标题： 垂直轴涡轮机的近尾流动力学 显示英文标题

标题： Near-Wake Dynamics of a Vertical-Axis Turbine

摘要： 横流或垂直轴涡轮是一种有前途的技术，用于捕捉风或流动水中的动能，其固有的非定常流体力学为单独转子或阵列的控制优化提供了独特的机会。 为了探索阵列中涡轮之间有益相互作用的潜力，使用平面立体粒子图像测速技术在水槽中研究单个双叶片横流涡轮尾流中的相干结构。 首先，描述了这种高弦长与半径比转子的平均尾流结构，并与以往的研究进行比较，提出了对观察到的尾流偏转的简单解释。 其次，通过三重分解分析非定常流，利用传统技术和优化动态模态分解的新实现方法提取周期分量。 后一种方法被证明优于条件平均和傅里叶方法，并揭示了远尾流中可能向钝体脱落过渡的频率。 第三，旋度和有限时间李雅普诺夫指数被用来进一步分析振荡尾流部分。 在尾流的两侧识别出涡街，并讨论了它们的形成机制及其对平均流的影响。 在转子后退侧的涡旋结构中观察到强烈的轴向（垂直）流动，其中叶片向下游移动。 对这些涡旋进行了时间分辨跟踪，结果表明，在一个直径下游，涡旋轨迹在旋转之间有显著的变化。 这种变化表明，在更远的下游距离处，捕获或避免这些结构将具有挑战性。 显示英文摘要

摘要： Cross-flow, or vertical-axis, turbines are a promising technology for capturing kinetic energy in wind or flowing water and their inherently unsteady fluid mechanics present unique opportunities for control optimization of individual rotors or arrays. To explore the potential for beneficial interactions between turbines in an array, coherent structures in the wake of a single two-bladed cross-flow turbine are examined using planar stereo particle image velocimetry in a water channel. First, the mean wake structure of this high chord-to-radius ratio rotor is described, compared to previous studies, and a simple explanation for observed wake deflection is presented. Second, the unsteady flow is then analyzed via the triple decomposition, with the periodic component extracted using a combination of traditional techniques and a novel implementation of the optimized dynamic mode decomposition. The latter method is shown to outperform conditional averaging and Fourier methods, as well as uncover frequencies suggesting a transition to bluff-body shedding in the far wake. Third, vorticity and finite-time Lyapunov exponents are then employed to further analyze the oscillatory wake component. Vortex streets on both sides of the wake are identified, and their formation mechanisms and effects on the mean flow are discussed. Strong axial (vertical) flow is observed in vortical structures shed on the retreating side of the rotor where the blades travel downstream. Time-resolved tracking of these vortices is performed, which demonstrates that vortex trajectories have significant rotation-to-rotation variation within one diameter downstream. This variability suggests it would be challenging to harness or avoid such structures at greater downstream distances.