3,244 research outputs found
The length scale for sub-grid-scale parameterization with anisotropic resolution
Use of the Smagorinsky eddy-viscosity formulation and related schemes for subgrid-scale parameterization of large eddy simulation models requires specification of a single length scale, earlier related by Lilly to the scale of filtering and/or numerical resolution. An anisotropic integration of the Kolmogoroff enstrophy spectrum allows generalization of that relationship to anisotropic resolution. It is found that the Deardorff assumption is reasonably accurate for small anisotropies and can be simply improved for larger values
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Ejection Forces and Friction Coefficients from Injection Molding Experiments Using Rapid Tooling Inserts
Experiments have been performed with injection mold inserts made using solid freeform
fabrication processes in an effort to further study such applications for economic production of
small quantities of parts. Static friction coefficients were determined for HDPE and HIPS against
P-20 steel, sintered LaserForm ST-100, and stereolithography SL 5170 using the ASTM D 1894
standard. Injection mold inserts were constructed of the same three materials and were used to
inject cylindrical parts using HDPE and HIPS. Ejection forces were measured, and a model was
used to calculate ejection forces and apparent coefficients of static friction. Statistical analyses
were used to determine the effects of packing time, cooling time and packing pressure on
ejection force for the three insert types. This paper compares experimental and calculated
ejection forces, compares standard friction test results to calculated apparent coefficients of
friction, summarizes the statistical results, and comments on the feasibility of using rapid tooled
inserts for injection molding.Mechanical Engineerin
Pressure, Resistance, and Current Activation of Anisotropic Compressible Hall States
Thermodynamic and electric properties of anisotropic compressible Hall states
at higher Landau levels are studied using a mean field theory on the von
Neumann lattice basis. It is shown that resistances agree with the recent
experiments of anisotropic compressible states and the states have negative
pressure. As implications, the collapse phenomena of the integer quantum Hall
effect are discussed.Comment: 4 pages, 5 figures, to be published in Physica
Simulation of guiding of multiply charged projectiles through insulating capillaries
Recent experiments have demonstrated that highly charged ions can be guided
through insulating nanocapillaries along the direction of the capillary axis
for a surprisingly wide range of injection angles. Even more surprisingly, the
transmitted particles remain predominantly in their initial charge state, thus
opening the pathway to the construction of novel ion-optical elements without
electric feedthroughs. We present a theoretical treatment of this
self-organized guiding process. We develop a classical trajectory transport
theory that relates the microscopic charge-up with macroscopic material
properties. Transmission coefficients, angular spread of transmitted particles,
and discharge characteristics of the target are investigated. Partial agreement
with experiment is found
New Physics in High Landau Levels
Recent magneto-transport experiments on ultra-high mobility 2D electron
systems in GaAs/AlGaAs heterostructures have revealed the existence of whole
new classes of correlated many-electron states in highly excited Landau levels.
These new states, which appear only at extremely low temperatures, are
distinctly different from the familiar fractional quantum Hall liquids of the
lowest Landau level. Prominent among the recent findings are the discoveries of
giant anisotropies in the resistivity near half filling of the third and higher
Landau levels and the observation of re- entrant integer quantum Hall states in
the flanks of these same levels. This contribution will survey the present
status of this emerging field.Comment: 8 pages, 9 figures. To be published in the Proceedings of the 13th
International Conference on the Electronic Properties of Two-Dimensional
System
Reply to Simon's Comment on "Evidence for an Anisotropic State of Two-Dimensional Electrons in High Landau Levels"
We recently reported [PRL 82, 394 (1999)] large transport anisotropies in a
two-dimensional electron gas in high Landau levels. These observations were
made utilizing both square and Hall bar sample geometries. Simon recently
commented [cond-mat/9903086] that a classical calculation of the current flow
in the sample shows a magnification of an underlying anisotropy when using a
square sample. In this reply we present more recent data obtained with a very
high mobility sample, and reiterate that, with or without magnification, an
anisotropic state develops in high Landau levels at very low temperatures.Comment: 1 page, 1 figur
The Onset of Anisotropic Transport of Two-Dimensional Electrons in High Landau Levels: An Isotropic-to-Nematic Liquid Crystal Phase Transition?
