A New Hybrid Free-Wake Model for Wind Turbine Aerodynamics with Application to Wake Steering

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2017

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Abstract

Wind energy has emerged as one of the most promising and rapidly growing renewable energy technologies in the United States and over the world. The offshore wind energy is of special interest because it has more consistent and faster wind speed, and is usually close to large population areas that are along the coast. However, wake shielding on offshore wind farms substantially reduces the efficiency of downstream wind turbines due to the interaction with the energy-depleted wakes from upwind turbines. This research considers a method to mitigate the wake shielding effect by tilting the turbine axes upward, which causes streamwise vorticity in the near wake so that the energy depleted wakes transport upward alleviating shielding, and pumping more energetic fluid into downstream turbines.

The wake simulations in this research employ a specially developed hybrid free-wake method for wind turbine wakes, that utilizes Vortex Lattice Method (VLM) for near wake representation with appropriate stall and unsteady models, and Constant Circulation Contours Method (CCCM) for turbine far wake representation with a large degree of downwind vorticity diffusion. This approach has been implemented to capture the natural behavior of multi-filament multi-blade complex turbine wakes in relatively short computation time, with the capability to simulate wake interaction with downstream turbines. It is validated through comparison to two wind tunnel tests, NREL/NASA-Ames Wind Tunnel Test and MEXICO, and two turbine wake numerical models, BEM and QBlade.

The wake steering effect for tilted turbines is verified and the degree of effectiveness is assessed. Detailed turbine wake structure is studied to obtain insights into how to strengthen the steering effect and decrease wake velocity deficit. Inline two turbine simulations where one turbine operates in the wake of the other have been performed to assess the advantage of wake steering in power generation of a system of turbines. Beyond the single rotor tilted turbine, an intermeshed rotor wind turbine configuration, consisting of two partially overlapping counter-rotating rotors, has been studied to assess its potential to strength wake steering effect and to intensify wake deficit recovery. These two turbine configurations are compared along with a discussion of potential advantages and challenges. Several model refinements for more robust turbine wake simulation are under development or considered as future research goals.

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Su, Keye (2017). A New Hybrid Free-Wake Model for Wind Turbine Aerodynamics with Application to Wake Steering. Dissertation, Duke University. Retrieved from https://hdl.handle.net/10161/16390.

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