Catégorie : en

M. Vial, L. Bellon and R. H. Hernandez, Experiments in Fluids 37 168-176 (2004)

doi: 10.1007/s00348-004-0796-0

We report experimental results of the forced wake of a thin symmetric flat plate, placed parallel to an uniform air stream, in the range of thickness-based Reynolds number 50< Re (e)<200. External wake forcing was introduced by small harmonic oscillations of a moving flap, placed at the trailing-edge of the flat plate. When the flap remains in a fixed horizontal position, the mean velocity profiles obtained by hot wire measurements, for different Reynolds numbers, are self similar. In the presence of harmonic forcing, within a certain range of the forcing frequency, the mean velocity profiles change and coherent structures are formed in the wake. Two independent flow-type resonances were observed: (i) when the inverse of the forcing frequency matches the flight time of the fluid particles along the flap. (ii) when the forcing frequency of the flap equals one half of the vortex shedding frequency of the flat plate and flap system. Implications of the two observed resonances on the wake structure are important. The first resonance (i) is associated to a wide but less intense (energy fluctuations) wake flow and the second resonance (ii) generates a thin but intense resultant wake flow.

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L. Buisson, L. Bellon and S. Ciliberto, Journal of Physics: Condensed Matter 15 S1163-S1179 (2003)

doi: 10.1088/0953-8984/15/11/336

The fluctuation-dissipation relation is measured on the dielectric properties of a gel (Laponite) and of a polymer glass (polycarbonate). For the gel it is found that during the transition from a fluid-like to a solid-like state the fluctuation-dissipation theorem is strongly violated. The amplitude and the persistence time of this violation are decreasing functions of frequency. Around 1 Hz it may persist for several hours. A very similar behaviour is observed in polycarbonate after a quench below the glass transition temperature. In both cases the origin of this violation is a highly intermittent dynamics characterized by large fluctuations. The relevance of these results for recent models of ageing is discussed.

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L. Bellon, L. Buisson, S. Ciliberto, F. Vittoz, Review of Scientific Instruments 73 3286-3290 (2002)

doi: 10.1063/1.1499210

In order to test the fluctuation-dissipation relation on rheological properties of soft materials, we built an experiment to measure thermally excited strain in a sample and compare it to the classical response to an external stress. The rheometer is based on a cylindrical Couette geometry. We use differential interferometry to achieve better than 1E-10 rad/root(Hz) sensitivity in angular position above 0.5 Hz. The forcing method, based on electrostatic interaction in a capacitor, generates torques comparable to that of thermal noise. Experiments on a calibrated silicon oil show good agreement between response and fluctuation measurements.

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L. Bellon and S. Ciliberto, Physica D-Nonlinear Phenomena 168 325-335 (2002)

doi: 10.1016/S0167-2789(02)00520-1

The validity of fluctuation dissipation relations (FDRs) in an aging system is studied in a colloidal glass during the transition from a fluid-like to a solid-like state. The evolution of the rheological and electrical properties is analyzed in the range 1-40 Hz. It is found that at the beginning of the transition the FDR is strongly violated in electrical measurements. The amplitude and the persistence time of this violation are decreasing functions of frequency. At the lowest frequencies of the measuring range it persists for times which are about 5% of the time needed to form the colloidal glass. This phenomenology is quite close to the recent theoretical predictions done for the violation of the FDR in glassy systems. In contrast in the rheological measurements no violation of the FDR is observed. The reasons of this large difference between the electrical and rheological measurements are discussed.

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L. Bellon, S. Ciliberto, H. Boubaker and L. Guyon, Optics Communications 207 49-56 (2002)

doi: 10.1016/S0030-4018(02)01475-X

We present a new design of the Nomarski interferometer, which can measure displacements of several microns, with a resolution better than 1E-13 m/root(Hz). In the standard design this sensitivity can be achieved only within a 100 nm displacement range. One main advantage of this new set-up of the interferometer is the total independence of the measure sensitivity on the interferometer thermal drifts.

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L. Bellon, S. Ciliberto and C. Laroche, European Physical Journal B 25 223-231 (2002)

doi: 10.1140/epjb/e20020025

A new kind of memory effect oil low frequency dielectric measurements oil plexiglass (PMMA) is described. These measurements show that cooling and heating the sample at constant rate give an hysteretic dependence on temperature of the dielectric constant epsilon. A temporary stop of cooling produces a downward relaxation of epsilon. Two main features are observed (i) when cooling is resumed epsilon goes back to the values obtained without the cooling stop (i.e. the low temperature state is independent of the cooling history) (ii) upon reheating epsilon keeps the memory of all the cooling stops (Advanced memory). The dependence of this effect on frequency and oil the cooling rate is analyzed. The memory deletion is studied too. Finally the results are compared with those of similar experiments done in spin glasses and with the famous experiments of Kovacs.

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R. H. Hernandez, M. Vial, L. Bellon and C. Baudet,  in Instabilities and Nonequilibrium Structures IX, O. Descalzi, J. Martínez, S. Rica (Eds.), Kluwer Academic Publishers. Series: Nonlinear Phenomena and Complex Systems 9 195-205 (2004)

ISBN: 978-1-4020-1950-0

We report experimental measurements of the wake behavior of a thin flat plate submitted to an external harmonic forcing. Two slightly different configurations are examined. Classical hot wire measurements of the velocity field downstream the plate and sound scattering experiments of the near wake demonstrates that the flat plate wake displays a kind of inertial resonance when the inverse of the forcing frequency matches the flying time of fluid particles along the moving part of the plate.

Proceedings of the Ninth International Workshop on Instabilities and Nonequilibrium Structures, Viña del Mar, Chile, December 2001. 

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