Andreï Kolmogorov

Résumé

Andreï Nikolaïevitch Kolmogorov ( ; à Tambov – à Moscou) est un mathématicien russe et soviétique qui a apporté des contributions significatives en mathématiques, notamment en théorie des probabilités, topologie, turbulence, mécanique classique, logique intuitionniste, théorie algorithmique de l'information et en analyse de la complexité des algorithmes. Biographie Enfance Kolmogorov est né à Tambov en 1903. Sa mère Maria Iakovlevna Kolmogorova (1871-1903), célibataire, meurt en accouchant. Elle est la benjamine d'une fratrie dont l'aînée est décédée elle aussi. Andreï est récupéré par son grand-père maternel, un homme noble et prospère qui n'avait pas accepté, semble-t-il, le mariage de sa fille avec un simple ingénieur agronome issu d'une famille modeste. Son père Nikolaï Matveïevitch Kataïev, lui rend visite de temps à autre, mais il est déporté de Saint-Pétersbourg pour sa participation au mouvement révolutionnaire et disparaît, probablement tué pendant la

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Concepts associés (29)

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Séances de cours associées (10)

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We study the behavior of solutions to the incompressible 2d Euler equations near two canonical shear flows with critical points, the Kolmogorov and Poiseuille flows, with consequences for the associated Navier-Stokes problems. We exhibit a large family of new, non-trivial stationary states of analytic regularity, that are arbitrarily close to the Kolmogorov flow on the square torus

\T^2

. This situation contrasts strongly with the setting of some monotone shear flows, such as the Couette flow: in both cases the linearized problem exhibits an "inviscid damping" mechanism that leads to relaxation of perturbations of the base flows back to nearby shear flows. While this effect persists nonlinearly for suitably small and regular perturbations of some monotone shear flows, for the Kolmogorov flow our result shows that this is not possible. Our construction of these stationary states builds on a degeneracy in the global structure of the Kolmogorov flow on

\T^2

. In this regard both the Kolmogorov flow on a rectangular torus and the Poiseuille flow in a channel are very different, and we show that the only stationary states near them must indeed be shears, even in relatively low regularity

H^3

resp.

H^{5+}

. In addition, we show that this behavior is mirrored closely in the related Navier-Stokes settings: the linearized problems near the Poiseuille and Kolmogorov flows both exhibit an enhanced rate of dissipation. Previous work by us and others shows that this effect survives in the full, nonlinear problem near the Poiseuille flow and near the Kolmogorov flow on rectangular tori, provided that the perturbations lie below a certain threshold. However, we show here that the corresponding result cannot hold near the Kolmogorov flow on

\T^2

2020

Chargement

, ,

The ensemble Kalman filter is a computationally efficient technique to solve state and/or parameter estimation problems in the framework of statistical inversion when relying on a Bayesian paradigm. Unfortunately its cost may become moderately large for systems de- scribed by nonlinear time-dependent PDEs, because of the cost entailed by each PDE query. In this paper we propose a reduced basis ensemble Kalman filter technique to address the above problems. The reduction stage yields intrinsic approximation errors, whose propagation through the filtering process might affect the accuracy of state/parameter estimates. For an efficient evaluation of these errors, we equip our reduced basis ensemble Kalman filter with a reduction error model (or error surrogate). The latter is based on ordinary kriging for functional-valued data, to gauge the effect of state reduction on the whole filtering process. The accuracy and efficiency of the resulting method is then verified on the estimation of uncer- tain parameters for a FitzHugh-Nagumo model and uncertain fields for a Fisher-Kolmogorov model.

MATHICSE2019

Chargement

In this paper, we consider large eddy simulations (LES) for human stenotic carotids in presence of atheromasic plaque, a pathological condition where transitional effects to turbulence may occur, with relevant clinical implications such as plaque rupture. We provide a reference numerical solution obtained at high resolution without any subgrid scale model, to be used to assess the accuracy of LES simulations. In the context we are considering, ie, hemodynamics, we cannot refer to a statistically homogeneous, isotropic, and stationary turbulent regime; hence, the classical Kolmogorov theory cannot be used. For this reason, a mesh size and a time step are deemed fine enough if they allow to capture all the features of the velocity field in the shear layers developed after the bifurcation. To assess these requirements, we consider a simplified model of the evolution of a 2D shear layer, a relevant process in the formation of transitional effects in our case. Then, we compare the results of LES sigma model (both static and dynamic) and mixed LES models (where also a similarity contribution is considered). In particular, we consider a realistic scenario of a human carotid, and we use the reference solution as gold standard. The results highlight the accuracy of the LES sigma models, especially for the static model.

Wiley2017

Chargement

\T^2

\T^2

H^3

resp.

H^{5+}

\T^2

2020