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

本应用的主要目的是深化最近获得的关于飞行条件下自然层流-湍流转变的第一个非线性阶段的知识，并将其转化为通过扩展的EN方法进行的改进的转变预测。在一个联合项目中采用了实验、弱非线性理论和直接数值模拟。将开发一种程序来说明在弱非线性阶段中低频模式的自产生和随后的放大。该方法可以类似于可变(简化)n-因子法，或者等效地，可以被设计成通过考虑低频模式的n-Boost来在较短的顺流运行长度的转变处获得n的标准值。EN方法是最广泛使用的过渡预测方法，尽管尝试使用具有感受性、生长和击穿模块的基于幅度的工具。然而，后者需要一个准确的迎面而来的干扰谱，这是非常难以预测的，而且在安静的飞行情况下往往是徒劳的。EN方法的改进将对层流翼型的设计产生重大影响，满足对环境负责的航空和提高风力涡轮机生产可再生能源的效率的要求。

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.