François Daviaud

Instabilities of a liquid layer locally heated on its free surface

We report experimental results concerning patterns in a model experiment built to study buoyant-thermocapillary-driven flows. The fluid is situated in a cooled cylindrical container and locally heated on its free surface. The resulting temperature gradient induces a basic flow which draws the surface fluid from the hot center toward the cold boundary. When the gradient is increased and depending on the height of liquid, the basic flow destabilizes into different stationary patterns. Above a second threshold, the patterns become time-dependent. These different instabilities are characterized and compared to recent theoretical results.

Experimental measurement of the scale-by-scale momentum transport budget in a turbulent shear flow

We report measurements linking velocity fluctuations with the turbulent drag in a turbulent closed flow, namely the von Karman flow. Making use of the angular momentum balance equation in integral form, we obtain a simple expression for the torque applied by the forcing mechanism, which we check against quantitative laser Doppler velocimetry measurements. We then decompose the angular momentum flux into contributions coming from the different spectral components of the flow. We provide evidence of the fact that the turbulent drag is dominantly generated by coherent structures at the largest scales of the flow.

TSF EXPERIMENT FOR COMPARISON OF HlGH REYNOLDS NUMBER TURBULENCE IN BOTH HE I AND HE II: FIRST RESULTS

Superfluid turbulence (TSF) project uses liquid helium for the fundamental study of turbulent phenomena behind a passive grid and is able to work both in HeI and in HeII. The helium flow is generated by a cold Barber and Nichols circulating pump, whereas helium flow temperature is kept constant by means of a heat exchanger immersed in a saturated bath. This experiment takes profit of the CEA Grenoble refrigerator (nominal capacity of 400 Watt at 1.8 K) to remove the heat due to pressure losses in this high Reynolds number experiment. In order to resolve the Kolmogorov scale associated with high Re flow, local instrumentation (e.g. sub-micrometer anemometer) was developed. The difficulties encountered with this local and fragile instrumentation in a quasi industrial environment are discussed and the adopted solutions are also described. Finally, first results (permanent mass flow rate of a few hundreds g/s) obtained both in the two phases of helium are presented.

MHD in von Kármán Swirling Flows

The magnetism of many astrophysical objects, such as various stars or planets, galaxies, the intergalactic medium, etc, is attributed to the motion of conducting fluid in their interiors. It has been first proposed by Larmor [17] that a flow of conducting fluid generates the magnetic field of the sun by maintaining the corresponding electric current against ohmic dissipation. Such a generation of electromagnetic energy from mechanical work using a self-excited dynamo has been known since Siemens [34] and is the most basic mechanism of electrical engineering. However, in industrial dynamos the path of the electric currents are constrained by a complex wiring, which even in the most elementary device, the homopolar dynamo [3], breaks mir ror symmetry. In addition, magnetic field lines are usually canalized using a high magnetic permeability material. No such well controlled external constraints on the field or on the current lines exist in“natural” dynamos, and for a long time, it has been far from obvious that the dynamo effect was the correct explanation for solar or earth magnetism. It has been even shown that a lot of flow and / or field configurations with enough symmetries cannot behave as fluid dynamos [for a review on anti-dynamo theorems, see Kaiser et al., these proceedings].