Time-symmetry breaking in turbulence

In three-dimensional turbulent flows, the flux of energy from large to small scales breaks time symmetry. We show here that this irreversibility can be quantified by following the relative motion of several Lagrangian tracers. We find by analytical calculation, numerical analysis and experimental observation that the existence of the energy flux implies that, at short times, two particles separate temporally slower forwards than backwards, and the difference between forward and backward dispersion grows as $t^3$. We also find the geometric deformation of material volumes, surrogated by four points spanning an initially regular tetrahedron, to show sensitivity to the time-reversal with an effect growing linearly in $t$. We associate this with the structure of the strain rate in the flow.Comment: 5 pages, 4 figure

Fast and Adjustable Resolution Grazing Incidence x-ray Liquid Surfaces Diffraction achieved through 2D Detector

We developed a setup using a two dimensional camera for Grazing Incidence x-ray Diffraction (GIXD) on Langmuir monolayers and more generally for surface diffraction on two dimensional powders. Compared to the classical setup using a linear detector combined with Soller's slits, the acquisition time is reduced of a factor of at least 10 (from more than one hour to a few minutes) using the same x-ray source (synchrotron bending magnet) with a comparable signal to noise ratio. Moreover, using an horizontal gap slit, the experimental resolution can be adjusted and for small values of the gap, better resolution can be achieved compared to the one obtained with the linear detector.Comment: 37pages, 11 figure

Collision rate of ice crystals with water droplets in turbulent flows

International audienceno abstrac

Settling and collision between small ice crystals in turbulent flows

International audienc

Single‑mode scannable nanosecond Ti:sapphire laser forhigh‑resolution two‑photon absorption laser‑induced fluorescence (TALIF)

International audienceA pulsed Ti:sapphire laser has been developed so as to operate over a wide range of frequencies, even far from the optimum wavelength (790 nm), as a narrow-band light source for TALIF experiments on O, Cl, N and H. The coupling of the optical cavity, both to its injection seeder and to the laser output beam, relies on a reflecting plate, which makes it fundamentally easier to control the coupling coefficient over a wider spectral range than with an ordinary transmission coupler. Two intra- cavity prisms are used to bring the green pumping light longitudinally coincident with the cavity axis, inside the Ti:sapphire crystal. Seeding by a CW Ti:sapphire laser has made it possible to obtain single-mode emission over the whole range of tunability, thanks to the spectral selection of the prisms and to a specifically developed digital/analog controller. Experiments carried out with the system on oxygen atoms inside an oxygen plasma show that the experimental bandwidth is limited essentially by the collisional dephasing rate and the finite pulse duration