1,668 research outputs found
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
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
Fuel quality/processing study. Volume 4: On site processing studies
Fuel treated at the turbine and the turbine exhaust gas processed at the turbine site are studied. Fuel treatments protect the turbine from contaminants or impurities either in the upgrading fuel as produced or picked up by the fuel during normal transportation. Exhaust gas treatments provide for the reduction of NOx and SOx to environmentally acceptable levels. The impact of fuel quality upon turbine maintenance and deterioration is considered. On site costs include not only the fuel treatment costs as such, but also incremental costs incurred by the turbine operator if a turbine fuel of low quality is not acceptably upgraded
Time-stepping approach for solving upper-bound problems: Application to two-dimensional Rayleigh-Benard convection
An alternative computational procedure for numerically solving a class of variational problems arising from rigorous upper-bound analysis of forced-dissipative infinite-dimensional nonlinear dynamical systems, including the Navier-Stokes and Oberbeck-Boussinesq equations, is analyzed and applied to Rayleigh-Benard convection. A proof that the only steady state to which this numerical algorithm can converge is the required global optimal of the relevant variational problem is given for three canonical flow configurations. In contrast with most other numerical schemes for computing the optimal bounds on transported quantities (e.g., heat or momentum) within the "background field" variational framework, which employ variants of Newton's method and hence require very accurate initial iterates, the new computational method is easy to implement and, crucially, does not require numerical continuation. The algorithm is used to determine the optimal background-method bound on the heat transport enhancement factor, i.e., the Nusselt number (Nu), as a function of the Rayleigh number (Ra), Prandtl number (Pr), and domain aspect ratio L in two-dimensional Rayleigh-Benard convection between stress-free isothermal boundaries (Rayleigh's original 1916 model of convection). The result of the computation is significant because analyses, laboratory experiments, and numerical simulations have suggested a range of exponents alpha and beta in the presumed Nu similar to (PrRa beta)-Ra-alpha scaling relation. The computations clearly show that for Ra <= 10(10) at fixed L = 2 root 2, Nu <= 0.106Pr(0)Ra(5/12), which indicates that molecular transport cannot generally be neglected in the "ultimate" high-Ra regime.NSF DMS-0928098 DMS-1515161 DMS-0927587 PHY-1205219Simons FoundationNSFONRInstitute for Computational Engineering and Sciences (ICES
Rocket measurement of the secondary electron spectrum in an aurora
Secondary electron emission measurement in aurora using Aerobee rocket vehicl
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
Temperature control of a liquid helium cooled Eigler-style scanning tunneling microscope
A procedure for operating an Eigler-style, low temperature scanning tunneling microscope (STM) at variable temperatures has been developed. A critical exchange gas pressure regime was found to allow for controlled variation of the STM temperature while it is encapsulated in a liquid helium Dewar. The sensitivity of various parameters to the ability to generate stable variable temperatures above 4 K is discussed
Crossover from Rate-Equation to Diffusion-Controlled Kinetics in Two-Particle Coagulation
We develop an analytical diffusion-equation-type approximation scheme for the
one-dimensional coagulation reaction A+A->A with partial reaction probability
on particle encounters which are otherwise hard-core. The new approximation
describes the crossover from the mean-field rate-equation behavior at short
times to the universal, fluctuation-dominated behavior at large times. The
approximation becomes quantitatively accurate when the system is initially
close to the continuum behavior, i.e., for small initial density and fast
reaction. For large initial density and slow reaction, lattice effects are
nonnegligible for an extended initial time interval. In such cases our
approximation provides the correct description of the initial mean-field as
well as the asymptotic large-time, fluctuation-dominated behavior. However, the
intermediate-time crossover between the two regimes is described only
semiquantitatively.Comment: 21 pages, plain Te
Diffusion-Limited Aggregation Processes with 3-Particle Elementary Reactions
A diffusion-limited aggregation process, in which clusters coalesce by means
of 3-particle reaction, A+A+A->A, is investigated. In one dimension we give a
heuristic argument that predicts logarithmic corrections to the mean-field
asymptotic behavior for the concentration of clusters of mass at time ,
, for . The total
concentration of clusters, , decays as at . We also investigate the problem with a localized steady source of
monomers and find that the steady-state concentration scales as
, , and , respectively,
for the spatial dimension equal to 1, 2, and 3. The total number of
clusters, , grows with time as , , and
for = 1, 2, and 3. Furthermore, in three dimensions we
obtain an asymptotic solution for the steady state cluster-mass distribution:
, with the scaling function
and the scaling variable .Comment: 12 pages, plain Te
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