Interactions fortes Ondes de surface/ tourbillons et autres diffuseurs – Tourbillonde

Study of water wave propagation is a working area by itself with numerous applications in oceanic or coastal engineering for instance. This wave propagation is also often used as an equivalent of acoustic wave propagation in fluids (hydraulic analogy). On the experimental point of view, compared to acoustics there are two advantages of working with water waves: first the acoustic waves is directly visualized by the deformation of the free surface; second the wavelength (large) and speed (small) of surface waves allow us to explore regimes of strong interaction specially in the transsonic domain (for instance, when size and velocity are of the order of those of a vortex). Our group has a strong background in this domain since a similar experimental set-up has already been built few years ago thanks to a CNRS "jeune équipe" grant. In October 2006, Pablo Cobelli has started his PhD thesis on the study of the interaction between a surface wave and a vortex. These are the two main axis we want to develop. At the moment, they both produce an intense excitement in the scientific community. • Multiple scattering of water waves: In the context of the development of research on non-polluting energy, swell is still an unexploited energy reservoir. Recently, research groups, essentially Asiatic (Xinhua Hu at Hong Kong, Zi at Shangaï, Jeong at Séoul…), have shown thanks to theories and tools of solid physics, that it is possible to focalize a wave on a point where its energy could be exploited. For instance, it has been shown that a periodical distribution of cylindrical scatterers (a crystal of scatterers) can focalize water waves. In this context, we want to test the robustness of the focalization for a disordered repartition of scatterers. In addition of the interest of this study by itself, the goal is to have an experimental set-up and the associate tools to study a less known system: multiple scattering by a collection of vortices. The equivalence between this and an effective medium has never been validated experimentally. A random distribution of vortices modelizes quite well a turbulent flow and this study would allow us to propose a new technique to investigate these turbulent flows. • Water wave black hole analogues: In the last ten years, numerous theoreticians in general relativity have been interested in black hole analogues in non-relativistic physics. It has appeared, thanks to these analogies, that it is possible to design experiments in fluid mechanics that simulate light wave propagation around black holes. The simplest of these experiments is the very experiment which study the propagation of water wave near a bathtub vortex. Theoretical literature (more and more numerous) expects a super-resonance phenomenon in the transsonic domain of the bathtub vortex. It clearly appears in this literature the request for experiments to feed the theory and which would confirm the resonance mechanism in the physics of black holes. Our goal is to realize the first experimental proof of this phenomenon. The motivation to built together - at ESPCI (A. Maurel/LOA & P. Petitjeans/ PMMH) and at the Université du Mans (V. Pagneux/LAUM) - this experiment is older since the advantages of working with water waves are a good solution for other study in which we are interested. • Trapped modes for water waves : The existence of trapped modes around one or several cylinders has been recently intensively studied, specially by English groups (D. Evans at Bristol, D. Porter and P. Chamberlain at Reading, R. Porter at Bristol, P. McIver at Loughborough). Nevertheless, there is not yet any experimental validation of this theoretical prediction. • Time reversal of surface waves : This is a project with the LOA. The LOA has developed a strong activity on time reversal ultrasonic waves and on the application of this technique to other domains of wave physics. There is a real challenge to test the robustness of time reversal of water waves which are dispersive. We have with our experimental set-up the requested configuration of the source, of the receptors as well as of the measurements techniques of spatial fields to be able to carry through this experiment.