112 research outputs found
Spectral theory for the failure of linear control in a nonlinear stochastic system
We consider the failure of localized control in a nonlinear spatially
extended system caused by extremely small amounts of noise. It is shown that
this failure occurs as a result of a nonlinear instability. Nonlinear
instabilities can occur in systems described by linearly stable but strongly
nonnormal evolution operators. In spatially extended systems the nonnormality
manifests itself in two different but complementary ways: transient
amplification and spectral focusing of disturbances. We show that temporal and
spatial aspects of the nonnormality and the type of nonlinearity are all
crucially important to understanding and describing the mechanism of nonlinear
instability. Presented results are expected to apply equally to other physical
systems where strong nonnormality is due to the presence of mean flow rather
than the action of control.Comment: Submitted to Physical Review
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Seasonal tropical cyclone forecasts
Seasonal forecasts of tropical cyclone activity in various regions have been developed since the first attempts in the early 1980s by Neville Nicholls (1979) for the Australian region and William Gray (1984(a), (b)) for the North Atlantic region. Over time, forecasts for different regions, using differing methodologies, have been developed. Tourism in various regions, such as the US Gulf and East Coasts and the Caribbean, is impacted by these seasonal forecasts. Insurance and re-insurance companies also make use of seasonal forecasts in their policy decisions. It is fundamental to provide these users with information about the accuracy of seasonal forecasts. Seasonal forecasts have limited use for emergency managers, because of the lack of skill in predicting impacts at the city or county level
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ONSET OF THE RAINY SEASON OVER TROPICAL BRAZIL IN OBSERVATIONS AND A GENERAL CIRCULATION MODEL SIMULATION
The intent of this paper is to present and document the hypothesis that the influence of sea surface temperature (SST) on rainfall in tropical Brazil is largely through its influence on onset and end of the rainy season, rather than on rain rates during the rainy season itself. The observations are compared with one 50 year run of the Euro- pean Centre / Hamburg Atmospheric Model ver- sion 4.5 general circulation model (GCM), although in this abstract, although only the obser- vations are shown
Stochastic magnetohydrodynamic turbulence in space dimensions
Interplay of kinematic and magnetic forcing in a model of a conducting fluid
with randomly driven magnetohydrodynamic equations has been studied in space
dimensions by means of the renormalization group. A perturbative
expansion scheme, parameters of which are the deviation of the spatial
dimension from two and the deviation of the exponent of the powerlike
correlation function of random forcing from its critical value, has been used
in one-loop approximation. Additional divergences have been taken into account
which arise at two dimensions and have been inconsistently treated in earlier
investigations of the model. It is shown that in spite of the additional
divergences the kinetic fixed point associated with the Kolmogorov scaling
regime remains stable for all space dimensions for rapidly enough
falling off correlations of the magnetic forcing. A scaling regime driven by
thermal fluctuations of the velocity field has been identified and analyzed.
The absence of a scaling regime near two dimensions driven by the fluctuations
of the magnetic field has been confirmed. A new renormalization scheme has been
put forward and numerically investigated to interpolate between the
expansion and the double expansion.Comment: 12 pages, 4 figure
Design-controlled synthesis of IrO2 sub-monolayers on Au nanoflowers: Marrying plasmonic and electrocatalytic properties
We develop herein plasmonic–catalytic Au–IrO2 nanostructures with a morphology optimized for efficient light harvesting and catalytic surface area; the nanoparticles have a nanoflower morphology, with closely spaced Au branches all partially covered by an ultrathin (1 nm) IrO2 shell. This nanoparticle architecture optimizes optical features due to the interactions of closely spaced plasmonic branches forming electromagnetic hot spots, and the ultra-thin IrO2 layer maximizes efficient use of this expensive catalyst. This concept was evaluated towards the enhancement of the electrocatalytic performances towards the oxygen evolution reaction (OER) as a model transformation. The OER can play a central role in meeting future energy demands but the performance of conventional electrocatalysts in this reaction is limited by the sluggish OER kinetics. We demonstrate an improvement of the OER performance for one of the most active OER catalysts, IrO2, by harvesting plasmonic effects from visible light illumination in multimetallic nanoparticles. We find that the OER activity for the Au–IrO2 nanoflowers can be improved under LSPR excitation, matching best properties reported in the literature. Our simulations and electrocatalytic data demonstrate that the enhancement in OER activities can be attributed to an electronic interaction between Au and IrO2 and to the activation of Ir–O bonds by LSPR excited hot holes, leading to a change in the reaction mechanism (rate-determinant step) under visible light illumination
De narizes extraÃdos por Machado: eugenias raciais, traços faciais e teorias psiquiátricas no Brasil oitocentista
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