Computational analysis of receptivity and growth mechanisms in laminar boundary layers

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The relationship between free-stream vortical disturbances and the resulting perturbations in boundary layers is investigated. For example in the flow through turbines, the presence of external disturbances induces a rapid breakdown process, commonly known as bypass transition, which transforms the orderly laminar flow into a chaotic turbulent state. Turbulent flows are associated with increased levels of viscous drag and often entail lower efficiency. In order to optimize efficiency, the design process of airfoils thus often aims to delay transition to turbulence as far as possible. In other cases, such as heat exchangers, it is sought to efficiently obtain a turbulent state while keeping viscous losses at a minimum. By means of an optimization approach, the present study identifies for the first time the most dangerous external disturbances, which are most effective in triggering the amplification of perturbations in pre-transitional boundary layers. This improved knowledge about the flow physics of transition due to external disturbances can help to improve efficiency and reduce emissions in a wide range of applications. Furthermore, the computational framework developed in this context will provide the basis for software tools which can be directly integrated into the industrial design process and which predict the potential for perturbation growth due to external disturbances in various flow configurations.