77 research outputs found
Pour une politique ambitieuse des données publiques
Ce rapport prĂ©sente une Ă©tude sur la rĂ©utilisation des donnĂ©es publiques, menĂ©e pour la DĂ©lĂ©gation aux usages de lâInternet du MinistĂšre de lâEnseignement supĂ©rieur et de la Recherche dans le cadre du Master dâAction Publique de lâĂcole des Ponts ParisTech.
Il met en perspective la problĂ©matique et les enjeux de lâOpen Data, propose un Ă©tat des lieux de la rĂ©utilisation des donnĂ©es publiques en France, et dessine trois scĂ©narios prospectifs pour lâĂ©volution future de ce mouvement. Elle prĂ©sente seize propositions pour une politique nationale ambitieuse dâouverture et de rĂ©utilisation des donnĂ©es publiques. Quatre Ă©lĂšves de lâĂcole des Ponts ParisTech, Pierre-Henri Bertin, Romain Lacombe, François Vauglin et Alice Vieillefosse ont menĂ© cette analyse de
septembre 2010 Ă janvier 2011, en rencontrant les acteurs clĂ©s de la rĂ©utilisation des donnĂ©es publiques, en prenant part Ă des colloques internationaux, et en sâappuyant sur la bibliographie existante
Recent Fluid Deformation closure for velocity gradient tensor dynamics in turbulence: time-scale effects and expansions
In order to model pressure and viscous terms in the equation for the
Lagrangian dynamics of the velocity gradient tensor in turbulent flows,
Chevillard & Meneveau (Phys. Rev. Lett. 97, 174501, 2006) introduced the Recent
Fluid Deformation closure. Using matrix exponentials, the closure allows to
overcome the unphysical finite-time blow-up of the well-known Restricted Euler
model. However, it also requires the specification of a decorrelation time
scale of the velocity gradient along the Lagrangian evolution, and when the
latter is chosen too short (or, equivalently, the Reynolds number is too high),
the model leads to unphysical statistics. In the present paper, we explore the
limitations of this closure by means of numerical experiments and analytical
considerations. We also study the possible effects of using time-correlated
stochastic forcing instead of the previously employed white-noise forcing.
Numerical experiments show that reducing the correlation time scale specified
in the closure and in the forcing does not lead to a commensurate reduction of
the autocorrelation time scale of the predicted evolution of the velocity
gradient tensor. This observed inconsistency could explain the unrealistic
predictions at increasing Reynolds numbers.We perform a series expansion of the
matrix exponentials in powers of the decorrelation time scale, and we compare
the full original model with a linearized version. The latter is not able to
extend the limits of applicability of the former but allows the model to be
cast in terms of a damping term whose sign gives additional information about
the stability of the model as function of the second invariant of the velocity
gradient tensor.Comment: 11 pages, 14 figures, submitted to the special issue "Fluids and
Turbulence" of Physica
Ion-ion correlations: an improved one-component plasma correction
Based on a Debye-Hueckel approach to the one-component plasma we propose a
new free energy for incorporating ionic correlations into Poisson-Boltzmann
like theories. Its derivation employs the exclusion of the charged background
in the vicinity of the central ion, thereby yielding a thermodynamically stable
free energy density, applicable within a local density approximation. This is
an improvement over the existing Debye-Hueckel plus hole theory, which in this
situation suffers from a "structuring catastrophe". For the simple example of a
strongly charged stiff rod surrounded by its counterions we demonstrate that
the Poisson-Boltzmann free energy functional augmented by our new correction
accounts for the correlations present in this system when compared to molecular
dynamics simulations.Comment: 5 pages, 2 figures, revtex styl
Lagrangian evolution of velocity increments in rotating turbulence: The effects of rotation on non-Gaussian statistics
The effects of rotation on the evolution of non-Gaussian statistics of velocity increments in rotating turbulence are studied in this paper. Following the Lagrangian evolution of the velocity increments over a fixed distance on an evolving material element, we derive a set of equations for the increments which provides a closed representation for the nonlinear interaction between the increments and the Coriolis force. Applying a restricted-Euler-type closure to the system, we obtain a system of ordinary differential equations which retains the effects of nonlinear interaction between the velocity increments and the Coriolis force. A priori tests using direct numerical simulation data show that the system captures the important dynamics of rotating turbulence. The system is integrated numerically starting from Gaussian initial data. It is shown that the system qualitatively reproduces a number of observations in rotating turbulence. The statistics of the velocity increments tend to Gaussian when strong rotation is imposed. The negative skewness in the longitudinal velocity increments is weakened by rotation. The model also predicts that the transverse velocity increment in the plane perpendicular to the rotation axis will have positive skewness, and that the skewness will depend on the Rossby number in a non-monotonic way. Based on the system, we identify the dynamical mechanisms leading to the observations. (c) 2010 Elsevier B.V. All rights reserved
