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

Cavitation erosion is a complex phenomenon involving the interaction of hydrodynamical, mechanical, metallurgical and chemical factors. A mathematical model is proposed to predict the erosion rate as a function of cavitation conditions and material properties.

In this paper, a systematic procedure is developed to obtain the stationary probability density function for the response of a general single-degree-of-freedom (SDOF) nonlinear oscillators under parametric and external Gaussian white-noise excitations. Wang and Zhang (1998) expressed the nonlinear function of oscillators by a polynomial formula. The nonlinear system described here has the following form: $\text{x}\text{̈}\text{+g(x,}\text{x}\text{̇}\text{)=k}{}_1\text{ξ}{}_1\text{(t)+k}{}_2\text{xξ}{}_2\text{(t)}$ , where $\text{g(x,}\text{x}\text{̇}\text{)=}\text{∑}\text{i=0}\text{∞}\text{g}{}_{\mathrm{i}}\text{(x)}\text{x}\text{̇}{}^{\mathrm{i}}$ and ξ₁,ξ₂ are Gaussian white noises. Thus, this paper is a generalization for the results obtained by Wang and Zhang (1998). The reduced Fokker–Planck (FP) equation is employed to get the governing equation of the probability density function. Based on this procedure, the exact stationary probability densities of many nonlinear stochastic oscillators are obtained, and it is shown that some of the exact stationary solutions described in the literature are only particular cases of the presented generalized results.

The noise radiated by a mixing layer is computed by an hybrid approach using linearized Euler's equations (LEE) as a wave operator. The acoustic source terms are obtained by substituting the aerodynamic fluctuating velocity field computed by the Large Eddy Simulation (LES) method into the right-hand side of the LEE. The radiated sound field as calculated in this way is in good agreement with the direct calculation obtained by LES. This study provides support to the validity of the acoustic analogy based on the LEE, both for the noise source terms and the mean flow effects on sound propagation.

We investigate the oscillation phenomena occurring when a two-dimensional plane jet emerges from a rectangular nozzle into a cavity with a body inside. It is shown that far from the lower oscillation limit, the frequencies of self-sustained oscillations are only governed by geometrical parameters. The evolutions of the Strouhal versus Reynolds number show that the effect of the geometry downstream the bluff-body is important for the control of the oscillation mechanism.

One of the sources of inaccuracy of a structural acoustic radiation calculation with a boundary element method is the inaccuracy of the vibration data used as boundary limits. When vibration data come from a finite element method calculation, these inaccuracies can be made, in particular, on the structural eigenfrequencies prevision. In this note, we propose a first approach of the problem for this effect of eigenfrequencies inaccuracies on the acoustic field calculation.

We study the nature of the instability of a binary mixture in a porous medium heated from below in the presence of an horizontal flow. The asymptotic response of the system to a localized perturbation in the both directions of the horizontal plane is evaluated. At the threshold of absolute instability, the rotational symmetry is broken by the presence of the horizontal flow and among infinity of unstable spatio–temporal modes, the system selects the traveling waves which propagate in the flow direction. We also show that in binary mixtures, contrary to pure fluids, the flow may suppress the transition from convective to absolute instability.

Using the finite element method, we study the cyclic single slip of well-oriented ferritic grains at the surface of a polycrystal. We explain some usual observations, such as an increased plastic strain and a reduced number of activated slip systems. But these computations, which do not consider a smoother hardening near the free surface, seem to underestimate the real surface effect.

In a previous paper (Boutin and Auriault 1993) the homogenization technique of multiscale expansions was used for investigating how acoustic waves may propagate in a bubbly fluid at finite concentration. Three different equivalent macroscopic behaviours where derived, for “large”, “medium” and “small” bubble systems, respectively. In the present paper, we extend the analysis by taking into consideration possible phase change effects. We show that phase change effects are negligible in the case of “large” bubbles, whereas they strongly modify the “medium” bubble system behaviour by decreasing the bulk modulus of several orders of magnitude. For “small” bubbles, capillary effects are dominating.

From experimental data obtained at a limited number of reference locations, a new method permitting to estimate the two-point spatial correlation tensor of the velocity on any mesh is presented. Firstly, a Gram-Charlier expansion is used to interpolate this tensor between reference points. Secondly, by using Proper Orthogonal Decomposition properties, this tensor is extrapolated on a larger mesh than the experimental one. An application of this procedure based on experimental turbulent plane mixing layer data is then shown.

This paper deals with a new solution suggested for dynamic balancing (shaking force and shaking moment balancing) of spatial mechanisms. The conditions for balancing are formulated by the minimization of the root-mean-square value of the shaking moment. There are considered two cases: mechanism with the input link by constant angular velocity and mechanism with the input link by variable angular velocity. The method is realized by displacement of the axis of rotation of the input link connected with the counterweight. The efficiency of the suggested method is illustrated by numerical example.