.Comment: 9 pages, 8 figures, 2 table
Nonuniqueness and derivative discontinuities in density-functional theories for current-carrying and superconducting systems
Current-carrying and superconducting systems can be treated within
density-functional theory if suitable additional density variables (the current
density and the superconducting order parameter, respectively) are included in
the density-functional formalism. Here we show that the corresponding conjugate
potentials (vector and pair potentials, respectively) are {\it not} uniquely
determined by the densities. The Hohenberg-Kohn theorem of these generalized
density-functional theories is thus weaker than the original one. We give
explicit examples and explore some consequences.Comment: revised version (typos corrected, some discussion added) to appear in
Phys. Rev.
The ring-opening polymerization of D,L-lactide in the melt initiated with tetraphenyltin
Melt polymerization conditions for D,L-lactide initiated with tetraphenyltin were studied with regard to polymer molecular weight and weight distributions. "Single" polymerization, "multiple"polymerization (four or eight reactions at the same time), and time-dependent studies are described. Single polymerizations using constant initiator concentrations resulted in a broad scattering of nonreproducible molecular weight values. Multiple polymerizations at constant initiator concentrations, however, resulted in nearly identical molecular weight profiles. Multiple polymerizations at different initiator concentrations did not show an inverse dependency of initiator concentration on polymer molecular weight. Both the single and multiple melt polymerizations resulted in rather broad molecular weight distributions. The presence of hydrolysis products of lactide during the melt polymerization most likely has a detrimental effect on molecular weight. After a short induction period the rather slow polymerization of D,L-lactide resulted in a maximal molecular weight followed by a slight decrease in molecular weight to a constant value. It is concluded that the polymerization of D,L-lactide in the melt initiated with tetraphenyltin does not proceed through a "living" mechanism
The Band-Gap Problem in Semiconductors Revisited: Effects of Core States and Many-Body Self-Consistency
A novel picture of the quasiparticle (QP) gap in prototype semiconductors Si
and Ge emerges from an analysis based on all-electron, self-consistent, GW
calculations. The deep-core electrons are shown to play a key role via the
exchange diagram --if this effect is neglected, Si becomes a semimetal.
Contrary to current lore, the Ge 3d semicore states (e.g., their polarization)
have no impact on the GW gap. Self-consistency improves the calculated gaps --a
first clear-cut success story for the Baym-Kadanoff method in the study of
real-materials spectroscopy; it also has a significant impact on the QP
lifetimes. Our results embody a new paradigm for ab initio QP theory
Total energy global optimizations using non orthogonal localized orbitals
An energy functional for orbital based calculations is proposed, which
depends on a number of non orthogonal, localized orbitals larger than the
number of occupied states in the system, and on a parameter, the electronic
chemical potential, determining the number of electrons. We show that the
minimization of the functional with respect to overlapping localized orbitals
can be performed so as to attain directly the ground state energy, without
being trapped at local minima. The present approach overcomes the multiple
minima problem present within the original formulation of orbital based
methods; it therefore makes it possible to perform calculations for an
arbitrary system, without including any information about the system bonding
properties in the construction of the input wavefunctions. Furthermore, while
retaining the same computational cost as the original approach, our formulation
allows one to improve the variational estimate of the ground state energy, and
the energy conservation during a molecular dynamics run. Several numerical
examples for surfaces, bulk systems and clusters are presented and discussed.Comment: 24 pages, RevTex file, 5 figures available upon reques
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