A Generalization of the Stillinger-Lovett Sum Rules for the Two-Dimensional Jellium
In the equilibrium statistical mechanics of classical Coulomb fluids, the
long-range tail of the Coulomb potential gives rise to the Stillinger-Lovett
sum rules for the charge correlation functions. For the jellium model of mobile
particles of charge immersed in a neutralizing background, the fixing of
one of the -charges induces a screening cloud of the charge density whose
zeroth and second moments are determined just by the Stillinger-Lovett sum
rules. In this paper, we generalize these sum rules to the screening cloud
induced around a pointlike guest charge immersed in the bulk interior of
the 2D jellium with the coupling constant ( is the
inverse temperature), in the whole region of the thermodynamic stability of the
guest charge . The derivation is based on a mapping technique of
the 2D jellium at the coupling = (even positive integer) onto a
discrete 1D anticommuting-field theory; we assume that the final results remain
valid for all real values of corresponding to the fluid regime. The
generalized sum rules reproduce for arbitrary coupling the standard
Z=1 and the trivial Z=0 results. They are also checked in the Debye-H\"uckel
limit and at the free-fermion point . The generalized
second-moment sum rule provides some exact information about possible sign
oscillations of the induced charge density in space.Comment: 16 page
Geometrical statistics of fluid deformation: Restricted Euler approximation and the effects of pressure
The geometrical statistics of fluid deformation are analyzed theoretically within the framework of the restricted Euler approximation, and numerically using direct numerical simulations. The restricted Euler analysis predicts that asymptotically a material line element becomes an eigenvector of the velocity gradient regardless its initial orientation. The asymptotic stretching rate equals the intermediate eigenvalue of the strain rate tensor. Analyses of numerical data show that the pressure Hessian is the leading cause to destroy the alignment between the longest axis of the material element and the strongest stretching eigendirection of the strain rate. It also facilitates the alignment between the longest axis of the element and the intermediate eigendirection of the strain rate during initial evolution, but tends to oppose the alignment later
Vorticity alignment results for the three-dimensional Euler and Navier-Stokes equations
We address the problem in Navier-Stokes isotropic turbulence of why the
vorticity accumulates on thin sets such as quasi-one-dimensional tubes and
quasi-two-dimensional sheets. Taking our motivation from the work of Ashurst,
Kerstein, Kerr and Gibson, who observed that the vorticity vector
{\boldmath\omega} aligns with the intermediate eigenvector of the strain
matrix , we study this problem in the context of both the three-dimensional
Euler and Navier-Stokes equations using the variables \alpha =
\hat{{\boldmath\xi}}\cdot S\hat{{\boldmath\xi}} and {\boldmath\chi} =
\hat{{\boldmath\xi}}\times S\hat{{\boldmath\xi}} where
\hat{{\boldmath\xi}} = {\boldmath\omega}/\omega. This introduces the
dynamic angle , which lies between
{\boldmath\omega} and S{\boldmath\omega}. For the Euler equations a
closed set of differential equations for and {\boldmath\chi} is
derived in terms of the Hessian matrix of the pressure . For
the Navier-Stokes equations, the Burgers vortex and shear layer solutions turn
out to be the Lagrangian fixed point solutions of the equivalent
(\alpha,{\boldmath\chi}) equations with a corresponding angle .
Under certain assumptions for more general flows it is shown that there is an
attracting fixed point of the (\alpha,\bchi) equations which corresponds to
positive vortex stretching and for which the cosine of the corresponding angle
is close to unity. This indicates that near alignment is an attracting state of
the system and is consistent with the formation of Burgers-like structures.Comment: To appear in Nonlinearity Nov. 199
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