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A Chiffaudel

Publications and source records attributed to A Chiffaudel.

7 recordsLinked to original sources
PubMed2006

Transport of magnetic field by a turbulent flow of liquid sodium.

We study the effect of a turbulent flow of liquid sodium generated in the von Kármán geometry, on the localized field of a magnet placed close to the frontier of the flow. We observe that the field can be transported by the flow on distances larger than its integral length scale. In the most turbulent configurations, the mean value of the field advected at large distance vanishes. However, the rms value of the fluctuations increases linearly with the magnetic Reynolds number. The advected field is strongly intermittent.

Journal Article↗
PubMed2006

Properties of steady states in turbulent axisymmetric flows.

We experimentally study the properties of mean and most probable velocity fields in a turbulent von Kármán flow. These fields are found to be described by two families of functions, as predicted by a recent statistical mechanics study of 3D axisymmetric flows. We show that these functions depend on the viscosity and on the forcing. Furthermore, when the Reynolds number is increased, we exhibit a tendency for Beltramization of the flow, i.e., a velocity-vorticity alignment. This result provides a first experimental evidence of nonlinearity depletion in nonhomogeneous nonisotropic turbulent flow.

Journal Article↗
PubMed2003

Nonlinear magnetic induction by helical motion in a liquid sodium turbulent flow.

We report an experimental study of the magnetic field B--> induced by a turbulent swirling flow of liquid sodium submitted to a transverse magnetic field B-->(0). We show that the induced field can behave nonlinearly as a function of the magnetic Reynolds number, R(m). At low R(m), the induced mean field along the axis of the flow, , and the one parallel to B-->(0), , first behave like R(2)(m), whereas the third component, , is linear in R(m). The sign of is determined by the flow helicity. At higher R(m), B--> strongly depends on the local geometry of the mean flow: decreases to zero in the core of the swirling flow but remains finite outside. We compare the experimental results with the computed magnetic induction due to the mean flow alone.

Journal Article↗
PubMed2001

Nonlinear Transition to a Global Mode for Traveling-Wave Instability in a Finite box.

Using hydrothermal waves in a long narrow channel as an ideal supercritical nonlinear wave system, we study the critical behavior at onset for a finite cell with poorly reflecting boundaries. We observe a spatially nonuniform global eigenmode localized near the end of the cell and associated with a wave front invading the cell. The onset of this mode-corresponding to the absolute instability transition-is shifted above the value corresponding to convective instability, measured in an annular channel with periodic boundary conditions. Experimental critical exponents are discussed in the framework of existing theoretical descriptions and quantitative comparison with the complex Ginzburg-Landau model is attempted.

Journal Article↗
PubMed1994

Alignment and sensitive detection of DNA by a moving interface.

In a process called "molecular combining," DNA molecules attached at one end to a solid surface were extended and aligned by a receding air-water interface and left to dry on the surface. Molecular combing was observed to extend the length of the bacteriophage lambda DNA molecule to 21.5 +/- 0.5 micrometers (unextended length, 16.2 micrometers). With the combing process, it was possible to (i) extend a chromosomal Escherichia coli DNA fragment (10(6) base pairs) and (ii) detect a minute quantity of DNA (10(3) molecules). These results open the way for a faster physical mapping of the genome and for the detection of small quantities of target DNA from a population of molecules.

DNA↗