16,950 research outputs found
An update on the double cascade scenario in two-dimensional turbulence
Statistical features of homogeneous, isotropic, two-dimensional turbulence is
discussed on the basis of a set of direct numerical simulations up to the
unprecedented resolution . By forcing the system at intermediate
scales, narrow but clear inertial ranges develop both for the inverse and for
direct cascades where the two Kolmogorov laws for structure functions are, for
the first time, simultaneously observed. The inverse cascade spectrum is found
to be consistent with Kolmogorov-Kraichnan prediction and is robust with
respect the presence of an enstrophy flux. The direct cascade is found to be
more sensible to finite size effects: the exponent of the spectrum has a
correction with respect theoretical prediction which vanishes by increasing the
resolution
Taxation and the Location of U.S. Investment Abroad
Tax policy toward the overseas income of U.S. firms is an important issue since foreign investment accounts for a sizabLe fraction of total investment by U.S. firms. At present there is no consensus on the degree to which U.S. firms respond to tax incentives when making international investment decisions. This paper seeks to shed light on this issue. Because the tax systems of (at least) two countries are involved,the specification of tax incentives is far from trivial. For example, U.S.treatment is based on the foreign tax credit mechanism. In its purest form,this mechanism would insure that the net tax rate on all income of U.S. firms would be equal to the U.S. rate, rendering the tax rates in the host countries irrelevant. In fact, actual U.S. tax practice is far removed from an idealized foreign tax credit mechanism. For instance the U.S. tax is not collected until income is repatriated from abroad; section I points out that deferral changes the incentive effects in fundamental ways. Foreign income tax rates definitely do matter in theory; in fact, they may be of overriding importance.The remainder of the paper seeks to test these theoretical considerations. First,we describe the cross-section data that were collected for this purpose. Then, we report the result that U.S. firms respond to net rates of return in general and to properly specified tax rates in particular.
Performance characteristics of wind profiling radars
Doppler radars used to measure winds in the troposphere and lower stratosphere for weather analysis and forecasting are lower-sensitivity versions of mesosphere-stratosphere-troposphere radars widely used for research. The term wind profiler is used to denote these radars because measurements of vertical profiles of horizontal and vertical wind are their primary function. It is clear that wind profilers will be in widespread use within five years: procurement of a network of 30 wind profilers is underway. The Wave Propagation Laboratory (WPL) has operated a small research network of radar wind profilers in Colorado for about two and one-half years. The transmitted power and antenna aperture for these radars is given. Data archiving procedures have been in place for about one year, and this data base is used to evaluate the performance of the radars. One of the prime concerns of potential wind profilers users is how often and how long wind measurements are lacking at a given height. Since these outages constitute an important part of the performance of the wind profilers, they are calculated at three radar frequencies, 50-, 405-, and 915-MHz, (wavelengths of 6-, 0.74-, and 0.33-m) at monthly intervals to determine both the number of outages at each frequency and annual variations in outages
Computational coarse graining of a randomly forced 1-D Burgers equation
We explore a computational approach to coarse graining the evolution of the
large-scale features of a randomly forced Burgers equation in one spatial
dimension. The long term evolution of the solution energy spectrum appears
self-similar in time. We demonstrate coarse projective integration and coarse
dynamic renormalization as tools that accelerate the extraction of macroscopic
information (integration in time, self-similar shapes, and nontrivial dynamic
exponents) from short bursts of appropriately initialized direct simulation.
These procedures solve numerically an effective evolution equation for the
energy spectrum without ever deriving this equation in closed form.Comment: 21 pages, 7 figure
Identification of the Atomic Scale Structures of the Gold-Thiol Interfaces of Molecular Nanowires by Inelastic Tunneling Spectroscopy
We examine theoretically the effects of the bonding geometries at the
gold-thiol interfaces on the inelastic tunneling spectra of propanedithiolate
(PDT) molecules bridging gold electrodes and show that inelastic tunneling
spectroscopy combined with theory can be used to determine these bonding
geometries experimentally. With the help of density functional theory, we
calculate the relaxed geometries and vibrational modes of extended molecules
each consisting of one or two PDT molecules connecting two gold nanoclusters.
