Improved Understanding of Vertical Mixing in the Lower Atmospheric Boundary Layer in the Presence of Wind Turbines via Numerical Simulations and Measurements

在过去的几十年里，风能在美国和世界范围内稳步增长，预计在不久的将来将继续增长。风能有无数的好处：它是可再生的和丰富的，不产生任何有害的污染物或温室气体排放，并且技术先进，往往在经济上与其他传统能源竞争。然而，人们越来越担心风力涡轮机在下边界层可能产生的不良影响，特别是对近地表温度的影响。关于这些影响可能是什么，文献存在很大分歧。数值模拟表明：白天和夜晚都很温暖;夜间升温，白天降温;升温或降温取决于大气稳定性;降温;无显著影响。这些相互矛盾的发现突出了风力涡轮机和大气之间反馈的内在复杂性。然而，只有一种机制被广泛接受（但从未经过测试）来解释风力涡轮机如何影响下边界层，即风力涡轮机尾流中产生的湍流增强了垂直混合。尾流是风力涡轮机下风处的羽状体积，其特征在于风速较低，湍流度高于未受干扰的逆风气流。 增强的垂直混合将表现为增加的垂直动量通量（从而增加风切变）和增加近地面的湍流动能（TKE）。测量风力涡轮机近地表特性变化的少数观测活动存在缺陷或不确定性，并且没有直接测量垂直动量通量变化。 因此，没有直接证据的增强垂直混合机制是今天。此外，增强的垂直混合并不能解释所有观察到的效应。智力上的优点：本项目提出的研究假设是，垂直混合一般不会增强，但更有可能在风力涡轮机的下风向附近的表面附近减少。 垂直风廓线由于在由涡轮机叶片跨越的直径为D的转子盘周围的区域中由风力涡轮机提取动能而被极大地改变，但是在转子下方，在地面附近产生的风切变通常不变或减小，而不是增强。因此，近地表的热量、水分和动量通量，以及最终的温度，一般不直接受尾流湍流的影响，而是受减少的而不是增强的风切变、大气稳定度和未扰动流特性的组合的影响。该测量活动名为VERTEX for VERTical Enhanced Mixing，将在刘易斯的特拉华州Gamesa大学2兆瓦风力涡轮机附近进行，使用12 - 15个表面通量塔，60米气象塔和2个扫描激光雷达。VERTEX将量化由于风涡轮机引起的动量和热通量、空气温度和地面附近的TKE的变化。 风力涡轮机和大气流之间的相互作用将使用至少两种最先进的数值代码进行模拟，这些代码可以将湍流尾流的细节解析到几米的分辨率。来自VERTEX的数据将用于验证模拟。更广泛的影响：这项研究将提供一个了解，如果以及如何风力涡轮机可以改变近地表的属性在任何显着的方式。这种理解很重要，因为它将加强风能部署，或者如果这种变化被认为过于剧烈，则会减缓风能部署。 研究结果将传达给：1）科学界通过参加会议和同行评审的论文; 2）通过有针对性的外联工作，包括在特拉华州的海岸日和新闻稿的年度大学展览一般公众;和3）大学生通过新开发的研究生和本科生课程"大气中的能源"。

Wind energy has been growing steadily in the U.S. and worldwide in the past decades and it is projected to continue its growth in the near future. Wind energy has countless benefits: it is renewable and abundant, does not produce any harmful pollutant or greenhouse gas emissions, and is so technologically advanced that it is often economically competitive with other traditional energy sources. However, concerns are rising about possible undesirable effects of wind turbines in the lower boundary layer, especially on near-surface temperature. The literature is highly divided about what these effects could be. Numerical simulations have shown: warming during day and night; warming at night and cooling during the day; warming or cooling depending on atmospheric stability; cooling; and no significant effects. These contradictory findings highlight the inherent complexity of the feedbacks between wind turbines and atmosphere.Only one mechanism, however, has been widely accepted (but never tested) to explain how wind turbines affect the lower boundary layer, namely that turbulence generated in wind turbine wakes enhances vertical mixing. Wakes are plume-like volumes downwind of wind turbines that are characterized by lower wind speeds and higher turbulence than the undisturbed upwind flow. Enhanced vertical mixing would manifest as increased vertical momentum fluxes (thus increased wind shear) and increased turbulent kinetic energy (TKE) near the ground. The few observational campaigns that have measured changes in near-surface properties by wind turbines have been flawed or inconclusive and have not measured directly vertical momentum flux changes. Therefore, no direct evidence of the enhanced vertical mixing mechanism is available today. In addition, enhanced vertical mixing does not explain all the observed effects.Intellectual Merit:The research hypothesis put forward in this project is that vertical mixing is generally not enhanced, but, more likely, reduced near the surface downwind of wind turbines. The vertical wind profile is greatly altered due to the extraction of kinetic energy by wind turbines in the region around the rotor disk of diameter D spanned by the turbine blades, but the resulting wind shear near the ground is generally unaltered or reduced, not enhanced, below the rotor. Near-surface heat, moisture, and momentum fluxes, and eventually temperature, are therefore generally not directly affected by wake turbulence, but rather by a combination of the reduced, not enhanced, wind shear, atmospheric stability, and undisturbed flow properties.To test this hypothesis, a combined observational and numerical approach will be carried out. The measurement campaign, named VERTEX for VERTical Enhanced MiXing, will be conducted near the University of Delaware Gamesa 2-MW wind turbine in Lewes, using 12-15 surface flux towers, a 60-m meteorological towers, and 2 scanning lidars. VERTEX will quantify changes in momentum and heat fluxes, air temperature, and TKE near the ground due to the wind turbine. The interactions between the wind turbine and the atmospheric flow will be simulated with at least two state-of-the-art numerical codes that can resolve the details of the turbulent wake down to a few meter resolution. Data from VERTEX will be used to validate the simulations. The sensitivity to a variety of inflow and atmospheric stability conditions will also be assessed.Broader Impacts:This research will provide an understanding if and how wind turbines can alter near-surface properties in any significant way. This understanding is important because it will either enhance wind energy deployment or slow it if such changes are considered too dramatic. Research findings will be communicated to: 1) the scientific community via participation to conferences and peer-reviewed papers; 2) the general public via targeted outreach efforts, including an exhibit at the annual University of Delaware's Coast Day and press releases; and 3) university students via a newly developed graduate and undergraduate course on "Energy in the atmosphere".