Comptes Rendus de l'Académie des Sciences - Series IIB - Mechanics-Physics-Chemistry-Astronomy最新文献

The comparison of energy levels is a classical problem in elastic buckling. In plastic buckling, this comparison can also be done through the notion of energy expense, which is an extension of the potential energy of conservative systems to dissipative systems. The difference of the energy expenses associated with the fundamental and the bifurcated branches just after bifurcation is discussed here in standard plasticity. The principal difficulty concerns bifurcation points satisfying the conditions of tangent modulus and total loading. In this particular situation, bifurcation may occur with or without elastic unloading. It is established in all cases that, as in elastic bifurcation with increasing load, the difference of energy expenses is of order 3 with respect to the modal component and that bifurcated branch has a lower energy expense under a certain condition of symmetry.

Using infrared image processing, the evolution of the heat source's distribution is determined on steel samples during monotone tension tests. We first show that one or more narrow and dissipative bands propagate along the gauge part of the sample during the plastic plateau that precedes the strain hardening. We then describe the progressive concentration of strong dissipation zones during the strain hardening, which prefigures the localized necking of the sample

The study of two-phase flows, such as liquid-solid suspensions, requires the development of adequate experimental tools. They must allow the determination of both velocity distributions for the two species mixed together. The use of the ultrasound velocimeter will provide us with the possibility of determining both fluid particles and solid suspension velocities.

The transition to unsteadiness is studied by direct numerical simulation. A direct transition from steady to chaotic behavior is observed for a Reynolds number 1 % above threshold. The convective instability of the stator boundary layer seems to be related to the direct transition to chaos. Low spatial resolution can trigger the radial convective instability of the boundary layer. It is shown that the curvature induces sharp gradients which require high spatial resolution. Nevertheless, the Hopf bifurcation responsible for the transition to periodic solutions has been located independently of mesh refinement.

After a brief review of the most important observational facts of the solar activity cycle, the basic mechanisms which are thought to be responsible for cycle generation are described with particular reference to the fundamental equations of magnetohydrodynamics. The empirical Babcock's model is illustrated as an instructive example introductory to the modern α-ω dynamo theories of the solar cycle, based on the mutual interaction of rotation with convection and magnetic fields. Different classes of α-ω dynamo theories operating in the convection zone are analyzed, starting from linear or kinematic dynamos, based on mean field electrodynamics, and ending with the description of fully hydromagnetic dynamos in which all the magnetohydrodynamical equations are numerically solved at once. The most recent efforts for modelling the boundary layer dynamos are also discussed. Conclusions are moderately optimistic.

The impingement of 15 round air jets on a plane wall leads to the formation of cells revealed by the spreading-over method on the impinged surface. The network of reattachment and detachment lines is the direct consequence of vortical structures appearing during the impingement. Their main characteristics are their horse-shoe shape and their three-dimensional aspect which is revealed by laser sheet visualisations. All these results allow us to make a schematic diagram of the flow.

With the addition of some particular layers, characterized in this note, a periodic non-absorbing optical multilayer allows us to keep the reflectance (or the transmittance) unchanged whatever the number of sequences in the coating. This invariant property, called isoreflectance (or isotransmittance) can be obtained only for a wavelength located inside the passband of the filter. Isoreflectance stretches the buffer single layer concept to multilayer and can be useful in the design of optical coatings.

This paper describes the development of a homogenization method for nonlinear elastic composites and that of an algorithm allowing us to obtain the researched homogenized behaviour without any numerical overcosts. © Académie des Sciences/Elsevier, Paris

Hypo-elasticity is a basic constituent of elastoplasticity. It is considered here in the framework of plane cyclic deformation. The variations of the stress components are studied depending on the objective derivative employed. Their value remains bounded in the general case with the Jaumann derivative. In contrast, the Green-Naghdi derivative leads to a linear increase with the number of cycles.

We present a justification of the nonlinear Koiter's shallow-shell model using an asymptotic approach. Our method needs no a priori assumptions. Putting equilibrium equations into a nondimensional form, dimensionless numbers naturally appear. These numbers characterize level forces and shell geometry. In the case of a shallow-shell subjected to moderate level forces, the nonlinear Koiter's shallow-shell model is obtained.