The understanding of a turbulent flow downstream of a horizontal circular cylinder submitted to a transverse air flow in the presence of significant buoyancy effects, is the main objective of the present Ph.D. Experimental and numerical approaches are used to supplement the knowledge of the turbulent mixed convection regime. Apart from the loss of symmetry of the wake induced by buoyancy effects, the flow studied here is characterized by the development of two main instabilities. In the lower shear-layer region one assists to the development of intermittent Kelvin-Helmholtz instabilities. We show that in presence of significant flottability effects the transition to turbulence is accelerated. At the rear of the cylinder the development of a secondary flow is initiated for the studied configuration because of buoyant forces. This secondary flow is highly three-dimensional and gives rise to dipole-like vorticity structures in the upper part of the wake. These structures are free to evolve in the wake or to pair with the turbulent wake vortices. Finally, in the presence of significant buoyancy effects, there is an increase of the turbulent activity in the wake of the cylinder which is characterized by a higher three-dimensional topology. Under these conditions temperature acts as an active scalar which is able to accelerate the transition to turbulence.
