11,450 research outputs found
Propagation of sound in turbulent media
Perturbation methods commonly used to study the propagation of acoustic waves in turbulent media are reviewed. Emphasis is on those techniques which are applicable to problems involving long-range propagation in the atmosphere and ocean. Characteristic features of the various methods are illustrated by applying them to particular problems. It is shown that conventional perturbation techniques, such as the Born approximation, yield solutions which contain secular terms, and which therefore have a relatively limited range of validity. In contrast, it is found that solutions obtained with the aid of the Rytov method or the smoothing method do not contain secular terms, and consequently have a much greater range of validity
Impressions of convexity - An illustration for commutator bounds
We determine the sharpest constant such that for all complex
matrices and , and for Schatten -, - and -norms the inequality
is valid. The main theoretical
tool in our investigations is complex interpolation theory.Comment: 32 pages, 88 picture
Influence of structural disorder and large-scale geometric fluctuations on the Coherent Transport of Metallic Junctions and Molecular Wires
Structural disorder is present in almost all experimental measurements of
electronic transport through single molecules or molecular wires. To assess its
influence on the conductance is computationally demanding, because a large
number of conformations must be considered. Here we analyze an approximate
recursive layer Green function approach for the ballistic transport through
quasi one-dimensional nano-junctions. We find a rapid convergence of the method
with its control parameter, the layer thickness, and good agreement with
existing experimental and theoretical data. Because the computational effort
rises only linearly with system size, this method permits treatment of very
large systems. We investigate the conductance of gold- and silver wires of
different sizes and conformations. For weak electrode disorder and imperfect
coupling between electrode and wire we find conductance variations of
approximately 20%. Overall we find the conductance of silver junctions well
described by the immediate vicinity of narrowest point in the junction, a
result that may explain the observation of well-conserved conductance plateaus
in recent experiments on silver junctions. In an application to flexible
oligophene wires, we find that strongly distorted conformations that are
sterically forbidden at zero temperature, contribute significantly to the
observed average zero-bias conductance of the molecular wire
Analysis of dynamic inlet distortion applied to a parallel compressor model
An investigation of surge was conducted by using a parallel compressor model of the J85-13 compressor implement on an analog computer. Surges were initiated by various types of dynamic disturbances in inlet pressure. The compressor model was less sensitive to disturbances of short duration, high frequency, and long duration where the compressor discharge pressure could react. Adding steady distortion to dynamic disturbances reduced the amount of dynamic disturbance required to effect surge. Steady and unsteady distortions combined linearly to reduce surge margin
Technical aspects of a demonstration tape for three-dimensional sound displays
This document was developed to accompany an audio cassette that demonstrates work in three-dimensional auditory displays, developed at the Ames Research Center Aerospace Human Factors Division. It provides a text version of the audio material, and covers the theoretical and technical issues of spatial auditory displays in greater depth than on the cassette. The technical procedures used in the production of the audio demonstration are documented, including the methods for simulating rotorcraft radio communication, synthesizing auditory icons, and using the Convolvotron, a real-time spatialization device
A comparative study of higher order Bragg gratings: Coupled-mode theory versus mode expansion modeling
Monte Carlo simulations of the directional-ordering transition in the two-dimensional classical and quantum compass model
A comprehensive study of the two-dimensional (2D) compass model on the square
lattice is performed for classical and quantum spin degrees of freedom using
Monte Carlo and quantum Monte Carlo methods. We employ state-of-the-art
implementations using Metropolis, stochastic series expansion and parallel
tempering techniques to obtain the critical ordering temperatures and critical
exponents. In a pre-investigation we reconsider the classical compass model
where we study and contrast the finite-size scaling behavior of ordinary
periodic boundary conditions against annealed boundary conditions. It is shown
that periodic boundary conditions suffer from extreme finite-size effects which
might be caused by closed loop excitations on the torus. These excitations also
appear to have severe effects on the Binder parameter. On this footing we
report on a systematic Monte Carlo study of the quantum compass model. Our
numerical results are at odds with recent literature on the subject which we
trace back to neglecting the strong finite-size effects on periodic lattices.
The critical temperatures are obtained as and
for the classical and quantum version, respectively,
and our data support a transition in the 2D Ising universality class for both
cases.Comment: 8 pages, 7 figures, differs slightly from published versio
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