Session A02: Mini-Symposium: Wind Energy Fluid Mechanics

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Progress in wind energy depends on the scientific understanding of the fundamental physics that drive the systems and our ability to translate that understanding to actionable design decisions. A recent gathering of experts from around the world discussed the challenges that limit our ability to drive innovation and create wind systems able to supply half or more of our electricity demand by midcentury. Three grand challenges were identified: i) understanding the physics of the atmosphere in the critical zone of wind turbine and plant operation; ii) aeroelasticity, system dynamics and manufacturing of the largest rotating machines ever built; and iii) controlling and optimizing wind plants to support the reliability and resilience of the future, renewables-dominated grid. One major theme is the tremendous range of scales in both time and space involved in computing wind plant performance, driving technology innovation, and operating the electrical grid. Bridging the fluid-dynamic scales, from the atmospheric weather systems down to the boundary layer of the airfoils, runs through the center of the grand challenges. Education that develops both a deep understanding of the physical phenomena and the systems interplay from the atmosphere to the grid is discussed.   

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Our recent paper highlighted three grand challenges to drive innovation to meet future demand and functionality of wind energy. The first of these challenges, improved understanding of the physics of atmospheric flow and wind power plant flow physics, requires collaboration between the atmospheric science and wind energy engineering communities to enable improved wind energy forecasting, operations, and wind turbine/wind plant design. Wind turbines reside in the lower levels (e.g. 300 m) of the atmosphere, and so experience not only global meteorological phenomena, manifested as pressure gradient and Coriolis forces, but also localized friction and buoyant forces from the surface. This presentation will highlight advances and gaps in our knowledge of flows in complex terrain, varying stability conditions, and in the offshore environment. We also highlight recent developments in coupled mesoscale-microscale modeling, so that design and operations of wind plants can incorporate both “weather” and localized forcing. Finally, wakes from individual turbines and from wind plants themselves exert impacts on downwind turbines as well as local micrometeorological environments. Recent advances in wake modeling, prediction, measurement, and manipulation will be presented.   

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The monotonic growth of wind energy brings numerous and new challenges in several scientific and engineering disciplines. Trends point towards designing larger wind farms with larger units in complex terrains, harsh environments, and offshore with fixed and floating units. Understanding the atmospheric boundary layer flow, unsteady flow-structure interaction, wake dynamics within wind farms, as well as loads and power fluctuations, are crucial to making this renewable energy resource sustainable and competitive (see Veers et al., DOI: 10.1126/science.aau2027). Several factors challenge the required characterization of related phenomena at laboratory and field scales. Scaling effects and representing multiscale processes are some barriers in laboratory experiments, whereas cost, fidelity, and flexibility are usually limiting factors in field measurements. Here, I will briefly illustrate current approaches used to capture the underlying flow physics in wind plants and will discuss measurement limitations and challenges at the laboratory and field settings.   

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The performance of large wind farms depends on the development of turbulent wind turbine wakes and the interaction between these wakes. Turbulence also plays a crucial role in transporting kinetic energy from the large-scale geostrophic winds in the atmospheric boundary layer towards heights where wind farms can harvest this energy. High-resolution large eddy simulations (LES) are ideally suited to understand these flow phenomena. Much has been learned from wind farm simulations, which initially focused on 'idealized' situations. Nowadays, the community increasingly focuses on modeling more complex situations, such as the effect of complex terrain and different atmospheric stability conditions. As wind farms become larger, the need to improve their design and develop control strategies to mitigate wake effects increases. However, due to the large separation of length scales and the number of cases, it is unfeasible to use LES for wind farm design. Therefore, LES are used to further develop computationally more tractable modeling approaches ranging from Reynolds Average Navier Stokes (RANS) models to analytical modeling approaches. With the increasing size of wind turbines and wind farms, an emerging challenge will be to also account for mesoscale flow phenomena.   

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A recent publication in Science has identified aerodynamics as a grand challenge for the continued growth and development of wind turbines. Current design practice uses blade element momentum (BEM) theory to derive blade forces from aerodynamic inputs. BEM is computational efficiency and has been used for the majority of blade designs, but it does have limitations. The assumptions used by BEM are challenged by modern and future wind turbine blades where lengths exceed 100m. As blades get more flexible, and include bend-twist coupling effects, the ability to accurately model unsteady aerodynamics is more important than ever. Recent advances in high-performance computing can give important insights into the phenomena. Large very flexible blades have large out of plane deflections that violate BEM assumptions. Recent studies at the National Renewable Energy Laboratory have been conducted using vortex methods to quantify these effects on the loads of large highly flexible blades and compare results to BEM. Further, vortex methods may be used to understand the interaction of these very flexible blades with their own wakes. This talk will give an overview of the current state-of-the-art in wind turbine blade aerodynamic modeling and will highlight current challenges and areas of recent and future work.   

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The fluid physics of offshore wind plants offer unique challenges that distinguish them from onshore wind plants. Offshore wind plants developed in the coming decade are likely to have enormous spatial scales compared to onshore plants, with hundreds of turbines operating, each with rotor diameters exceeding 200 m. In this talk, several the fluid physics challenges for offshore wind plants will be discussed, including modeling, simulation, and measurement challenges. These include: the interaction between the atmospheric flow physics and the wind plant, which spans the mesoscale and microscale and creates significant numerical modeling challenges; the prevalence of extreme events such as hurricanes, with unique flow physics that produce wind conditions that differ fundamentally from normal operation; the role of wind turbine wakes and their impact on wind plant performance, as well as plant to plant interactions; the complex unsteady aerodynamics of floating wind turbines, which experience dynamic platform motion.