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Natural convection is investigated numerically and experimentally in a cylindrical annulus. The governing equations based on primitive variables are solved using Chorin’s method. In addition to the unicellular flows reported in the literature, depending on initial conditions, bicellular flows are observed for high Rayleigh numbers. The bifurcation point is determined numerically. The velocity field for unicellular flows is measured by laser-Doppler anemometry in an air-filled annulus. A perturbation solution is also presented. The experimental results are in good agreement with numerical predictions and the perturbation solution.

This work makes explicit an algebraic expression giving the matrix of transient influence coefficients associated with a one-dimensional semi-confined aquifer model. The domain studied is divided into a series of connected and completely mixed compartments over which the governing equation is discretized. The discrete equations obtained are solved for the compartmental hydraulic head and used to derive the algebraic expression in question. The basic properties of the so-called algebraic influence coefficients are investigated. In particular, their consistency with the exact Green function is highlighted. Finally, the newly derived influence coefficients are applied to a simplified aquifer system in order to formulate and solve the problem of identifying illegal groundwater pumping.

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De la société humaine, Ibn Khaldun propose un modèle très simple, mais dont on n'a pas jusqu'ici tiré toutes les conséquences. Deux états sont distingués, qui sont en même temps les deux grands stades d'évolution de l'Homme : la badàwa (état de la société agro-pastorale), première et originelle — logiquement et historiquement —, proche de la nature (at-tab' ou at-tabi'a), simple et ne réalisant que le nécessaire (ad-darûri), en un mot, « être en puissance » ; la hadâra (état de la société urbaine), seconde et dérivée, complexe, préoccupée du superflu, où s'accomplit l' « être en acte » de la société. Le destin du 'umrân (civilisation dans le sens large) est d'osciller, selon une loi implacable, entre ces deux stades.

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