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音对当前和未来，甚至更强大的风力涡轮机至关重要。最主要的噪声源是尾缘噪声。它是由动叶尾缘边界层压力波动引起的。频率特性和声级在很大程度上取决于边界层中的相干涡状结构。目前减少尾缘噪声的方法显示出矛盾的结果，因为它们对相干结构的影响既没有得到充分的理解，也没有得到可靠的建模。目前对自由剪切流动和边界层流动的研究清楚地表明，线性稳定性和解析分析可以用来系统地描述相干结构的形成和控制。因此，在低噪声项目中，这些非常成功和新颖的方法被应用于典型的风力涡轮机翼型的流场。中心目标是一个描述尾缘噪声基本机制的物理低维模型。建模分为几个步骤。相干结构近似为平均流场的线性模态，可以通过稳定性分析或解析分析来确定。然后通过线性模态的数据同化来确定机翼表面的压力波动。因此，尾缘噪声是机翼表面压力场积分的结果。该方法的核心创新是利用线性稳定性和解析理论对声源进行建模。一方面，与目前的低维模型相比，这使得可以在更少的经验输入变量下定量确定尾缘噪声。另一方面，该模型提供了相干结构形成的因果机制，从而提供了尾缘噪声的因果机制，从而可以优化现有的控制措施并开发新的更有效的控制措施。低噪声项目从整个流场的大涡模拟开始。从流场中可以提取出相干结构的速度和压力波动，并推导出经验的低维模型。在模拟平均流场的基础上，建立了稳定性和可解性理论模型。通过风洞内的压力和声压测量，对模拟结果和模型进行了验证。最后，对尾缘噪声的控制影响进行了基于模型的分析。在《低噪声》中开发的模型将展示风力涡轮机尾缘噪声产生和有效降低的基本物理关系。此外，在低噪声中开发的模型和概念可以应用于各种其他壁面流动。

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.