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

The cable model describes the electric behavior of a biological cell membrane. For positive membrane conductance its potential is stable. This model does not explicitly take into account diffusion effects. We show that if the diffusion of the different ionic species is taken into account, an instability can set in for a positive global differential conductance. The instability is then characterized by a spatial modulation of the membrane potential and the occurrence of ionic circulations.

We consider the Maxwell operator $\text{A}$ that describes 3D electromagnetic wave propagation in an infinite cylindrical wave guide with a rectangular section, where the medium is made of two homogeneous parts separated by a vertical interface. The spectral analysis of this operator points out a countable family of real numbers in the spectrum called thresholds, and the resolvent operator R_$\text{A}$(z) = ($\text{A}$ − z)⁻¹, Im (z)) ≠ 0, can be extended (‘limiting absorption principle’) continuously to the lower or upper half-planes (origin excepted) in a suitable weighted L²-topology. In particular, this continuity holds at the thresholds.

The paper includes a qualitative study of the equation of radial, nonlinear, nonadiabatic pulsations of the magnetic polytropes. The magnetic field is considered to be purely toroidal and weak. The nonadiabatic effects are described by two parameters μ related to the sources of energy and λ proportional with the damping of the energy. We find that there is only one equilibrium point for this equation. Its character is determined by μ. Whatever the values of the parameters involved, periodic orbits exist if μ > 0. These are unique and stable only if μ and λ are small. We note that in our qualitative analysis a major role is played by μ, λ is assumed to be small based on observational grounds. Numerical examples confirm and complete the qualitative analysis.

We study the flow of a liquid metal driven by thermal buoyancy in a cylindrical container. It is shown that an instability of the mixing-layer type is responsible for fluctuations of velocity and temperature when a critical value of the Grashof number is reached. This instability can be suppressed when a moderate rotating magnetic field is applied. It is shown that the Ekman pumping is responsible for the stabilisation of the initial buoyancy driven flow.

Fracture and fatigue experiments we have performed under mixed mode (I + III) and (I + II + III) loadings have shown that as the crack extends, the front gradually twists around the direction of propagation to reach a pure opening mode. This implies that the kink angle of the crack extension at the beginning of the propagation is variable along the front. The aim of this paper is to determine, in the context of linear elastic fracture mechanics, the value of this kink angle along the whole front. The values obtained are compared to the experimental ones.

Bright points, small and short lifetime structures, appear permanently in the atmosphere and could be associated with magnetic reconnections, potential sources of coronal heating. Such structures have been observed with the coronal instruments on-board SOHO and a statistical analysis was carried out in order to find the signature of very small scale structures.

A segment of the upper surface of a laminar airfoil model is submitted to forced vibrations at controlled frequency by means of piezo-electric actuators. The experiment carried out in a wind tunnel demonstrated an appreciable diminution of the profile drag for some ranges of the angle of attack and Reynolds number.

From the shallow water gravity wave theory, an analytic transformation reconstructs the velocity field in all the fluid domains. The solution obtained is more realistic and non-divergent.

Mechanisms involved in cryogenic flame stabilization are important in liquid rocket propulsion applications. These mechanisms are studied experimentally in this article by considering a single jet-flame formed by a coaxial injector fed by liquid oxygen (LOX) and gaseous hydrogen (GH₂). The flame is analyzed by imaging the light emitted by hydroxyl (OH) radicals. To characterize the reaction zone structure one takes the average of the light emission signal. The result of this operation is then treated by numerical tomography (based on the Abel transform) to get the local volumetric light intensity distribution. This method is used to study the effect of the LOX tube recess on the structure of the flame in the vicinity of the injection plane. It is shown that the recess augments the flame expansion angle and the size of the volume where the reaction takes place.

We consider slender (homogeneous, Isotropie) elastic cylinders submitted to conservative forces the intensity of which is a parameter. In the framework of the nonlinear three dimensional theory of elastostatics, by using asymptotic expansion methods, we obtain a hierarchy-depending on the order of magnitude of the applied forces-of five asymptotic unidimensional models, starting from the linear theory of flexible beams to the nonlinear theory of extensible strings. © Académie des Sciences/Elsevier, Paris