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-

Parametric excitation of surface waves via forced vertical oscillation of a container filled with fluid (the Faraday instability) is investigated experimentally in a small-depth large-aspect-ratio system, with a viscous fluid and with two simultaneous forcing frequencies. The asymptotic pattern observed just above the threshold for the first instability of the flat surface is found to depend strongly on the frequency ratio and the amplitudes and phases of the two sinusoidal components of the driving acceleration. Parallel lines, squares, and hexagons are observed. With viscosity 100 cS, these stable standing-wave patterns do not exhibit strong sidewall effects, and are found in containers of various shapes including an irregular shape. A ‘quasi-pattern’ of twelvefold symmetry, analogous to a two-dimensional quasi-crystal, is observed for some even/odd frequency ratios. Many of the experimental phenomena can be modelled via cubic-order amplitude equations derived from symmetry arguments.

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We report the observation of dynamo action in the VKS experiment, i.e., the generation of magnetic field by a strongly turbulent swirling flow of liquid sodium. Both mean and fluctuating parts of the field are studied. The dynamo threshold corresponds to a magnetic Reynolds number Rm ∼ 30. A mean magnetic field of order 40 G is observed 30 % above threshold at the flow lateral boundary. The rms fluctuations are larger than the corresponding mean value for two of the components. The scaling of the mean square magnetic field is compared to a prediction previously made for high Reynolds number flows.

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 91.25.Cw -Origins and models of the magnetic field; dynamo theories PACS 47.65.+a -Magnetohydrodynamics and electrohydrodynamicsAbstract. -We report the first experimental observation of reversals of a dynamo field generated in a laboratory experiment based on a turbulent flow of liquid sodium. The magnetic field randomly switches between two symmetric solutions B and −B. We observe a hierarchy of time scales similar to the Earth's magnetic field: the duration of the steady phases is widely distributed, but is always much longer than the time needed to switch polarity. In addition to reversals we report excursions. Both coincide with minima of the mechanical power driving the flow. Small changes in the flow driving parameters also reveal a large variety of dynamo regimes.Dynamo action is the instability mechanism by which mechanical energy is partially converted into magnetic energy by the motion of an electrically conducting fluid [1]. It is believed to be at the origin of the magnetic fields of planets and most astrophysical objects. One of the most striking features of the Earth's dynamo, revealed by paleomagnetic studies [2], is the observation of irregular reversals of the polarity of its dipole field. This behaviour is allowed from the constitutive equations of magnetohydrodynamics [1] and has been observed in numerical models [3]. On the other hand, industrial dynamos routinely generate currents and magnetic fields from mechanical motions. In these devices, pioneered by Siemens [4], the path of the electrical currents and the geometry of the (solid) rotors are completely prescribed. As it cannot be the case for planets and stars, experiments aimed at studying dynamos in the laboratory have evolved towards relaxing these constraints. Solid rotor experiments [5] showed that a dynamo state could be reached with prescribed motions but currents free to self-organize. A landmark was reached in 2000, when the experiments in Riga [6] and Karlsruhe [7] showed that fluid dynamos could be generated by organizing favourable sodium flows, the electrical currents being again free to self-organize. For these experiments, the self-sustained dynamo fields had simple time dynamics (a steady field in Karlsruhe and an oscillatory field in Riga). No further dynamical evolution was observed. The search for more complex dynamics, such as exhibited by natural objects, has motivated most teams working on the dynamo problem to design experiments with less constrained flows and a higher level of turbulence [8]. The von Kármán sodium experiment (VKS) is one of them. It has recently shown regimes where a statistically stationary dynamo self-generates [9]. We report here the existence of other dynamical regimes and describe below the occurence of irregular reversals and excursions.

We report the existence of structurally stable pulse-like solutions in the vicinity of an inverted Hopf bifurcation. These localized structures correspond to droplets in first order phase transitions, where they are known to be unstable. We show that the stabilisation mechanism is a non variational effect, i.e. is due to the non existence of a "free-energy" to minimize in the instability problem we consider. We propose this mechanism as an explanation for the existence of localized waves in shear flows or in convection experiments in binary fluid mixtures

We report a study of a fingering instability which occurs during the spreading of a spinning drop. We measure the time evolution of the drop profile, the critical radius for instability onset, and the fastest growing instability wavelength. The experimental results are compatible with the predictions of lubrication theory when the centrifugal eA'ect is not too large compared to the capillary one.