Université de Lille Espace 1894 – Institut Pasteur, Lille – October 5 and 6, 2024
Stand Effet poisson Stefano Berti and Enrico Calzavarini
The festival theme was ocean of knowledge this year. We talked about different types of fluid motion occurring at the surface and in the interior of the ocean: surface waves, internal waves, chaotic motion and turbulence. The experimental characterization and theoretical understanding of these processes are still incomplete, and represent a major challenge for research in oceanography and fluid mechanics. Forecasting ocean currents is also important for a number of societal issues, including coastal protection, biodiversity preservation, the control of pollutant and plastics dispersion. However, such predictions are often made difficult by the typically irregular and chaotic nature of oceanic flows. In a chaotic system, a tiny change (e.g. the flapping of a fish’s fin) can be amplified to produce a huge consequence (e.g. the formation of a sea storm), limiting predictability over long timescales. Four PhD students of our lab (Unité de Mécanique de Lille), Matthieu Hallaert, Michael Maalouly, Saad Raza, Luz Andrea Silva Torres helped us to present this topic on our stand.
Enrico finalizing the preparation of our stand; from left to right: water tank for surface waves, two containers filled with stratified fluids (different color means different density) where internal gravity waves take place, vortex generator with heavy/light solid particles, chaotic waterwheel. Internal waves propagating in a tank. Initially half of the container is filled with salty, heavier water (blue) and half with fresh, lighter water (yellow). When the membrane separating them is removed, salty water flows in the bottom part of the channel and fresh water in the upper part. The density contrast between these two fluids generates waves at the interface, which propagate along the channel. As it can be seen, the wave height can be important in the interior of the stratified fluid, while the surface remains almost quiescent. In the second part of the video, a fluid of intermediate density (colored in green) is obtained by enclosing the two-layer fluid in part of the channel and stirring. When free to move again, this fluid of intermediate density sets at the height matching its density, meaning at the interface between the blue and yellow layers. Internal waves in a glass filled with water (heavier, colored in red) and vaseline oil (lighter, tranparent). In this slow-motion video, the large amplitude, in the interior of the two-layer fluid, of waves generated by stirring can be appreciated. Luz and Matthieu explaining the phenomenon of preferential concentration: in a vortex, particles lighter than the fluid get trapped inside the vortex, while those heavier than the fluid are expelled from it. In the back: myself showing internal gravity waves by shaking a box containing a stratified fluid made of two layers: (heavier) water below and (lighter) vaseline oil above.Myself illustrating particle preferential concentration in a vortex flow to a family of visitors. On the other side of our stand, Luz and Matthieu discuss with other visitors.
In this vortex, generated by a magnetic stirrer, particles lighter than water (white) concentrate in the vortex center, particles heavier than water (yellow) remain outside it.
Michael illustrating the chaotic waterwheel, a mechanical analogue of Lorenz system, which provides a simplified description of thermal convection in the atmosphere. In Lorenz system, the dynamics amplify small perturbations to large values, so that after a certain time it is no longer possible to predict the state of the system. This strong dependence on initial conditions is often illustrated in terms of the butterfly effect: the movement of a butterfly somewhere in the atmosphere may produce a storm somewhere else. Pushing the analogy further, and playing with words, we transposed this concept to oceanic flows to highlight their turbulent nature, due to interactions between different processes, and their limited predictability. The chaotic waterwheel: the buckets fill from above but they have a hole on their bottom, from which they empty. The distribution of mass of the water inside the buckets determines whether the wheel turns clockwise or counter-clockwise. For appropriate values of the filling and emptying rates the dynamics become chaotic, as seen from the unpredictable changes of rotation direction of the wheel.A group photo, from left to right: Matthieu, Enrico, Luz, Saad, myself.
