Nichtlineare Stadien der Transition in einer verzögerten Flügel-Grenzschichtströmung bei kontrolliertem, kontinuierlichen und bei natürlichen Störspektrum

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The main goal of the present application is to deepen the recently gained knowledge about the first nonlinear stages of natural laminar-turbulent transition at flight conditions and to transfer it into an improved transition prediction by an extended en-method. Experiments, weakly nonlinear theory, and direct numerical simulations are employed within a joint project. A procedure will be developed to account for the self-generation of low frequency modes and subsequent amplification during the weakly non-linear stage. This approach can be similar to the variable (reduced-) n-factor method, or equivalently, can be designed to achieve standard values of n at transition at a shorter streamwise running length by accounting for an n-boost of low frequency modes. The en-method is the most widely used transition prediction method despite attempts to employ amplitude based tools with receptivity-, growth-, and breakdown modules. However the latter need an accurate oncoming disturbance spectrum that is very difficult to predict, and, moreover, often is futile in quiet flight situations. Improvements of the en-method will have a significant impact on the design of laminar wing profiles, meeting the requirements of environmentally responsible aviation and increased efficiency of renewable energy production by wind turbines.