An Experimental Investigation on the Effects of Turbine Rotation Directions on the Wake Interference of Wind Turbines

An experimental study was conducted to investigate on the effects of the relative rotation directions of two tandwm wind turbines on the power production performance and flow characteristics in the wakes of two wind turbines in tandem. The experimental study was performed in a large-scale Aerodynamics/tmospheric Boundary Layer (AABL) Wind Tunnel located at the Aerospace Engineering Department of Iowa State University. While the oncoming Atmospheric Boundary Layer (ABL) wind was kept in constant during the experiments, the turbine power outputs, the dynamic wind loads (i.e., aerodynamic forces) acting on the wind turbines, and the flow characteristics in the wakes the wind turbines were measured and compared quantitatively with the turbines operating in either co-rotating (i.e., the downstream wind turbine has the same rotation direction as the upstream turbine) and counter-rotating configurations (i.e., the downstream wind turbine has an opposite rotation direction in relation to the upstream wind turbine). It was found that an obvious azimuthal flow velocity component, i.e., so-called “pre-rotating” effect, would be generated in the wake flow of the upstream turbine. When the downstream turbine operates in the co-rotating configuration, due to the effects of the “pre-rotating” azimuthal flow velocity, the effective angle of attack of the oncoming ABL wind approaching to the second wind turbine would be decreased, thereby, a smaller lift to drive the rotor of the downstream turbine. However, when the downstream turbine operated in the counter-rotating configuration, the “prerotating” azimuthal flow velocity would result in a greater effective angle of attack for the oncoming wind to approach the second wind turbine, thereby a larger lift to drive the rotor of the downstream turbine. As a result, the downstream wind turbine was found to be able to harvest more wind energy when it was operated in counter-rotating configuration (up to 17% more power output were achieved based according to the measurement results of the present study), compared with that in counter-rotating configuration. The benefits of the counter-rotating configuration in power production were found to decrease gradually as the distance between the two wind turbines increases.