991 research outputs found
Energy potential of a tidal fence deployed near a coastal headland
Enhanced tidal streams close to coastal headlands appear to present ideal locations for the deployment of tidal energy devices. In this paper, the power potential of tidal streams near an idealized coastal headland with a sloping seabed is investigated using a near-field approximation to represent a tidal fence, i.e. a row of tidal devices, in a two-dimensional depth-averaged numerical model. Simulations indicate that the power extracted by the tidal fence is limited because the flow will bypass the fence, predominantly on the ocean side, as the thrust applied by the devices increases. For the dynamic conditions, fence placements and headland aspect ratios considered, the maximum power extracted at the fence is not related in any obvious way to the local undisturbed kinetic flux or the natural rate of energy dissipation due to bed friction (although both of these have been used in the past to predict the amount of power that may be extracted). The available power (equal to the extracted power net of vertical mixing losses in the immediate wake of devices) is optimized for devices with large area and small centre-to-centre spacing within the fence. The influence of energy extraction on the natural flow field is assessed relative to changes in the M2 component of elevation and velocity, and residual bed shear stress and tidal dispersion
Modelling tidal energy extraction in a depth-averaged coastal domain
An extension of actuator disc theory is used to describe the properties of a tidal energy device, or row of tidal energy devices, within a depth-averaged numerical model. This approach allows a direct link to be made between an actual tidal device and its equivalent momentum sink in a depth-averaged domain. Extended actuator disc theory also leads to a measure of efficiency for an energy device in a tidal stream of finite Froude number, where efficiency is defined as the ratio of power extracted by one or more tidal devices to the total power removed from the tidal stream. To demonstrate the use of actuator disc theory in a depth-averaged model, tidal flow in a simple channel is approximated using the shallow water equations and the results are compared with the published analytical solutions. © 2010 © The Institution of Engineering and Technology
On the Applicability of Temperature and Precipitation Data from CMIP3 for China
Global Circulation Models (GCMs) contributed to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) and are widely used in global change research. This paper assesses the performance of the AR4 GCMs in simulating precipitation and temperature in China from 1960 to 1999 by comparison with observed data, using system bias (B), root-mean-square error (RMSE), Pearson correlation coefficient (R) and Nash-Sutcliffe model efficiency (E) metrics. Probability density functions (PDFs) are also fitted to the outputs of each model. It is shown that the performance of each GCM varies to different degrees across China. Based on the skill score derived from the four metrics, it is suggested that GCM 15 (ipsl_cm4) and GCM 3 (cccma_cgcm_t63) provide the best representations of temperature and precipitation, respectively, in terms of spatial distribution and trend over 10 years. The results also indicate that users should apply carefully the results of annual precipitation and annual temperature generated by AR4 GCMs in China due to poor performance. At a finer scale, the four metrics are also used to obtain best fit scores for ten river basins covering mainland China. Further research is proposed to improve the simulation accuracy of the AR4 GCMs regarding China
Legendrian Distributions with Applications to Poincar\'e Series
Let be a compact Kahler manifold and a quantizing holomorphic
Hermitian line bundle. To immersed Lagrangian submanifolds of
satisfying a Bohr-Sommerfeld condition we associate sequences , where is a
holomorphic section of . The terms in each sequence concentrate
on , and a sequence itself has a symbol which is a half-form,
, on . We prove estimates, as , of the norm
squares in terms of . More generally, we show that if and
are two Bohr-Sommerfeld Lagrangian submanifolds intersecting
cleanly, the inner products have an
asymptotic expansion as , the leading coefficient being an integral
over the intersection . Our construction is a
quantization scheme of Bohr-Sommerfeld Lagrangian submanifolds of . We prove
that the Poincar\'e series on hyperbolic surfaces are a particular case, and
therefore obtain estimates of their norms and inner products.Comment: 41 pages, LaTe
Evolution of particle-scale dynamics in an aging clay suspension
Multispeckle x-ray photon correlation spectroscopy was employed to
characterize the slow dynamics of a colloidal suspension formed by
highly-charged, nanometer-sized disks. At scattering wave vectors
corresponding to interparticle length scales, the dynamic structure factor
follows a form ], where
1.5. The characteristic relaxation time increases with the sample age
approximately as and decreases with
approximately as . Such a compressed exponential decay with
relaxation time that varies inversely with is consistent with recent models
that describe the dynamics in disordered elastic media in terms of strain from
random, local structural rearrangements. The amplitude of the measured decay in
varies with in a manner that implies caged particle motion at
short times. The decrease in the range of this motion and an increase in
suspension conductivity with increasing indicate a growth in the
interparticle repulsion as the mechanism for internal stress development
implied by the models.Comment: 4 pages, includes 4 postscript figures; accepted for publication in
Phys Rev Let
Semiclassical almost isometry
Let M be a complex projective manifold, and L an Hermitian ample line bundle
on it. A fundamental theorem of Gang Tian, reproved and strengthened by
Zelditch, implies that the Khaeler form of L can be recovered from the
asymptotics of the projective embeddings associated to large tensor powers of
L. More precisely, with the natural choice of metrics the projective embeddings
associated to the full linear series |kL| are asymptotically symplectic, in the
appropriate rescaled sense. In this article, we ask whether and how this result
extends to the semiclassical setting. Specifically, we relate the Weinstein
symplectic structure on a given isodrastic leaf of half-weighted
Bohr-Sommerfeld Lagrangian submanifolds of M to the asymptotics of the the
pull-back of the Fubini-Study form under the semiclassical projective maps
constructed by Borthwick, Paul and Uribe.Comment: exposition improve
Computational NMR investigation of mixed-metal (Al,Sc)-MIL-53 and its phase transitions
Funding: The authors would like to thank the ERC (Advanced Grant 787073 ADOR) and the Allan Handsel Postgraduate Research Scholarship for Chemistry for studentship funding for ZHD and EALB, respectively. We also acknowledge support from the Collaborative Computational Project on NMR Crystallography (CCP-NC) funded by EPSRC (EP/T026642/1) and from the UK Materials and Molecular Modelling Hub (Young), which is partially funded by EPSRC (EP/T022213/1, EP/W032260/1 and EP/P020194/1) for which access was obtained via the UKCP consortium and funded by EPSRC (EP/P022561/1).Compositionally complex metal-organic frameworks (MOFs) have properties that depend on local structure that is often difficult to characterise. In this paper a density functional theory (DFT) computational study of mixed-metal (Al,Sc)-MIL-53, a flexible MOF with several different forms, was used to calculate the relative energetics of these forms and to predict NMR parameters that can be used to evaluate whether solid-state NMR spectroscopy can be used to differentiate, identify and characterise the forms adopted by mixed-metal MOFs of different composition. The NMR parameters can also be correlated with structural features in the different forms, giving fundamental insight into the nature and origin of the interactions that affect nuclear spins. Given the complexity of advanced NMR experiments required, and the potential need for expensive and difficult isotopic enrichment, the computational work is invaluable in predicting which experiments and approaches are likely to give the most information on the disorder, local structure and pore forms of these mixed-metal MOFs.Publisher PDFPeer reviewe
Eigenvalues of Laplacian with constant magnetic field on non-compact hyperbolic surfaces with finite area
We consider a magnetic Laplacian on a
noncompact hyperbolic surface \mM with finite area. is a real one-form
and the magnetic field is constant in each cusp. When the harmonic
component of satifies some quantified condition, the spectrum of
is discrete. In this case we prove that the counting function of
the eigenvalues of satisfies the classical Weyl formula, even
when $dA=0.
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