11,567 research outputs found
Relation between shear parameter and Reynolds number in statistically stationary turbulent shear flows
Studies of the relation between the shear parameter S^* and the Reynolds
number Re are presented for a nearly homogeneous and statistically stationary
turbulent shear flow. The parametric investigations are in line with a
generalized perspective on the return to local isotropy in shear flows that was
outlined recently [Schumacher, Sreenivasan and Yeung, Phys. Fluids, vol.15, 84
(2003)]. Therefore, two parameters, the constant shear rate S and the level of
initial turbulent fluctuations as prescribed by an energy injection rate
epsilon_{in}, are varied systematically. The investigations suggest that the
shear parameter levels off for larger Reynolds numbers which is supported by
dimensional arguments. It is found that the skewness of the transverse
derivative shows a different decay behavior with respect to Reynolds number
when the sequence of simulation runs follows different pathways across the
two-parameter plane. The study can shed new light on different interpretations
of the decay of odd order moments in high-Reynolds number experiments.Comment: 9 pages, 9 Postscript figure
Analysis of the High-Altitude Cooling of the Ranger SGV-770 D-4 Engine in the Bell XP-77 Airplane
No abstract availabl
Electron-spin beat susceptibility of excitons in semiconductor quantum wells
Recent time-resolved differential transmission and Faraday rotation
measurements of long-lived electron spin coherence in quantum wells displayed
intriguing parametric dependencies. For their understanding we formulate a
microscopic theory of the optical response of a gas of optically incoherent
excitons whose constituent electrons retain spin coherence, under a weak
magnetic field applied in the quantum well's plane. We define a spin beat
susceptibility and evaluate it in linear order of the exciton density. Our
results explain the many-body physics underlying the basic features observed in
the experimental measurements
On statistically stationary homogeneous shear turbulence
A statistically stationary turbulence with a mean shear gradient is realized
in a flow driven by suitable body forces. The flow domain is periodic in
downstream and spanwise directions and bounded by stress free surfaces in the
normal direction. Except for small layers near the surfaces the flow is
homogeneous. The fluctuations in turbulent energy are less violent than in the
simulations using remeshing, but the anisotropy on small scales as measured by
the skewness of derivatives is similar and decays weakly with increasing
Reynolds number.Comment: 4 pages, 5 figures (Figs. 3 and 4 as external JPG-Files
Relativistic corrections to the electromagnetic polarizabilities of compound systems
The low-energy amplitude of Compton scattering on the bound state of two
charged particles of arbitrary masses, charges and spins is calculated. A case
in which the bound state exists due to electromagnetic interaction (QED) is
considered. The term, proportional to , is obtained taking into
account the first relativistic correction. It is shown that the complete result
for this correction differs essentially from the commonly used term
, proportional to the r.m.s. charge radius of the system. We
propose that the same situation can take place in the more complicated case of
hadrons.Comment: 19 pages, LaTe
Sub-Kolmogorov-Scale Fluctuations in Fluid Turbulence
We relate the intermittent fluctuations of velocity gradients in turbulence
to a whole range of local dissipation scales generalizing the picture of a
single mean dissipation length. The statistical distribution of these local
dissipation scales as a function of Reynolds number is determined in numerical
simulations of forced homogeneous isotropic turbulence with a spectral
resolution never applied before which exceeds the standard one by at least a
factor of eight. The core of the scale distribution agrees well with a
theoretical prediction. Increasing Reynolds number causes the generation of
ever finer local dissipation scales. This is in line with a less steep decay of
the large-wavenumber energy spectra in the dissipation range. The energy
spectrum for the highest accessible Taylor microscale Reynolds number
R_lambda=107 does not show a bottleneck.Comment: 8 pages, 5 figures (Figs. 1 and 3 in reduced quality
Yang-Mills equation for stable Higgs sheaves
We establish a Kobayashi-Hitchin correspondence for the stable Higgs sheaves
on a compact Kaehler manifold. Using it, we also obtain a Kobayashi-Hitchin
correspondence for the stable Higgs G-sheaves, where G is any complex reductive
linear algebraic group
Pooling versus model selection for nowcasting with many predictors: an application to German GDP
This paper discusses pooling versus model selection for now- and forecasting in the presence of model uncertainty with large, unbalanced datasets. Empirically, unbalanced data is pervasive in economics and typically due to di¤erent sampling frequencies and publication delays. Two model classes suited in this context are factor models based on large datasets and mixed-data sampling (MIDAS) regressions with few predictors. The specification of these models requires several choices related to, amongst others, the factor estimation method and the number of factors, lag length and indicator selection. Thus, there are many sources of mis-specification when selecting a particular model, and an alternative could be pooling over a large set of models with different specifications. We evaluate the relative performance of pooling and model selection for now- and forecasting quarterly German GDP, a key macroeconomic indicator for the largest country in the euro area, with a large set of about one hundred monthly indicators. Our empirical findings provide strong support for pooling over many specifications rather than selecting a specific model. --casting,forecast combination,forecast pooling,model selection,mixed - frequency data,factor models,MIDAS
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