PHD : Caractérisation du mélange des écoulements à densité variable

Impacts of buoyancy effects on the mixing at river confluences: Water mixing at river confluences strongly affects the ecological habitat and the water uses. Two simultaneous processes determine the mixing processes and efficiency at river conflu-ences: the turbulent diffusion and the dispersion. The first one gathers all the structures induced by the shear at the interface between both inflows, such as Kelvin-Helmholtz coherent turbu-lent structures, while the second deals with large secondary currents such as gravity currents, large helicoïdal cells or streamwise-oriented vortices. In case of a sufficient density difference between the inflows, mainly due to the temperature and/or the suspended sediments concentra-tion difference, dispersion due to buoyancy effects can become dominant: the interface be-tween the two inflows becomes tilted, and eventually horizontal, with the lighter inflow located above the heavier one. This strongly affects the length necessary for a good mixing. The present work focusses on the mixing of two open-channel flows with different densities. Our first goal is to find characteristic threshold values of dimensionless numbers permitting to asses correctly the dominant type of mixing: turbulent diffusion or dispersion caused by density differences. The so-called Gamma Parameter, so as the Richardson and the Densimetric Froude numbers are possible candidates for such a number. For 15 new field experiments and 50 con-figurations taken from the literature, these parameters are evaluated and their correlation with mixing patterns is assessed. The second goal is to parametrize the transverse mixing coefficient downstream from confluences. This will permit to improve the parametrization of mixing pro-cesses in 1D calculations. The present field experiments were conducted (i) at the Rhône-Saône confluence (a 40° angle confluence in Lyon, France) and at the Rhône-Arve confluence (a 10° angle confluence in Ge-neva, Switzerland) and (ii) downstream of 2 thermal releases of nuclear power plants: Bugey and Saint-Alban located near Lyon (France). The 3D velocity, temperature and conductivity fields were measured along selected cross-sections with an ADCP and CTD probes. First re-sults indicate that density driven secondary currents dominate the mixing at the Rhône-Saône confluence, with a mixing rate increasing when the density difference between the two inflows increases. On the contrary, at Rhône-Arve the turbulent diffusion at the interface dominates with a high velocity difference and the density difference between the inflows remains limited. Finally, the Densimetric Froude number seems a good parameter to predict the mixing pattern, but more work is still required to quantify a threshold value to distinguish the governing mixing process.

MASTER Sciences de l'Eau - HYDRE (Apprentissage)

Licence Professionnelle Aménagement et Gestion des Ressources en Eau (Apprentissage)

BTSA Gestion et Maitrise de l'Eau (Apprentissage)

BTA Gestion de la Faune Sauvage

Responsable de la modelisation hydraulique

Utilisation courante de QGIS, PortEau. Encadrement des techniciens et alternants. Suivi de chantier et présentation des résultats des études aux élus.

Conducteur de travaux / Chef de chantier

Chargé d'affaire Eau potable et Assainissement

Conducteur de travaux / Chef de chantier

Apprenti Chargé d'affaires

Apprenti technicien eau potable et assainissement

Fontainier

Peintre en bâtiment

Vendeur en magasin de ski

Chasseur Alpin