6,539 research outputs found
Computing aerodynamic sound using advanced statistical turbulence theories
It is noted that the calculation of turbulence-generated aerodynamic sound requires knowledge of the spatial and temporal variation of Q sub ij (xi sub k, tau), the two-point, two-time turbulent velocity correlations. A technique is presented to obtain an approximate form of these correlations based on closure of the Reynolds stress equations by modeling of higher order terms. The governing equations for Q sub ij are first developed for a general flow. The case of homogeneous, stationary turbulence in a unidirectional constant shear mean flow is then assumed. The required closure form for Q sub ij is selected which is capable of qualitatively reproducing experimentally observed behavior. This form contains separation time dependent scale factors as parameters and depends explicitly on spatial separation. The approximate forms of Q sub ij are used in the differential equations and integral moments are taken over the spatial domain. The velocity correlations are used in the Lighthill theory of aerodynamic sound by assuming normal joint probability
Atmospheric-wake vortex interactions
The interactions of a vortex wake with a turbulent stratified atmosphere are investigated with the computer code WAKE. It is shown that atmospheric shear, turbulence, and stratification can provide the dominant mechanisms by which vortex wakes decay. Computations included the interaction of a vortex wake with a viscous ground plane. The observed phenomenon of vortex bounce is explained in terms of secondary vorticity produced on the ground. This vorticity is swept off the ground and advected about the vortex pair, thereby altering the classic hyperbolic trajectory. The phenomenon of the solitary vortex is explained as an interaction of a vortex with crosswind shear. Here, the vortex having the sign opposite that of the sign of the vorticity in the shear is dispersed by a convective instability. This instability results in the rapid production of turbulence which in turn disperses the smoke marking the vortex
The Gaussian formula and spherical aberration of the static and moving curved mirrors from Fermat's principle
The Gaussian formula and spherical aberrations of the static and relativistic
curved mirrors are analyzed using the optical path length (OPL) and Fermat's
principle. The geometrical figures generated by the rotation of conic sections
about their symmetry axes are considered for the shapes of the mirrors. By
comparing the results in static and relativistic cases, it is shown that the
focal lengths and the spherical aberration relations of the relativistic
mirrors obey the Lorentz contraction. Further analysis of the spherical
aberrations for both static and relativistic cases have resulted in the
information about the limits for the paraxial approximation, as well as for the
minimum speed of the systems to reduce the spherical aberrations.Comment: 15 pages, 7 figures, uses iopart. Major revisions on the physical
interpretations of the results. Accepted for publication in J. Op
Coherent flash of light emitted by a cold atomic cloud
When a resonant laser sent on an optically thick cold atomic cloud is
abruptly switched off, a coherent flash of light is emitted in the forward
direction. This transient phenomenon is observed due to the highly resonant
character of the atomic scatterers. We analyze quantitatively its
spatio-temporal properties and show very good agreement with theoretical
predictions. Based on complementary experiments, the phase of the coherent
field is reconstructed without interferometric tools.Comment: Submitted to Phys. Rev. Let
Photoelastic force measurements in granular materials
Photoelastic techniques are used to make both qualitative and quantitative
measurements of the forces within idealized granular materials. The method is
based on placing a birefringent granular material between a pair of polarizing
filters, so that each region of the material rotates the polarization of light
according to the amount of local of stress. In this review paper, we summarize
past work using the technique, describe the optics underlying the technique,
and illustrate how it can be used to quantitatively determine the vector
contact forces between particles in a 2D granular system. We provide a
description of software resources available to perform this task, as well as
key techniques and resources for building an experimental apparatus
Measurement of atomic diffraction phases induced by material gratings
Atom-surface interactions can significantly modify the intensity and phase of
atom de Broglie waves diffracted by a silicon nitride grating. This affects the
operation of a material grating as a coherent beam splitter. The phase shift
induced by diffraction is measured by comparing the relative phases of serveral
interfering paths in a Mach-Zehnder Na atom interferometer formed by three
material gratings. The values of the diffraction phases are consistent with a
simple model which includes a van der Waals atom-surface interaction between
the Na atoms and the silicon nitride grating bars.Comment: 4 pages, 5 figures, submitted to PR
Phase Space Evolution and Discontinuous Schr\"odinger Waves
The problem of Schr\"odinger propagation of a discontinuous wavefunction
-diffraction in time- is studied under a new light. It is shown that the
evolution map in phase space induces a set of affine transformations on
discontinuous wavepackets, generating expansions similar to those of wavelet
analysis. Such transformations are identified as the cause for the
infinitesimal details in diffraction patterns. A simple case of an evolution
map, such as SL(2) in a two-dimensional phase space, is shown to produce an
infinite set of space-time trajectories of constant probability. The
trajectories emerge from a breaking point of the initial wave.Comment: Presented at the conference QTS7, Prague 2011. 12 pages, 7 figure
Observation of Brewster's effect for transverse-electric electromagnetic waves in metamaterials: Experiment and theory
We have experimentally realized Brewster's effect for transverse-electric
waves with metamaterials. In dielectric media, Brewster's no-reflection effect
arises only for transverse-magnetic waves. However, it has been theoretically
predicted that Brewster's effect arises for TE waves under the condition that
the relative permeability r is not equal to unity. We have designed an array of
split-ring resonators as a metamaterial with mu_r 1 using a finite-difference
time-domain method. The reflection measurements were carried out in a 3-GHz
region and the disappearance of reflected waves at a particular incident angle
was confirmed.Comment: 4 pages, 5 figure
Observing the Profile of an Atom Laser Beam
We report on an investigation of the beam profile of an atom laser extracted
from a magnetically trapped Rb Bose-Einstein condensate. The transverse
momentum distribution is magnified by a curved mirror for matter waves and a
momentum resolution of 1/60 of a photon recoil is obtained. We find the
transverse momentum distribution to be determined by the mean-field potential
of the residing condensate, which leads to a non-smooth transverse density
distribution. Our experimental data are compared with a full 3D simulation of
the output coupling process and we find good agreement.Comment: 4 pages, 4 figure
Average transmission probability of a random stack
The transmission through a stack of identical slabs that are separated by
gaps with random widths is usually treated by calculating the average of the
logarithm of the transmission probability. We show how to calculate the average
of the transmission probability itself with the aid of a recurrence relation
and derive analytical upper and lower bounds. The upper bound, when used as an
approximation for the transmission probability, is unreasonably good and we
conjecture that it is asymptotically exact.Comment: 10 pages, 6 figure
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