Traveling waves in a fluid layer submitted to a horizontal temperature gradient

We report experimental observations of traveling waves in a pure fluid with a free surface situated in a long container submitted to a horizontal temperature gradient perpendicular to its large extension. Above a critical value of the gradient and depending on the height of liquid $h,$ a source of waves is created in the container for small value of $h,$ while the system exhibits stationary patterns for larger values of $h.$ The spatio-temporal properties of the waves are studied and compared to theoretical predictions. Email contact: [email protected]: Saclay-S93/017 Email: [email protected]

"Barchan" dunes in the lab

We demonstrate the feasibility of studying dunes in a laboratory experiment. It is shown that an initial sand pile, under a wind flow carrying sand, flattens and gets a shape recalling barchan dunes. An evolution law is proposed for the profile and the summit of the dune. The dune dynamics is shown to be shape invariant. The invariant shape, the ``dune function'' is isolated.Comment: 3 pages, 4 figure

Numerical study of homogeneous dynamo based on experimental von Karman type flows

A numerical study of the magnetic induction equation has been performed on von Karman type flows. These flows are generated by two co-axial counter-rotating propellers in cylindrical containers. Such devices are currently used in the von Karman sodium (VKS) experiment designed to study dynamo action in an unconstrained flow. The mean velocity fields have been measured for different configurations and are introduced in a periodic cylindrical kinematic dynamo code. Depending on the driving configuration, on the poloidal to toroidal flow ratio and on the conductivity of boundaries, some flows are observed to sustain growing magnetic fields for magnetic Reynolds numbers accessible to a sodium experiment. The response of the flow to an external magnetic field has also been studied: The results are in excellent agreement with experimental results in the single propeller case but can differ in the two propellers case.Comment: 20 pages, 32 figure

Inhibition of phase turbulence close to onset of convection by permeable lateral boundary condition for the mean flow

International audienceWe show that the mechanisms which govern the onset of time dependence in usual Rayleigh-Bénard convection at low Prandtl number may be inhibited by a suitable choice of the lateral boundary condition for the sole mean flow. We first build a boundary condition which behaves like a rigid one for the roll flow and like a permeable one for the mean flow. We then observe that phase turbulence is inhibited close to the onset of convection. We understand this effect by solving the mean-flow field from the Cross-Newell equations. Our experimental result together with its interpretation demonstrates indirectly the existence of mean flows and enlightens the ways by which mean flows destabilize patterns

Ambivalent effects of added layers on steady kinematic dynamos in cylindrical geometry: application to the VKS experiment

The intention of the ''von Karman sodium'' (VKS) experiment is to study the hydromagnetic dynamo effect in a highly turbulent and unconstrained flow. Much effort has been devoted to the optimization of the mean flow and the lateral boundary conditions in order to minimize the critical magnetic Reynolds number and hence the necessary motor power. The main focus of this paper lies on the role of ''lid layers'', i.e. layers of liquid sodium between the impellers and the end walls of the cylinder. First, we study an analytical test flow to show that lid layers can have an ambivalent effect on the efficiency of the dynamo. The critical magnetic Reynolds number shows a flat minimum for a small lid layer thickness, but increases for thicker layers. For the actual VKS geometry it is shown that static lid layers yield a moderate increase of the critical magnetic Reynolds number by approximately 12 per cent. A more dramatic increase by 100 until 150 per cent can occur when some rotational flow is taken into account in those layers. Possible solutions of this problem are discussed for the real dynamo facility.Comment: 24 pages, 11 figures, minor changes, to appear in European Journal of Mechanics B/Fluid

Electrical conductance of a 2D packing of metallic beads under thermal perturbation

Electrical conductivity measurements on a 2D packing of metallic beads have been performed to study internal rearrangements in weakly pertubed granular materials. Small thermal perturbations lead to large non gaussian conductance fluctuations. These fluctuations are found to be intermittent and gathered in bursts. The distributions of the waiting time between to peaks is found to be a power law inside bursts. The exponent is independent of the bead network, the intensity of the perturbation and external stress. these bursts are interpreted as the signature of individual bead creep rather than collective vaults reorganisations. We propose a simple model linking the exponent of the waiting time distribution to the roughness exponent of the surface of the beads.Comment: 7 pages, 6 figure

Coupling of rotating water jets by surface waves

International audienceAbove a critical flow rate, a single jet impinging from below on a water-air interface oscillates while arrays of jets exhibit collective behaviors. The aim of the paper is to explain the physics governing such an array. First, a series of experiments shows that the instability mechanism leading to the oscillation of a single jet is due to a feedback effect between surface distortion and the unstable jet as in the ''jet-edge'' system and that several jets are coupled through surface waves. Then a modelization is proposed in which jets are considered as oscillators close to their limit cycle coupled by delayed action to take into account the wave propagation. A good agreement is obtained between the predictions of the model and experiments on sets of two or four jets. The long range of the surface waves and the large number of oscillators render this jet configuration very similar to idealized phase coupled nonlinear oscillators investigated in theoretical models but with a not instantaneous coupling ensured by traveling waves. This allows in particular the existence of various synchronization modes, which appear as geometrical pattern