International audienceWe study experimentally the propagation of high-amplitude compressional waves in a chain of beads in contact, submitted or not to a small static force. In such a system, solitary waves have been theoretically predicted by Nesterenko J. Appl. Mech. Tech. Phys. USSR 5, 733 1984. We have built an impact generator in order to create high-amplitude waves in the chain. We observe the propagation of isolated nonlinear pulses, measure their velocity as a function of their maximum amplitude, for different applied static forces, and record their shape. In all experiments, we find good agreement between our observations and the theoretical predictions of the above reference, without using any adjustable parameter in the data analysis. We also show that the velocity measurements taken at three different nonzero applied static forces all lie on a single curve, when expressed in rescaled variables. The size of the pulses is typically one-tenth the total length of the chain. All the measurements support the identification of these isolated nonlinear pulses with the solitary waves predicted by Nesterenko. S1063-651X9704211-

We report the observation of intermittency in gravity-capillary wave turbulence on the surface of mercury. We measure the temporal fluctuations of surface wave amplitude at a given location. We show that the shape of the probability density function of the local slope increments of the surface waves strongly changes across the time scales. The related structure functions and the flatness are found to be power laws of the time scale on more than one decade. The exponents of these power laws increase nonlinearly with the order of the structure function. All these observations show the intermittent nature of the increments of the local slope in wave turbulence. We discuss the possible origin of this intermittency.PACS numbers: 47.35.-i, 47.52.+j, 05.45.-a One of the most striking feature of turbulence is the occurrence of bursts of intense motion within more quiescent fluid flow. This generates an intermittent behavior [1,2]. One of the quantitative characterization of intermittency is given by the probability density function (PDF) of the velocity increments between two points separated by a distance r. Starting from a roughly Gaussian PDF at integral scale, the PDFs undergo a continuous deformation when r is decreased within the inertial range and develop more and more stretched exponential tails [3]. Deviation from the Gaussian shape can be quantified by the flatness of the PDF. The origin of nonGaussian statistics in three dimensional hydrodynamic turbulence has been ascribed to the formation of strong vortices since the early work of Batchelor and Townsend [1]. However, the physical mechanism of intermittency is still an open question that motivates a lot of studies in three dimensional turbulence [4]. Intermittency has been also observed in a lot of problems involving transport by a turbulent flow for which the analytical description of the anomalous scaling laws can be obtained [5].It has been known since the work of Zakharov and collaborators that weakly interacting nonlinear waves can also display Kolmogorov type spectra related to an energy flux cascading from large to small scales [6,7]. These spectra have been analytically computed using perturbation techniques, but can also be obtained by dimensional analysis using Kolmogorov-type arguments [8]. More recently, it has been proposed that intermittency corrections should be also taken into account in wave turbulence [9] and may be connected to singularities or coherent structures [8,10] such as wave breaking [11] or whitecaps [8] in the case of surface waves. However, intermittency in wave turbulence is often related to non Gaussian statistics of low wave number Fourier amplitudes [10], thus it is not obviously related to small scale intermittency of hydrodynamic turbulence. Surprisingly, there exist only a small number of experimental studies on wave turbulence [12,13,14,15,16] compared to hydrodynamic turbulence, and to the best of our knowledge, no experimental observation of intermittency has been reported in wave turbulence.In this letter, we repo...

PACS. 46.10.+z Mechanics of discrete systems - 83.70.Fn Granular solids,

We report an experimental study of a "gas" of inelastically colliding particles, excited by vibrations in low gravity. In the case of a dilute granular medium, we observe a spatially homogeneous gaslike regime, the pressure of which scales like the 3͞2 power of the vibration velocity. When the density of the medium is increased, the spatially homogeneous fluidized state is no longer stable but displays the formation of a motionless dense cluster surrounded by low particle density regions. 81.70.Ha, 83.10.Pp, 83.70.Fn Vibrated granular media display striking fluidlike properties: convection and heaping [1,2], period doubling instabilities [3], and parametric extended [4] or localized [5] surface waves. When the vibration is strong enough, the granular medium undergoes a transition to a fluidized state. It looks like a gas of particles that can be described using kinetic theory [6]. The "granular temperature," i.e., the mean kinetic energy per particle, is determined by the balance between the power input due to the external vibration and dissipation by inelastic collisions. Fluidization by vibrations has been studied experimentally [7,8] and numerically [8,9], but no agreement has been found so far for the dependence of the granular temperature on the amplitude and the frequency of external vibrations [10][11][12].One of the most interesting properties of such "granular gases" is the tendency to form clusters. Although this has probably been known since the early observation of planetary rings [13], there exist only a few recent laboratory experiments. One experiment, with a horizontally shaken two-dimensional layer of particles, displayed a cluster formation, but the coherent friction force acting on all the particles was far from being negligible [14]. We performed a similar three-dimensional experiment in the laboratory and observed clustering, but we could not rule out the possibility of a resonance mechanism between the time of flight under gravity and the excitation frequency [15]. Various cluster types in granular flows have also been observed numerically [16]. The mechanisms of cluster formation are an active subject of research that still deserves more study because of its relevance to technical, astrophysical [17], or geophysical [18] applications of granular media. At a more fundamental level, it is of a primary interest to understand the new qualitative behaviors due to inelasticity of collisions, i.e., nonconservation of energy, in kinetic theory.In this Letter, we report a study of the kinetic regimes of a granular medium, fluidized by vibrating its container in a low gravity environment. The motivation for low gravity is to achieve an experimental situation in which inelastic collisions are the only interaction mechanism. The aim of the experiment is to observe new phenomena which result from the inelasticity of the collisions and are thus absent in a usual gas. In the dilute case, we show that the pressure of a granular gas scales like the 3͞2 power of the vibration velocity. When the density o...