Natural and industrial flows, in geophysics, aeronautics or process engineering, are complex, unsteady, sometimes multiphase, and most often turbulent. Understanding and modeling these flows is a real challenge for both fundamental and practical reasons.
On a global scale, atmospheric and oceanic flows are subject to stratification and background rotation effects. These lead to the generation of internal waves, which have a profound influence on the flow dynamics, such as the emergence of eddies or coherent jets that can influence the mixing properties (heat, pollutants ...)
On a smaller scale, flows with interfaces (either between two liquids or between a liquid and a gas) provide other examples of such complex flows. The formation of ocean waves illustrates the wide range of open issues, from the origin of the first ripples generated by wind to their amplification to the mechanism of saturation and dissipation by wave breaking. Other examples are the coiling instability of "liquid ropes" that fall on a surface and the surprising morphology of the "liquid curtains" that form at the exit of a horizontal pipe.
In this research group, we develop model experiments in simple and controlled configurations that aim to reproduce these complex flows from the first stages of instability to fully turbulent situations.
Lift Crisis
After the pioneering work of G. Eiffel (1912) the « Drag crisis » is now a well known phenomena of fluid mechanics for a bluff body moving at large velocity. During this crisis the drag force becomes, surprisingly, a decreasing function of the relative velocity. We have shown that at the drag crisis, non-up/down symmetrical bodies can also experience a strong "lift crisis", i.e. a sharp transition or even an inversion in their lift force.
Wake of inertial waves in a rotating fluid
A remarkable property of rapidly rotating flows is their tendency to become
two-dimensional: a slowly moving object moves along with a fluid column,
called a Taylor column, aligned with the axis of rotation.
But when the velocity increases, the object can emit a wake of inertial waves,
similar to a boat emitting a wake of surface waves. We made precise
measurements of this particular wake on the rotating platform Gyroflow,
and obtained an excellent agreement with a theory based on a slender body
approximation.
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Linear and nonlinear regimes of an inertial wave attractor
Fluids subjected to a global rotation are the support of a specific class of waves, called inertial waves, found
in geophysical and astrophysical flows. As a result of the anomalous reflection laws
of these waves, which keep their inclination constant with respect to the horizontal, these systems can lead
in closed domains to singular modes, called wave attractors, focusing the energy on a limit cycle.
We report an experimental study of the non-linear regime of an inertial wave attractor revealing the
emergence of a triadic resonance instability with singular features. The scaling laws of the attractor wavelength and amplitude are shown
to be quantitativelly described by introducing a turbulent viscosity in the linear attractor model.
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Turbulence under rotation
Together with the effects of fluid stratification, the two-dimensional structuration of a turbulent flow by the Coriolis force
is a key mechanism for the understanding of geophysical flows (ocean, atmosphere, rotating stars etc.).
The Gyroflow rotating platform is an experimental tool for the study of
the fundamental processes emerging from the interaction between global rotation and fluid dynamics.
It is first of all dedicated to the study of turbulence under rotation.
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Turbulent windprint on a liquid surface
As soon as a light breeze blows on the surface of the water, well before the threshold of formation of the first capillary waves, we notice that the surface is not perfectly smooth like a mirror. Very small deformations (in the order of a few microns) appear, called wrinkles. We have shown that these wrinkles are the imprint of pressure fluctuations travelling in the turbulent boundary layer generated by the wind, which stripe the surface of small unsteady wakes. We study these wrinkles using wind tunnel experiments based on a highly sensitive optical method (Synthetic Schlieren) and numerical simulations.
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Liquid rope coiling
If you like honey on your toast at breakfast, you are ready to perform a simple and beautiful fluid mechanics experiment. Plunge a spoon into the honey jar, and then hold it vertically several inches above the toast. The falling honey builds a whirling corkscrew-shaped structure - a phenomenon called "liquid rope coiling".
Making folds by blowing on a liquid
When the wind blows on the surface of a liquid, it is well known that, above a critical wind velocity, a propagative wave is formed. But what happens when the fluid is very viscous (100 to 1000 times more viscous than water), to the point that these propagative waves become critically damped? Experimentally, we observe that the waves are violently destabilized in the form of sharp "liquid folds", like a fabric forming folds in front of an iron. These liquid folds then advance at high speed, pushed by the wind, and can interact with each other (coalescence, sheltering...)
Torricelli's curtain: Morphology of laminar jets under gravity
While the form of a fluid jet issuing horizontally from an orifice
was first studied by Torricelli (1643), this classic problem in fluid
mechanics still holds surprises. When a laminar jet issues from the
end of a pipe, it divides into primary and secondary
jets with a thin vertical curtain of fluid connecting them.
We are currently using laboratory experiments and numerical
simulations to study this unexpected behavior.
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Strange rotation in an orbitally shaken glass of beer
Swirling a glass of wine induces a rotating gravity wave along with a mean flow rotating in the direction of the applied swirl. Surprisingly, when the liquid is covered by a floating cohesive material, for instance a thin layer of foam in a glass of beer, the mean rotation at the surface can reverse. This intriguing counter-rotation can also be observed with coffee cream, tea scum, cohesive powder, provided that the wave amplitude is small and the surface covering fraction is large..
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See the movie on Youtube !
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