Similitude des phénomènes de diffusion géophysique

Simulation of geophysical diffusion effects to scale

¹ Ingénieur au C.N.B.S., Chef de la Section d'Etude des marées au Laboratoire de Mécanique des Fluides de l'Université de Grenoble.
² Maitre-Assistant à la Faculté des Sciences, Laboratoire de Mécanique des Fluides de l'Université de Grenoble.

Résumé

Where radio-active effluent is discharged into the sea it is necessary to study its dispersion over extensive areas and long periods of time. The authors have studied the horizontal diffusion effect experimentally to a small scale and have produced hydraulic and mathematical models capable of representing it to a very large scale, in both cases exclusively for tidal inshore areas. In small-scale studies carried out in-situ a general method is used, which is based on the dispersion of individual floats and whereby one can establish whether one is dealing with an 'inertia interval' as predicted by turbulence similarity theory and, if so, define its limits in time and space. The method finally leads to a closer numerical definition of the power relationships from which characteristic diffusion parameters are obtained. As any real-life experimental study is limited by the accuracy of its instrumentation, very large-scale diffusion was investigated on a rotating hydraulic model, which, in theory, seemed capable of reproducing the phenomenon satisfactorily to scale. After initial calibration of the model, its representativeness was confirmed by a number of medium-scale tests carried out in real life and on a model, whereupon it was used to study cases of large-scale diffusion not approachable by direct experimental investigation. Simulation of horizontal diffusion on a mathematical model of an inshore sea area was found to give a faithful reproduction of the current pattern and seems to be a suitable method for site supervision applications.