Michele Alessandro BUCCI, a doctoral student at the Dynfluid laboratory on the Arts et Métiers campus Arts et Métiers Paris, will defend his thesis on the topic "Subcritical and supercritical dynamics of incompressible flow around micro-roughness elements,"on Monday, December 4, 2017.
This thesis aims to highlight the limitations of passive control using miniaturized roughness elements. The flow topology induced by the presence of cylindrical roughness and miniaturized vortex generators was studied to analyze the dynamics for short and long time scales. Various supercritical bifurcations were examined using global stability analysis. The subcritical bifurcation is triggered by transient energy growth mechanisms or by the receptivity of stable global modes. Optimal 3D forcing structures are extracted to understand the spatial distribution related to the resonance frequency of the system. Direct perturbed numerical simulation reveals the central role of the least stable global mode in the observed nonlinear instabilities. A detailed analysis of the vortex structures shows that they are mainly related to the underlying linear mechanisms. The main global characteristic of the eigenmode is related to the presence of a separation zone downstream of the cylindrical roughness. Using miniaturized vortex generators, this separation zone is greatly reduced and no isolated global mode is then present. The flow dynamics are found to be driven not only by the Reynolds roughness number and its geometric aspect ratio, but also by the ratio between the roughness height and the boundary layer thickness.