The recently discovered anisotropy of the longitudinal resistance of
two-dimensional electrons near half filling of high Landau levels is found to
persist to much higher temperatures T when a large in-plane magnetic field B||
is applied. Under these conditions we find that the longitudinal resistivity
scales quasi-linearly with B||/T. These observations support the notion that
the onset of anisotropy at B||=0 does not reflect the spontaneous development
of charge density modulations but may instead signal an isotropic-to-nematic
liquid crystal phase transition.Comment: 5 pages, 4 figure
Oxygen Gas Abundances at 0.4<z<1.5: Implications for the Chemical Evolution History of Galaxies
We report VLT-ISAAC and Keck-NIRSPEC near-infrared spectroscopy for a sample
of 30 0.47<z<0.92 CFRS galaxies and five [OII]-selected, M_B,AB<-21.5, z~1.4
galaxies. We have measured Halpha and [NII] line fluxes for the CFRS galaxies
which have [OII], Hbeta and [OIII] line fluxes available from optical
spectroscopy. For the z~1.4 objects we measured Hbeta and [OIII] emission line
fluxes from J-band spectra, and Halpha line fluxes plus upper limits for [NII]
fluxes from H-band spectra. We derive the extinction and oxygen abundances for
the sample using a method based on a set of ionisation parameter and oxygen
abundance diagnostics, simultaneously fitting the [OII], Hbeta, [OIII], Halpha
and [NII] line fluxes. Our most salient conclusions are: a) the source of gas
ionisation in the 30 CFRS and in all z~1.4 galaxies is not due to AGN activity;
b) about one third of the 0.47<z<0.92 CFRS galaxies in our sample have
substantially lower metallicities than local galaxies with similar luminosities
and star formation rates; c) comparison with a chemical evolution model
indicates that these low metallicity galaxies are unlikely to be the
progenitors of metal-poor dwarf galaxies at z~0, but more likely the
progenitors of massive spirals; d) the z~1.4 galaxies are characterized by the
high [OIII]/[OII] line ratios, low extinction and low metallicity that are
typical of lower luminosity CADIS galaxies at 0.4<z<0.7, and of more luminous
Lyman Break Galaxies at z~3.1, but not seen in CFRS galaxies at 0.4<z<1.0; e)
the properties of the z~1.4 galaxies suggest that the period of rapid chemical
evolution takes place progressively in lower mass systems as the universe ages,
and thus provides further support for a downsizing picture of galaxy formation,
at least from z~1.4 to today.Comment: Proceedings contribution for "The Fabulous Destiny of Galaxies;
Bridging Past and Present", Marseille, 200
Optimal model parameters for multi-objective large-eddy simulations
A methodology is proposed for the assessment of error dynamics in large-eddy simulations. It is demonstrated that the optimization of model parameters with respect to one flow property can be obtained at the expense of the accuracy with which other flow properties are predicted. Therefore, an approach is introduced which allows to assess the total errors based on various flow properties simultaneously. We show that parameter settings exist, for which all monitored errors are "near optimal," and refer to such regions as "multi-objective optimal parameter regions." We focus on multi-objective errors that are obtained from weighted spectra, emphasizing both large- as well small-scale errors. These multi-objective optimal parameter regions depend strongly on the simulation Reynolds number and the resolution. At too coarse resolutions, no multi-objective optimal regions might exist as not all error-components might simultaneously be sufficiently small. The identification of multi-objective optimal parameter regions can be adopted to effectively compare different subgrid models. A comparison between large-eddy simulations using the Lilly-Smagorinsky model, the dynamic Smagorinsky model and a new Re-consistent eddy-viscosity model is made, which illustrates this. Based on the new methodology for error assessment the latter model is found to be the most accurate and robust among the selected subgrid models, in combination with the finite volume discretization used in the present study
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