Dynamics and Destabilisation of Helical Vortices

风力发电机尾流效应可以显著降低风电场的全球发电量。为了更快地恢复尾流速度赤字，人们提出了一种有吸引力的可能性，即对尾流进行激励，以诱导较早的尾流破裂。为了有效地开发此类尾流控制技术，有必要了解不稳定性及其驱动参数。UBeRT项目旨在研究螺旋尾流击穿的基本物理学。这将通过建立一个独特和新颖的实验和数值测试平台来研究不稳定性来实现。对尾迹激励措施的响应将使用三种涡轮模型进行探索：i)实验涡轮（UBeRT）， ii) UBeRT的运动学比例数值孪生（NumBeRT），将在空气中而不是在水中运行，iii)通用全尺寸15MW涡轮数值模型（IEA 15MW）。为了获得比传统风洞更高的雷诺数，UBeRT转子应在VWS （TU Berlin）的大型拖曳槽中运行。数值孪生数伯特的模拟应用于评估实验结果在空气中的可转移性。IEA 15MW风力机的模拟应提供有关可能的尺度效应的信息，并解决宽带入流湍流对尾流稳定性的影响。为了了解螺旋尾流击穿的基本物理特性，需要对长、短波不稳定性激励下螺旋尾流的模态和频率含量进行基本研究。通过相对流入速度的周期性变化，在数值和实验上引入长波不稳定性。短波不稳定性在实验中是通过叶尖段产生的二次涡来引入的，二次涡通过与叶尖涡的相互作用产生短波不稳定性。在数值上，它将通过在叶尖增加涡量来引入，这将在涡核中产生椭圆不稳定性。提出的项目旨在进一步确定湍流流入激发的不稳定模式以及流入湍流与螺旋涡之间发生的相互作用机制。尾迹发展的模态分析应揭示导致最快不稳定增长的扰动尺度。利用立体粒子图像测速技术采集了UBeRT旋翼下游尾迹发展和击穿的高分辨速度场。将使用流动求解器FLOWer对NumBeRT转子和IEA15MW涡轮机进行高保真的尺度解析模拟。这两个数据集将公开用于代码比较和验证低阶尾流模型，同时补充现有的涡轮机数据库，这些数据库没有明确关注尾流破坏现象。

Wind turbine wake effects can significantly reduce the global power production of wind parks. In order to achieve a quicker recovery of the wake velocity deficit, exciting the wake in order to induce earlier wake breakdown has been proposed as an attractive possibility to reduce these effects. For the efficient development of such wake control techniques, understanding instabilities and their driving parameters is necessary. The UBeRT Project aims to investigate the fundamental physics of helical wake breakdown. This shall be achieved by establishing a unique and novel experimental and numerical testbed for investigating instabilities. Response to wake excitation measures shall be explored using three turbine models: i) An experimental turbine (UBeRT), ii) a kinematically scaled numerical twin of the UBeRT (NumBeRT), that shall operate in air instead of water, and, iii) a generic full-scale 15 MW turbine numerical model (IEA 15MW). The UBeRT rotor shall be operated in the large towing tank at VWS (TU Berlin) in order to achieve higher Reynolds number than in a conventional wind tunnel. Simulations of the numerical twin NumBeRT shall serve to evaluate transferability of the experimental results in the air. Simulations of the IEA 15MW wind turbine shall provide information about possible scaling effects and address the effects of broadband inflow turbulence on wake stability. Fundamental investigation of modal and frequency content of helical wakes under excitation of long- and short-wave instabilities is required to understand the fundamental physics of helical wake breakdown. Long-wave instabilities shall be introduced numerically and experimentally via a periodic variation of the relative inflow velocity. Short-wave instabilities shall be introduced experimentally via a secondary vortex emerging from the blade tip section, which through interaction with the tip vortex shall give raise to a short-wave instability. Numerically, it shall be introduced via an added vorticity at the blade tip which shall create an elliptic instability in the vortex core. The proposed project aims further at identifying unstable modes excited through turbulent inflow and which interaction mechanisms occur between inflow turbulence and the helical vortex. Modal analysis of the wake development shall reveal the perturbation scales that lead to the quickest instability growth. Highly-resolved velocity fields of the wake development and breakdown downstream of the UBeRT rotor will be collected by means of stereo Particle Image Velocimetry. High-fidelity scale-resolving simulations of the NumBeRT rotor and the IEA15MW turbine will be performed with the flow solver FLOWer. Both data sets will be publicly available for code comparisons and validation of lower order wake models while complementing existing turbine databases, which do not explicitly focus on wake breakdown phenomena.