Wind turbine - atmospheric boundary layer interactions: model experiments and implications on numerical simulations

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With diameters on the order of 100 meters, horizontal axis wind-turbines are the largest rotating machines ever built. Unlike most turbomachines, their normal operating conditions are strongly influenced by natural variability. When deployed as large arrays in wind farms, wind turbines interact among themselves and with the atmospheric turbulent boundary layer. To optimize their siting and geometric arrangements, as well as to quantify the aggregate effects of wind-farms on the land-atmosphere exchanges at various scales, computer modeling of the atmospheric transport processes requires parameterizing the wind turbines as realistically as possible. Intellectual Merit: The proposed research is a wind-tunnel experimental study of flow and energetics in wind-turbine arrays, and multi-scale analysis of data motivated specifically to address computer modeling and parameterization issues. Much is already known about wind-turbine blade aerodynamics, but the knowledge on modeling the interactions of wind-turbines with the turbulent, highly variable environment is far less developed. The proposed research will address this gap using new-generation fluid dynamics instrumentation at a much increased resolution compared to prior work, coupled with new data analysis techniques. Stereo Particle-Image-Velocimetry will be used to measure high-resolution velocity maps in wind-tunnel models of wind-turbine arrays under neutral stratification. The data will be used address the following issues: (i) Model parameters (such as drag or power coefficients, effective roughness height, etc..), including their dependence on properties of the flow and array geometry. (ii) The role of coherent periodic structures in helical wakes, with specific emphasis on their impact on stresses. Borrowing from concepts developed for multi-stage turbomachinery, the data will be used to measure the "deterministic stresses". The relative importance of such stresses will be quantified and possible models proposed. (iii) The scaling behavior of mean velocity and stress profiles will be examined via a similarity approach. (iv) Energy fluxes across scales will be examined, using analysis tools newly developed in the context of Large Eddy Simulation and subgrid-scale modeling. Results will be condensed into useful parameterizations and models that may be used in micro, meso, and global scale simulations of wind-turbine arrays in the atmosphere. Broader impacts: The proposed research will contribute to advance the applicability, efficiency, and understanding of the long-term sustainability of wind energy extraction from the atmospheric environment. In terms of intellectual broader impacts, the proposed research on the interactions among large human-made machinery, coherent flow structures, and a random turbulent background may lead to new 'space-scale-time' modeling concepts and analysis tools. In terms of outreach, synergies with an existing NSF-funded Alliance for Graduate Education and Professoriate (AGEP) program will be developed. Current involvement in the Central New York-Puerto Rico (CNY-PR) Alliance for Graduate Education and the Professoriate at RPI focuses on recruiting and mentoring minority students for academic careers. An additional center is planned for Caguas, Puerto Rico - "The International Center on Energy and Sustainability". The proposed research will enable useful synergies between JHU, RPI, this planned Center, and AGEP partners University of Turabo and University of Puerto Rico-Mayaguez. Recruitment of undergraduate students from Puerto Rico to work during summer internships on this project is also planned, as well as mentorship of postdoctoral scholars for academic careers in the field of energy and sustainability.