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Predicting Wind Turbine Wake Breakdown Using a Free Vortex Wake Code

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When modeling wind turbine wake recovery, the location of wake breakdown plays a crucial role. The breakdown is caused by a rapid deformation of the helical near-wake vortex structure that is triggered by the pairing of successive blade tip vortices. In this paper, the capability of a cost-efficient lifting-line free vortex wake code to accurately predict the wake breakdown location and its underlying mechanisms is demonstrated and validated against simulation results of a large-eddy simulation solver and additional data from the literature. Furthermore, this work investigates a technique to accelerate the breakdown of wind turbine wakes. The onset of wake breakdown is caused by perturbations that travel along the helical structure of the wake and grow via mutual-induction interaction between neighboring vortex filaments. To accelerate wake breakdown, the blade tip vortices are perturbed at different frequencies via trailing-edge flaps located in the outboard region of the rotor blades. Through the evaluation of the perturbation growth rates and the analysis of velocity fields, it is shown that for a multi-megawatt wind turbine operating in a turbulent wind field, the wake breakdown position can be significantly affected by a moderate flap actuation amplitude if excited at an appropriate frequency.


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