SOGOOD

A soliton is a kind of solitary wave that travels without deforming, unlike normal waves that weaken as they move forward. They are found in many different fields, including water (in particular waves on the sea), light (in optical fibres) and even quantum physics. Until recent experimental observations, soliton gases were considered a purely theoretical model in non-linear statistical physics.

The SOGOOD project consortium comprises four university groups with expertise in experiments and numerical simulation of non-linear waves (PHLAM-Université de Lille, LHEEA - Centrale Nantes, MSC-Université Paris Diderot, LEGI-Université Grenoble-Alpes). The aim is to see whether these soliton gases obey very precise physical laws and whether this can help us to better understand other complex phenomena such as wave turbulence or the behaviour of quantum gases (important for technologies such as quantum computers).

The Hydrodynamic and Ocean Engineering Tank at Centrale Nantes recently played host to researchers from Sorbonne University, Université Paris Cité and the University of Liège, as part of the SOGOOD project. The teams conducted experiments on the diffusion of particles at the water surface under the influence of an isotropic wave field.

For these tests, three key ingredients are required to generate the targeted isotropic wave field:

1. The wave maker was controlled to produce multidirectional waves covering a wide angular spread.
2. The side walls reflected waves generated at large angles, sending them back toward the tank's center.
3. A "wall" was installed at the end of the tank, replacing the usual beach that absorbs waves. Like the side walls, this wall ensured complete wave reflection.

These three elements help to ensure that the waves converged from all directions at the centre of the tank, creating the desired isotropic wave field.

The researchers measured the waves generated using 17 probes laid out over a surface area of one square metre to determine the angular distribution of the waves and validate their isotropy. They then made video recordings of the movement of polystyrene balls of different sizes floating on the surface of the water in the same wave field. Long-duration tests were conducted to obtain reliable statistics on the movement of the floaters. Analysis of these movements will enable us to characterise the diffusion of particles floating on the surface of the water.
 

These experiments will provide a better understanding of how random waves move floating objects and, in particular, how the size of the objects in relation to the length of the waves influences their diffusion.