52,454 research outputs found
Local measurements of velocity fluctuations and diffusion coefficients for a granular material flow
Measurements were made of two components of the average and fluctuating velocities, and of the local self-diffusion coefficients in a flow of granular material. The experiments were performed in a 1 m-high vertical channel with roughened sidewalls and with polished glass plates at the front and the back to create a two-dimensional flow. The particles used were glass spheres with a nominal diameter of 3 mm. The flows were high density and were characterized by the presence of long-duration frictional contacts between particles. The velocity measurements indicated that the flows consisted of a central uniform regime and a shear regime close to the walls. The fluctuating velocities in the transverse direction increased in magnitude from the centre towards the walls. A similar variation was not observed for the streamwise fluctuations. The self-diffusion coefficients showed a significant dependence on the fluctuating velocities and the shear rate. The velocity fluctuations were highly anistropic with the streamwise components being 2 to 2.5 times the transverse components. The self-diffusion coefficients for the streamwise direction were an order-of-magnitude higher than those for the transverse direction. The surface roughness of the particles led to a decrease in the self-diffusion coefficients
Long-lived memory for electronic spin in a quantum dot: Numerical analysis
Techniques for coherent control of electron spin-nuclear spin interactions in
quantum dots can be directly applied in spintronics and in quantum information
processing. In this work we study numerically the interaction of electron and
nuclear spins in the context of storing the spin-state of an electron in a
collective state of nuclear spins. We take into account the errors inherent in
a realistic system: the incomplete polarization of the bath of nuclear spins
and the different hyperfine interactions between the electron and individual
nuclei in the quantum dot. Although these imperfections deteriorate the
fidelity of the quantum information retrieval, we find reasonable fidelities
are achievable for modest bath polarizations.Comment: RevTex, 10 pages, 9 EPS figure
The use of precession modulation for nutation control in spin-stabilized spacecraft
The relations which determine the nutation effects induced in a spinning spacecraft by periodic precession thrust pulses are derived analytically. By utilizing the idea that nutation need only be observed just before each precession thrust pulse, a difficult continuous-time derivation is replaced by a simple discrete-time derivation using z-transforms. The analytic results obtained are used to develop two types of modulated precession control laws which use the precession maneuver to concurrently control nutation. Results are illustrated by digital simulation of an actual spacecraft configuration
Spectrum of an open disordered quasi-two-dimensional electron system: strong orbital effect of the weak in-plane magnetic field
The effect of an in-plane magnetic field upon open quasi-two-dimensional
electron and hole systems is investigated in terms of the carrier ground-state
spectrum. The magnetic field, classified as weak from the viewpoint of
correlation between size parameters of classical electron motion and the gate
potential spatial profile is shown to efficiently cut off extended modes from
the spectrum and to change singularly the mode density of states (MDOS). The
reduction in the number of current-carrying modes, right up to zero in magnetic
fields of moderate strength, can be viewed as the cause of
magnetic-field-driven metal-to-insulator transition widely observed in
two-dimensional systems. Both the mode number reduction and the MDOS
singularity appear to be most pronounced in the mode states dephasing
associated with their scattering by quenched-disorder potential. This sort of
dephasing is proven to dominate the dephasing which involves solely the
magnetic field whatever level of the disorder.Comment: RevTeX-4 class, 12 pages, 5 eps figure
Simultaneous Spin-Charge Relaxation in Double Quantum Dots
We investigate phonon-induced spin and charge relaxation mediated by
spin-orbit and hyperfine interactions for a single electron confined within a
double quantum dot. A simple toy model incorporating both direct decay to the
ground state of the double dot and indirect decay via an intermediate excited
state yields an electron spin relaxation rate that varies non-monotonically
with the detuning between the dots. We confirm this model with experiments
performed on a GaAs double dot, demonstrating that the relaxation rate exhibits
the expected detuning dependence and can be electrically tuned over several
orders of magnitude. Our analysis suggests that spin-orbit mediated relaxation
via phonons serves as the dominant mechanism through which the double-dot
electron spin-flip rate varies with detuning.Comment: 5 pages, 3 figures, Supplemental Material (2 pages, 2 figures
Rayleigh scattering temperature measurements in a swirl stabilized burner
Rayleigh scattering temperature measurements were obtained in a turbulent reactive swirling coaxial jet discharged from a swirl-stabilized burner along the jet-flame centerline. They are reported up to 10 fuel nozzle diameters downstream of the burner exit at a Reynolds number of 29000. The effect of swirl numbers (S=0.3, 0.58, 1.07) on the temperature fields, the power spectral density of temperature fluctuations and on the probability density functions of the temperature fluctuations was determined
Experimental Assessment of ‘subgrid’ scale Probability Density Function Models for Large Eddy Simulation
Filtered density functions (FDFs) of mixture fraction are quantified by analyzing
experimental data obtained from two-dimensional planar laser-induced fluorescence scalar
measurements in the isothermal swirling flow of a combustor operating at a Reynolds number of
28,662 for three different swirl numbers (0.3, 0.58 and 1.07). Two-dimensional filtering using a
box filter was performed on the measured scalar to obtain the filtered variables used for
presumed FDF for Large Eddy Simulations (LES). A dependant variable
from the measured scalar, which was a pre-computed temperature, was integrated over the
experimentally obtained FDF as well as over the presumed beta or top-hat FDFs and a relative
error in temperature prediction was calculated. The experimentally measured FDFs depended on
swirl numbers and axial and radial positions in the flow. The FDFs were unimodal in the regions
of low variance and bimodal in the regions of high variance. The influence of the filter spatial dimension on the measured FDF was evaluated and consequences for subgrid modeling for LES discussed
- …