Exploring wake interactions in a counter-rotating dual-rotor system for different yaw configurations: an experimental study

As an opportunity to increase turbine and wind park efficiencies, multi- and dual-rotor turbines have received great attention among research and industry. The goal of this experimental study is to validate and add findings to numerical simulations on dual rotors. It will be shown that the rotational sense greatly influences the lateral interaction and therefore the downstream evolution of a dual wake. In the experiments, 19 hot wires on a downstream traversing array are used to acquire the velocities in the wake at high temporal and spatial resolution. Two laterally spaced counter-rotating model wind turbines with a diameter of 0.58 m (=1 D) generate the wake, which is analyzed between 0.75 and 10.0 D downstream. The benefits in dual-rotor wake control will be demonstrated through symmetrical yaw-alternation of the setup in dependency of the positioning of the turbines with different rotational sense. To achieve a beneficial wake evolution for a downwind turbine, two main strategies are presented. One is a wake centroid deflection away from a possible downstream turbine, causing a flow entrainment from above the dual-rotor setup. The other one is an induced wake collision, resulting in strong turbulent interaction between the two single wakes and faster wake recovery. The knowledge of these principles and their application to full scale show great potential for large efficiency gains for wind park operation.