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

The aim of this paper is the calculation, at high frequencies, of the diffracted pressure of an acoustic wave by axisymmetrical rigid surfaces. Using an integral formulation, we present herein a mixed approach based on the asymptotic expansion theory, in order to reduce the space of the integration domain, and on the collocation method, used to improve the numerical computation. This numerical software leads to an interesting saving of computation time. This approach is developed in the study of an analytical case.

The paper deals with a comparison between experimental data obtained by Blanc-Benon (1984) and theoretical predictions of the coherence function of a spherical sound wave after passing through a turbulent jet. The predictions are based on the theory of sound propagation in moving random media recently developed by Ostashev (1994, 1997), which correctly takes into account the effects of an isotropic vector random field of the medium velocity fluctuations on line-of-sight sound propagation. It is shown that the theoretical predictions fit the data well.

The concentration fluctuation PDF resulting from the transient turbulent mixing of a scalar blob exhibits rapidly a decreasing exponential shape, parametrised by a mixing time t_s = 0.12 d/u ln(5Sc) which depends on the injection time of the blob in the medium d/u and on the Schmidt number Sc.

We use the fractional dispersion concept to analyse the anomalous diffusion regime shown in a tracer dispersion in a hydrodynamical Taylor-Couette experiment. The anomalous diffusion is characterized with a parameter a (diffusion exponent), equal to two for the Gaussian diffusion. The results of the fractional advection-diffusion equa lion are compared to the concentration curves of the residence time distributions obtained in the experiment and to those from a coupled advection-diffusion model. The diffusion exponent, the generalized diffusion coefficient and the axial velocity are evaluated by fit.

The linear temporal three-dimensional instability of incompressible viscous flow in a rotating divergent channel is studied. The boundary between stable and unstable regions is plotted as a function of the different physical parameters of the problem. It is shown that the instability is caused by the channel rotation when the half-angle of the channel, α, is small and it is caused by the channel expansion for large α. When α is taken constant, we found that rotation has the same effect as in plane channel flow, where the basic flow is destabilized by weak rotation and stabilized by strong rotation.

A formulation is presented based on the reciprocity principle in which radiative budgets are expressed as sums of Net Exchange Rates between all volume and surface elements. On this basis, a Monte-Carlo Method has been developed that proves to be numerically very efficient compared to standard Analogue Monte-Carlo Methods. It was used for computation of radiative budgets inside a two-dimensional cavity field with a participating gas. Strongly irregular grids and a wide range of optical thicknesses can be studied with this method.

The static extreme-load approach, under certain conditions, appears to be the solution of an unconstrained discrete minimax problem. It is shown that it is possible to eliminate the operator Max in this problem. The subsequent problem involves minimizing a regularized function without constraints. An application to the determination of the macroscopic strength of a punched material exhibits the efficiency of this method.

Dynamic loadings produce high stress waves leading to the fragmentation of brittle materials such as ceramics. The main mechanism used to explain the change of the number of fragments with stress rate is an obscuration phenomenon. After the presentation of a probabilistic approach, the evolution of the number of nucleated flaws and a damage model are derived.

Chemical plugging by scale precipitation is a major problem with localised irrigation techniques. Chronoelectrogravimetry allows the evaluation of the incrusting character of waters. The method was applied to irrigation waters of the Cap Bon region.