| Resumo : |
The phenomenon of turbulent transition in boundary layers begins with the amplification of disturbances linked to the natural stability of the flow or generated by external forces. Despite the existence of theories describing the dynamics of laminar and turbulent boundary layers, the mechanisms of transition to turbulence, especially in environments subject to external disturbances, remain poorly understood. This, combined with the flow's high sensitivity to small amplitude disturbances, often results in imprecise predictions of the turbulent transition location. The research presented in this thesis studies flows subjected to high free-stream turbulence in cases ranging from a transitional boundary layer over a flat plate without a leading edge to more complex flows around fixed and movable airfoils. Employing modal decomposition techniques, such as spectral POD and resolvent analysis, the most energetic flow structures driving the transition process are extracted and studied. The work also addresses the challenges of modal decomposition, such as the need for extensive data and the limitations of stationary statistics assumptions. Additionally, it investigates the initial phase of the transition to turbulence, modelling the interaction of external disturbances with the flow through modal decomposition. The contributions include methodological advancements and the application of modal decomposition to identify receptivity mechanisms in transitional boundary layers. Methodological improvements include modifying the spectral POD algorithm to reduce the requirements on time series length and applying a space-time formulation of POD to flows with rapidly varying statistics over time. Besides, the receptivity mechanisms that induce bypass transition in environments of high free-stream turbulence are studied through modal decomposition. This involves applying the linearised Navier-Stokes equations to distinguish the linear response to external disturbances from structures amplified by nonlinear interactions. Subsequently, the identified nonlinear mechanisms are studied through a set of reduced nonlinear forcing modes related to POD modes through the resolvent operator, clarifying the role of nonlinear mechanisms in the amplification of instabilities in the boundary layer during the early stages of turbulent transition. |