Dynamics and Destabilisation of Helical Vortices

风力涡轮机尾迹效应可以显著减少全球风电场的发电量。为了更快地恢复尾迹速度赤字，激发尾迹以诱导更早的尾迹破裂被认为是一种有吸引力的减小这些影响的可能性。为了有效地发展这类尾流控制技术，了解不稳定性及其驱动参数是必要的。UberT项目旨在研究螺旋尾迹破裂的基本物理。这将通过建立一个独特的、新颖的实验和数值试验台来实现，用于研究不稳定性。应使用三个涡轮机模型来探索对尾流激励措施的响应：i)实验涡轮机(UberT)，ii)UberT的运动尺度数值孪生涡轮机(NumBeRT)，将在空气中而不是水中运行，以及iii)通用全尺寸15兆瓦涡轮机数值模型(IEA 15 MW)。UberT旋翼应在VWS(德国柏林工业大学)的大型拖曳水箱中运行，以获得比传统风洞中更高的雷诺数。对数值孪生NumBeRT的模拟将用于评估实验结果在空气中的可传递性。IEA 15兆瓦风力涡轮机的模拟应提供有关可能的尺度效应的信息，并解决宽带流入湍流对尾流稳定性的影响。为了了解螺旋尾迹破裂的基本物理过程，需要对长波和短波不稳定性激励下螺旋尾迹的模式和频率成分进行基础研究。长波不稳定性应通过相对流入速度的周期性变化在数值和实验中引入。短波不稳定性将通过从叶尖截面涌出的二次涡在实验中引入，通过与叶尖涡的相互作用，将产生短波不稳定性。在数值上，它将通过叶片尖端的附加涡量引入，这将在涡核中产生椭圆形不稳定。该项目的目的是进一步确定通过湍流入流激发的不稳定模式，以及入流湍流与螺旋涡之间发生的相互作用机制。对尾迹发展的模式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仪采集UberT旋翼下游尾迹发展和破裂的高分辨率速度场。将使用流动解算器FLOW对NumBeRT转子和IEA15 MW汽轮机进行高保真尺度分辨模拟。这两个数据集将公开用于低阶尾流模型的代码比较和验证，同时补充现有的涡轮机数据库，这些数据库并未明确关注尾流击穿现象。

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.