24,758 research outputs found
A framework for the evaluation of turbulence closures used in mesoscale ocean large-eddy simulations
We present a methodology to determine the best turbulence closure for an
eddy-permitting ocean model through measurement of the error-landscape of the
closure's subgrid spectral transfers and flux. We apply this method to 6
different closures for forced-dissipative simulations of the barotropic
vorticity equation on a f-plane (2D Navier-Stokes equation). Using a
high-resolution benchmark, we compare each closure's model of energy and
enstrophy transfer to the actual transfer observed in the benchmark run. The
error-landscape norms enable us to both make objective comparisons between the
closures and to optimize each closure's free parameter for a fair comparison.
The hyper-viscous closure most closely reproduces the enstrophy cascade,
especially at larger scales due to the concentration of its dissipative effects
to the very smallest scales. The viscous and Leith closures perform nearly as
well, especially at smaller scales where all three models were dissipative. The
Smagorinsky closure dissipates enstrophy at the wrong scales. The anticipated
potential vorticity closure was the only model to reproduce the upscale
transfer of kinetic energy from the unresolved scales, but would require
high-order Laplacian corrections in order to concentrate dissipation at the
smallest scales. The Lagrangian-averaged alpha-model closure did not perform
successfully for forced 2D isotropic Navier-Stokes: small-scale filamentation
is only slightly reduced by the model while small-scale roll-up is prevented.
Together, this reduces the effects of diffusion.Comment: 44 pages, 21 figures, 1 Appendix, submitted to Ocean Modelin
Oil Prices, Profits, and Recessions : An Inquiry Using Terrorism as an Instrumental Variable
Nearly all post-war recessions have been preceded by oil-price shocks, but is this because spikes in the price of petroleum cause economic downturns? Most research has ignored an identification problem : oil prices and the state of the world economy are endogenously determined. This paper uses terrorist incidents as an instrumental variable. In an international panel of industries, we show that after correction for simultaneity bias â though not before â the price of oil has large negative effects upon profitability. Our results seem to lend support to the claim that oil-price spikes can be a source of recessions.
Coherent exciton dynamics in the presence of underdamped vibrations
Recent ultrafast optical experiments show that excitons in large biological
light-harvesting complexes are coupled to molecular vibration modes. These
high-frequency vibrations will not only affect the optical response, but also
drive the exciton transport. Here, using a model dimer system, the frequency of
the underdamped vibration is shown to have a strong effect on the exciton
dynamics such that quantum coherent oscillations in the system can be present
even in the case of strong noise. Two mechanisms are identified to be
responsible for the enhanced transport efficiency: critical damping due to the
tunable effective strength of the coupling to the bath, and resonance coupling
where the vibrational frequency coincides with the energy gap in the system.
The interplay of these two mechanisms determines parameters responsible for the
most efficient transport, and these optimal control parameters are comparable
to those in realistic light-harvesting complexes. Interestingly, oscillations
in the excitonic coherence at resonance are suppressed in comparison to the
case of an off-resonant vibration
Cancellation exponent and multifractal structure in two-dimensional magnetohydrodynamics: direct numerical simulations and Lagrangian averaged modeling
We present direct numerical simulations and Lagrangian averaged (also known
as alpha-model) simulations of forced and free decaying magnetohydrodynamic
turbulence in two dimensions. The statistics of sign cancellations of the
current at small scales is studied using both the cancellation exponent and the
fractal dimension of the structures. The alpha-model is found to have the same
scaling behavior between positive and negative contributions as the direct
numerical simulations. The alpha-model is also able to reproduce the time
evolution of these quantities in free decaying turbulence. At large Reynolds
numbers, an independence of the cancellation exponent with the Reynolds numbers
is observed.Comment: Finite size box effects have been taken into account in the
definition of the partition function. This has resulted in a more clear
scaling in all figures. Several points are clarified in the tex
Systematic derivation of a rotationally covariant extension of the 2-dimensional Newell-Whitehead-Segel equation
An extension of the Newell-Whitehead-Segel amplitude equation covariant under
abritrary rotations is derived systematically by the renormalization group
method.Comment: 8 pages, to appear in Phys. Rev. Letters, March 18, 199
On the number of unlabeled vertices in edge-friendly labelings of graphs
Let be a graph with vertex set and edge set , and be a
0-1 labeling of so that the absolute difference in the number of edges
labeled 1 and 0 is no more than one. Call such a labeling
\emph{edge-friendly}. We say an edge-friendly labeling induces a \emph{partial
vertex labeling} if vertices which are incident to more edges labeled 1 than 0,
are labeled 1, and vertices which are incident to more edges labeled 0 than 1,
are labeled 0. Vertices that are incident to an equal number of edges of both
labels we call \emph{unlabeled}. Call a procedure on a labeled graph a
\emph{label switching algorithm} if it consists of pairwise switches of labels.
Given an edge-friendly labeling of , we show a label switching algorithm
producing an edge-friendly relabeling of such that all the vertices are
labeled. We call such a labeling \textit{opinionated}.Comment: 7 pages, accepted to Discrete Mathematics, special issue dedicated to
Combinatorics 201
Renormalization Group Method and Reductive Perturbation Method
It is shown that the renormalization group method does not necessarily
eliminate all secular terms in perturbation series to partial differential
equations and a functional subspace of renormalizable secular solutions
corresponds to a choice of scales of independent variables in the reductive
perturbation method.Comment: 5 pages, late
Derivation of Amplitude Equations by Renormalization Group Method
A proper formulation in the perturbative renormalization group method is
presented to deduce amplitude equations. The formulation makes it possible not
only avoiding a serious difficulty in the previous reduction to amplitude
equations by eliminating all of the secular terms but also consistent
derivation of higher-order correction to amplitude equations.Comment: 6 page, revte
Introduction to the Special Issue on "Satellite Altimetry: New Sensors and New Applications"
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