1,824 research outputs found
Variational bounds on the energy dissipation rate in body-forced shear flow
A new variational problem for upper bounds on the rate of energy dissipation
in body-forced shear flows is formulated by including a balance parameter in
the derivation from the Navier-Stokes equations. The resulting min-max problem
is investigated computationally, producing new estimates that quantitatively
improve previously obtained rigorous bounds. The results are compared with data
from direct numerical simulations.Comment: 15 pages, 7 figure
Exponentially growing solutions in homogeneous Rayleigh-Benard convection
It is shown that homogeneous Rayleigh-Benard flow, i.e., Rayleigh-Benard
turbulence with periodic boundary conditions in all directions and a volume
forcing of the temperature field by a mean gradient, has a family of exact,
exponentially growing, separable solutions of the full non-linear system of
equations. These solutions are clearly manifest in numerical simulations above
a computable critical value of the Rayleigh number. In our numerical
simulations they are subject to secondary numerical noise and resolution
dependent instabilities that limit their growth to produce statistically steady
turbulent transport.Comment: 4 pages, 3 figures, to be published in Phys. Rev. E - rapid
communication
Phase space dynamics of overdamped quantum systems
The phase space dynamics of dissipative quantum systems in strongly condensed
phase is considered. Based on the exact path integral approach it is shown that
the Wigner transform of the reduced density matrix obeys a time evolution
equation of Fokker-Planck type valid from high down to very low temperatures.
The effect of quantum fluctuations is discussed and the accuracy of these
findings is tested against exact data for a harmonic system.Comment: 7 pages, 2 figures, to appear in Euro. Phys. Let
An Evaluation of Computational Fluid Dynamics for Spillway Modelling
Computational Fluid Dynamics (CFD) is used extensively by engineers to model and analyse complex issues related hydraulic design, planning studies for future generating stations, civil maintenance, supply efficiency, and dam safety. The integrity of computed values from CFD models is of considerable economic importance in the design, upgrading and maintenance of hydroelectric generating stations. CFD models have the ability to predict many characteristics flow over a spillway and Manitoba Hydro has had good agreement with physical model results in the past. However, date there has not been a review that brings all the available information together for a comprehensive assessment. The objective of this research is to build upon previous investigations on the use of CFD modelling, by focusing specifically on the ability to accurately model spillways using CFD. This paper discusses three-dimensional numerical modelling of several different spillway configurations using the CFD software Flow-3D and compares the predicted rating curves, pressures, and water surface elevations to corresponding physical model experimental values. The numerical model results were generally in agreement with physical model data, however, the relative differences in discharges were found to have a P/Hd dependency. The accuracy for a given model resolution and associated computational time required was also considered
Superconducting Fluxon Pumps and Lenses
We study stochastic transport of fluxons in superconductors by alternating
current (AC) rectification. Our simulated system provides a fluxon pump,
"lens", or fluxon "rectifier" because the applied electrical AC is transformed
into a net DC motion of fluxons. Thermal fluctuations and the asymmetry of the
ratchet channel walls induce this "diode" effect, which can have important
applications in devices, like SQUID magnetometers, and for fluxon optics,
including convex and concave fluxon lenses. Certain features are unique to this
novel two-dimensional (2D) geometric pump, and different from the previously
studied 1D ratchets.Comment: Phys. Rev. Lett. 83, in press (1999); 4 pages, 5 .gif figures;
figures also available at http://www-personal.engin.umich.edu/~nori/ratche
Subdiffusion-limited reactions
We consider the coagulation dynamics A+A -> A and A+A A and the
annihilation dynamics A+A -> 0 for particles moving subdiffusively in one
dimension. This scenario combines the "anomalous kinetics" and "anomalous
diffusion" problems, each of which leads to interesting dynamics separately and
to even more interesting dynamics in combination. Our analysis is based on the
fractional diffusion equation
Coherent State path-integral simulation of many particle systems
The coherent state path integral formulation of certain many particle systems
allows for their non perturbative study by the techniques of lattice field
theory. In this paper we exploit this strategy by simulating the explicit
example of the diffusion controlled reaction . Our results are
consistent with some renormalization group-based predictions thus clarifying
the continuum limit of the action of the problem.Comment: 20 pages, 4 figures. Minor corrections. Acknowledgement and reference
correcte
Hammerhead, an ultrahigh resolution ePix camera for wavelength-dispersive spectrometers
Wavelength-dispersive spectrometers (WDS) are often used in synchrotron and
FEL applications where high energy resolution (in the order of eV) is
important. Increasing WDS energy resolution requires increasing spatial
resolution of the detectors in the dispersion direction. The common approaches
with strip detectors or small pixel detectors are not ideal. We present a novel
approach, with a sensor using rectangular pixels with a high aspect ratio
(between strips and pixels, further called "strixels"), and strixel
redistribution to match the square pixel arrays of typical ASICs while avoiding
the considerable effort of redesigning ASICs. This results in a sensor area of
17.4 mm x 77 mm, with a fine pitch of 25 m in the horizontal direction
resulting in 3072 columns and 176 rows. The sensors use ePix100 readout ASICs,
leveraging their low noise (43 e, or 180 eV rms). We present results
obtained with a Hammerhead ePix100 camera, showing that the small pitch (25
m) in the dispersion direction maximizes performance for both high and low
photon occupancies, resulting in optimal WDS energy resolution. The low noise
level at high photon occupancy allows precise photon counting, while at low
occupancy, both the energy and the subpixel position can be reconstructed for
every photon, allowing an ultrahigh resolution (in the order of 1 m) in
the dispersion direction and rejection of scattered beam and harmonics. Using
strixel sensors with redistribution and flip-chip bonding to standard ePix
readout ASICs results in ultrahigh position resolution (1 m) and low
noise in WDS applications, leveraging the advantages of hybrid pixel detectors
(high production yield, good availability, relatively inexpensive) while
minimizing development complexity through sharing the ASIC, hardware, software
and DAQ development with existing versions of ePix cameras.Comment: 8 pages, 6 figure
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