We formulate a perturbative theory of inelastic tunneling through molecules
bridging metal contacts in terms of elastic transmission amplitudes, and use
this theory to calculate the inelastic tunneling spectra of the gold-PDT-gold
extended molecules. We consider PDT molecules with both trans and gauche
conformations bound to the gold clusters at top, bridge and hollow bonding
sites. Comparing our results with the experimental data of Hihath et al. [Nano
Lett. 8, 1673 (2008)], we identify the most frequently realized conformation in
the experiment as that of trans molecules top-site bonded to both electrodes.
We find the switching from the 42 meV vibrational mode to the 46 meV mode
observed in the experiment to be due to the transition of trans molecules from
mixed top-bridge to pure top-site bonding geometries. Our results also indicate
that gauche molecular conformations and hollow site bonding did not contribute
significantly to the experimental inelastic tunneling spectra. For pairs of PDT
molecules connecting the gold electrodes in parallel we find total elastic
conductances close to twice those of single molecules bridging the contacts
with similar bonding conformations and small splittings of the vibrational mode
energies for the modes that are the most sensitive to the molecule-electrode
bonding geometries.Comment: 14 pages, 8 figures, 1 table. arXiv admin note: significant text
overlap with arXiv:1103.2378;
http://jcp.aip.org/resource/1/jcpsa6/v136/i1/p014703_s
Unusual conductance of polyyne-based molecular wires
We report a full self-consistent ab initio calculation of the current-voltage
curve and the conductance of thiolate capped polyynes in contact with gold
electrodes. We find the conductance of polyynes an order of magnitude larger
compared with other conjugated oligomers. The reason lies in the position of
the Fermi level deep in the HOMO related resonance. With the conductance weakly
dependent on the applied bias and almost independent of the length of the
molecular chain, polyynes appear as nearly perfect molecular wires.Comment: 4 pages, 5 figures, 3 table
Experimental evidence of accelerated energy transfer in turbulence
We investigate the vorticity dynamics in a turbulent vortex using scattering
of acoustic waves. Two ultrasonic beams are adjusted to probe simultaneously
two spatial scales in a given volume of the flow, thus allowing a dual channel
recording of the dynamics of coherent vorticity structures. Our results show
that this allows to measure the average energy transfer time between different
spatial length scales, and that such transfer goes faster at smaller scales.Comment: 5 pages, 5 figure
Consequences of a Change in the Galactic Environment of the Sun
The interaction of the heliosphere with interstellar clouds has attracted
interest since the late 1920's, both with a view to explaining apparent
quasi-periodic climate "catastrophes" as well as periodic mass extinctions.
Until recently, however, models describing the solar wind - local interstellar
medium (LISM) interaction self-consistently had not been developed. Here, we
describe the results of a two-dimensional (2D) simulation of the interaction
between the heliosphere and an interstellar cloud with the same properties as
currently, except that the neutral H density is increased from the present
value of n(H) ~ 0.2 cm^-3 to 10 cm^-3. The mutual interaction of interstellar
neutral hydrogen and plasma is included. The heliospheric cavity is reduced
considerably in size (approximately 10 - 14 AU to the termination shock in the
upstream direction) and is highly dynamical. The interplanetary environment at
the orbit of the Earth changes markedly, with the density of interstellar H
increasing to ~2 cm^-3. The termination shock itself experiences periods where
it disappears, reforms and disappears again. Considerable mixing of the shocked
solar wind and LISM occurs due to Rayleigh-Taylor-like instabilities at the
nose, driven by ion-neutral friction. Implications for two anomalously high
concentrations of 10Be found in Antarctic ice cores 33 kya and 60 kya, and the
absence of prior similar events, are discussed in terms of density enhancements
in the surrounding interstellar cloud. The calculation presented here supports
past speculation that the galactic environment of the Sun moderates the
interplanetary environment at the orbit of the Earth, and possibly also the
terrestrial climate.Comment: 23 pages, 2 color plates (jpg), 3 figures (eps
Universal decay of scalar turbulence
The asymptotic decay of passive scalar fields is solved analytically for the
Kraichnan model, where the velocity has a short correlation time. At long
times, two universality classes are found, both characterized by a distribution
of the scalar -- generally non-Gaussian -- with global self-similar evolution
in time. Analogous behavior is found numerically with a more realistic flow
resulting from an inverse energy cascade.Comment: 4 pages, 3 Postscript figures, submitted to PR
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