Turbulence des écoulements actifs

This thesis presents the work I have done in the last three years in the field of fluid mechanics. The main focus has been the study of incompressible fluids dynamics J have studied is not the one described by the pure Navier-Stokes equations, because other local properties of the flow are taken into account, such as polymer elongation and temperatures. Dissolving a small quantity of polymers in a fluid drastically changes its macroscopical properties. J here present two works that deal with instabilities of a laminar viscoelastic fluid. The system is the same in both cases, but the conditions are different : the first one treats the inset of inertial instabilities, and how they are modified by the presence of polymers. The second one deals with instabilities occurring at low Reynolds numbers, when the elasticity of the flow increases. The second part of the thesis deals with convective turbulence, originated by temperature fluctuations. A first work focuses on the study of the Lagrangian properties of a two-dimensional fluid in pure convection. In the end, J present the preliminary results obtained from the study of turbulence generated by Raileigh-Taylor instabilities, focusing on the system s global quantities and their time evolution.NICE-BU Sciences (060882101) / SudocSudocFranceItalyFRI