Complex terrain effects on wake characteristics of a parked wind turbine

In order to extend lifetime and enhance energy production of wind turbines in complex terrain it is important to learn about their wake characteristics. Hence, wind-tunnel experiments are carried out to analyze the wind-turbine wake downwind of a mountain. The wind-tunnel simulation of the neutrally stratified atmospheric boundary layer (ABL) developing above a flat terrain is first generated using the well established Counihan approach. Once a well developed ABL simulation is achieved for a flat terrain, three various mountain models are separately exposed to that ABL simulation, as well as the flat terrain as a reference case. Wake characteristics of a single wind-turbine model in the wake of those four various terrain models are then studied. The ratios between the height of different mountains to the wind-turbine hub height are 0, 0.417, 0.833, 0.833 (0.417) for the flat terrain, small mountain, large mountain, mountain with a bay, respectively. For the mountain with a bay, there are two different ratios between the mountain height and the wind-turbine hub height; 0.833 at the lateral edges of the mountain, 0.417 in the lateral center of the mountain. All three mountain models are 600 mm long in the main wind direction and 1000 mm wide laterally to the main wind direction. Small mountain model is 100 mm uniformly high, large mountain model is 200 mm uniformly high. The mountain with a bay model is 200 mm high with a normal slope to 100 mm height in the lateral center of the mountain model, whereas this cavity is 200 mm wide in the lateral direction. The calculated ABL simulation length scale factor is 1:300, and it is applied on mountain models and the wind-turbine model as well. The wind-turbine model is designed to correspond to commonly used prototype wind turbines. The experiments are carried out for the wind-turbine model in parking position to analyze trends expected in a strong wind situation, when there is no rotation of the rotor blades. Wake characteristics analyzed with respect to the mean wind velocity, turbulence intensity and velocity power spectra indicate several important findings. In particular, the observed flow retardation is more exhibited near the ground surface and the mountains. The mountain-induced flow disturbance enhances with increasing the size and complexity of the mountains. Turbulence intensity in the wake of the mountain and wind turbine is considerably larger than in the atmospheric boundary layer. Velocity power spectra are strongly influenced by the terrain complexity, whereas the effects of the mountain and surface roughness are mostly constrained up to the double mountain height. There is a strong energy content at frequencies corresponding to a free shear layer separating from the mountain ridge. Dominant turbulence structures are displaced vertically, as the wake is transported with the flow in the main wind direction.