An experimental study of surface waves parametrically excited by vertical vibrations is presented. The shape of the eigenmodes in a closed vessel, and the importance of the free-surface boundary conditions, are discussed. Stability boundaries, wave amplitude, and perturbation characteristic time of decay are measured and found to be in agreement with an amplitude equation derived by symmetry. The measurement of the amplitude equation coefficients explains why the observed transition is always supercritical, and shows the effect of the edge constraint on the dissipation and eigen frequency of the various modes. The fluid surface tension is obtained from the dispersion relation measurement. Several visualization methods in large-aspect-ratio cells are presented.

Gravitropism, the slow reorientation of plant growth in response to gravity, is a key determinant of the form and posture of land plants. Shoot gravitropism is triggered when statocysts sense the local angle of the growing organ relative to the gravitational field. Lateral transport of the hormone auxin to the lower side is then enhanced, resulting in differential gene expression and cell elongation causing the organ to bend. However, little is known about the dynamics, regulation, and diversity of the entire bending and straightening process. Here, we modeled the bending and straightening of a rod-like organ and compared it with the gravitropism kinematics of different organs from 11 angiosperms. We show that gravitropic straightening shares common traits across species, organs, and orders of magnitude. The minimal dynamic model accounting for these traits is not the widely cited gravisensing law but one that also takes into account the sensing of local curvature, what we describe here as a graviproprioceptive law. In our model, the entire dynamics of the bending/straightening response is described by a single dimensionless "bending number" B that reflects the ratio between graviceptive and proprioceptive sensitivities. The parameter B defines both the final shape of the organ at equilibrium and the timing of curving and straightening. B can be estimated from simple experiments, and the model can then explain most of the diversity observed in experiments. Proprioceptive sensing is thus as important as gravisensing in gravitropic control, and the B ratio can be measured as phenotype in genetic studies.perception | signaling | movement | morphogenesis

The nature of the transition between static and flowing regimes in granular media 1,2 provides a key to understanding their dynamics. When a pile of sand starts flowing, avalanches occur on its inclined free surface. Previously, studies 3 of avalanches in granular media have considered the time series of avalanches in rotating drums 4 , or in piles continuously fed with material. Here we investigate single avalanches created by perturbing a static layer of glass beads on a rough inclined plane. We observe two distinct types of avalanche, with evidence for different underlying physical mechanisms. Perturbing a thin layer results in an avalanche propagating downhill and also laterally owing to collisions between neighbouring grains, causing triangular tracks; perturbing a thick layer results in an avalanche front that also propagates upwards, grains located uphill progressively tumbling down because of loss of support. The perturbation threshold for triggering an avalanche is found to decrease to zero at a critical slope. Our results may improve understanding of naturally occurring avalanches on snow slopes 5 where triangular tracks are also observed.The experiments are done on an inclined plane covered with velvet cloth. This surface is chosen so that the glass beads (180-300 m in diameter), our granular material, have a larger friction with it than between themselves. A thin layer of grains can thus remain static on the plane up to a larger angle than if it were on a grain pile. We set the plane to an angle J (larger than the pile angle J 0 ) and pour glass beads abundantly at the top. The moving beads leave behind a static layer of uniform thickness h(J) (arrow leading to point a in Fig. 1; the geometry of the system is shown in Fig. 1 inset). This effect is explained by the variation of the friction of the successive grain layers with their distance to the surface of the inclined slope 6 : it is maximum for the bottom layer, and decreases continuously to the value for a thick pile. The top static layer is that which has a large enough friction coefficient on the underlying layers m(h) to come to a stop. At inclination angle J, the friction coefficient of this top static layer is then mðhÞ ¼ tanJ. The measurements h(J) of Fig. 1 thus give the variation of the coefficient of friction with depth 7 . We obtain a simple exponential decay as in ref. 7:

Cavitation in a liquid seeded with bubbles is used as a new visualization technique to single out the regions of very low pressure of a fully developed turbulent flow. By this means, the sudden appearance of high vorticity filaments is observed. These structures are very thin and short lived and display a high degree of temporal as well as spatial intermittency. They contribute to the flow organization: In particular their disintegration corresponds to the formation of large eddies.

Abstract. Almost fifty years of investigations of barchan dunes morphology and dynamics is reviewed, with emphasis on the physical understanding of these objects. The characteristics measured on the field (shape, size, velocity) and the physical problems they rise are presented. Then, we review the dynamical mechanisms explaining the formation and the propagation of dunes. In particular a complete and original approach of the sand transport over a flat sand bed is proposed and discussed. We conclude on open problems by outlining future research directions.

A new experiment can create small scale barchan dunes under water: some sand is put on a tray moving periodically and asymmetrically in a water tank, and barchans rapidly form. We measure basic morphological and dynamical properties of these dunes and compare them to field data. These favorable results demonstrate experimentally the relevance of the so-called "saturation length" for the control of the dunes physics.

In this paper results on the low-pressure filaments that appear spontaneously in three-dimensional turbulent flows are presented. An individual characterization of the filaments is first obtained by studying the correlations between the flow visualization and local measurements of the pressure and the velocity. Then, a statistical study of the time recordings of the pressure that exhibits intermittent short and deep depressions is presented. It is shown that the pressure histograms depend only on the square of the injection velocity, and that the rate of production of strong depressions is independent of the Reynolds number. These results impose severe constraints on the possible mechanisms of formation of the filaments; they are consistent with a simple model, in which the formation of the filaments results primarily from the partial rollup of stretched shear layers. In this model there is a difference between the hierarchies of pressure and vorticity filaments: the filaments with the largest depression are the thickest (and the longest), while the filaments with the strongest vorticity are likely to be the thinnest (and shortest).

We present in this paper a simplification of the dune model proposed by Sauermann et al. which keeps the basic mechanisms but allows analytical and parametric studies. Two kinds of purely propagative two dimensional solutions are exhibited: dunes and domes, which, by contrast to the former, do not show avalanche slip face. Their shape and velocity can be predicted as a function of their size. We recover in particular that dune profiles are not scale invariant (small dunes are flatter than the large ones), and that the inverse of the velocity grows almost linearly with the dune size. We furthermore get the existence of a critical mass below which no stable dune exists. However, the linear stability analysis of a flat sand sheet shows that it is unstable at large wavelengths and suggests a mechanism of dune initiation.