LowNoise: Linear Stability and Resolvent Analysis for Prediction and Mitigation of Wind Turbine Trailing-edge Noise

减轻风力涡轮机的流动噪声对于当前和未来甚至更强大的风力涡轮机至关重要。最重要的噪声源是后沿噪声。它是由转子叶片后缘处边界层中的压力波动引起的。频率特性和声级在很大程度上取决于边界层中的相干涡流状结构。目前的方法来减少后缘噪声显示矛盾的结果，因为它们对相干结构的影响既没有充分理解，也不可靠modeled.Current的自由剪切流和边界层流的研究清楚地表明，线性稳定性和预解式分析可以用来系统地描述相干结构的形成和控制。因此，在LowNoise项目中，这些非常成功和新颖的方法被应用于典型风力涡轮机翼型的流场。中心目标是一个物理低维模型，它描述了后缘噪声的基本机制。相干结构近似为平均流场的线性模式，可以通过稳定性或预解式分析来确定。机翼表面上的压力波动然后由线性模态的数据同化确定。因此，后缘噪声是机翼表面压力场积分的结果。该方法的核心创新之处是使用线性稳定性和预解式理论对声源进行建模。一方面，这使得能够在比当前低维模型少得多的经验输入变量下定量确定后缘噪声。另一方面，该模型提供了相干结构形成的因果机制，从而也提供了后缘噪声的因果机制，这使得优化现有的控制措施和开发新的更有效的控制措施成为可能。LowNoise项目从整个流场的大涡模拟开始。从流场的速度和压力波动的相干结构可以提取和经验的低维模型可以推导出来。基于模拟的平均流场，将开发稳定性和解决方案理论模型。通过风洞中的压力和声学测量，对模拟和模型进行了实验验证。该项目最后对后缘噪声的控制影响进行了基于模型的分析。在LowNoise中开发的模型将显示风力涡轮机后缘噪声产生和有效降低的基本物理关系。此外，LowNoise中开发的模型和概念可以应用于各种其他有壁流动。

The mitigation of flow noise from wind turbines is of central importance for current and future, even more powerful wind turbines. The most important noise source is the trailing-edge noise. It is caused by pressure fluctuations in the boundary layer at the trailing edge of the rotor blade. Frequency characteristics and sound levels depend largely on the coherent eddy-like structures in the boundary layer. Current methods to reduce trailing-edge noise show contradictory results, as their influence on the coherent structures is neither sufficiently understood nor reliably modelled.Current investigations on free shear flows and boundary layer flows clearly show that linear stability and resolvent analysis can be used to systematically describe the formation and control of coherent structures. Therefore, in the LowNoise project, these very successful and novel methods are applied to the flow field of a typical wind turbine airfoil. The central goal is a physical low-dimensional model which describes the essential mechanisms of trailing-edge noise.The modelling is done in several steps. The coherent structures are approximated as linear modes of the mean-flow field, which can be determined by the stability or resolvent analysis. The pressure fluctuations on the wing surface are then determined by data assimilation from the linear modes. The trailing-edge noise, then, results from the integral of the pressure field on the wing surface.The central innovation of this approach is the modelling of the sound sources using linear stability and resolvent theory. On the one hand, this enables the quantitative determination of the trailing-edge noise at significantly fewer empirical input variables than with current low-dimensional models. On the other hand, the model provides the causal mechanisms of the formation of the coherent structures, and thus, of the trailing-edge noise, which allows to optimise existing control measures and to develop new more effective control measures.The LowNoise project starts with large-eddy simulations of the entire flow field. From the flow fields the velocity and pressure fluctuations of the coherent structures can be extracted and an empirical low-dimensional model can be derived. Based on the mean-flow fields of the simulation the stability and resolvent theory model will be developed. The validation of the simulations and the model is done experimentally by means of pressure and acoustic measurements in a wind tunnel. The project ends with a model-based analysis of the control influences of the trailing-edge noise.The models developed in LowNoise will show the essential physical relationships for the generation and effective reduction of trailing-edge noise of wind turbines. Furthermore, the model and concepts developed in LowNoise can be applied to a variety of other wall-bounded flows.