Large-Eddy Simulation of Wind Turbine Blade and Wake Flow

Wind energy is considered a clean and renewable energy source to reduce the dependency on fossil fuels. Thus, electrical power produced by wind turbines is becoming increasingly important for the world energy supply. Due to the complex operating conditions, unsteady loads are critical in turbine design and the alleviation of the unsteady loads can directly prolong the overall turbine lifetime and reduce the energy generation costs. Dynamic stall, i.e., flow separation under a dynamically changing angle of attack, is the major source of unsteady loads on turbine blades. Therefore it is of fundamental interest to understand the involved mechanisms, which helps reducing the probability of fatigue failure and maintenance costs. In this thesis, large-eddy simulations are performed to investigate the dynamic stall mechanisms and the effects of freestream turbulence on the aerodynamic coefficients for dynamic stall conditions. The numerical analysis shows that the stall behaviors can be considerably altered by increasing the turbulence intensity in the boundary layer. Furthermore, this thesis focuses on two innovative and effective control techniques to combat unsteady loads. A novel load control concept, i.e., adaptive camber airfoil, is introduced and large-eddy simulations of the flow over adaptive camber blade sections are performed. A high load reduction of the adaptive camber airfoil, which interacts with large velocity fluctuations in the free stream, is achieved. Additionally, the novel concept of using oscillating flaps to modify the wake turbulent structures is considered. The capability of the oscillating flaps to perturb the helical path of the tip vortex system is evaluated by large-eddy simulations. The tip vortex system perturbed by the flaps breaks up further upstream into wake turbulence, such that a reduction of the structural loads on the wind turbines located in the wake can be expected.