Generating realistic turbulent wakes from scaled rotors in the lab: inflow control, rainfall and Coriolis forces

Within a worldwide effort to increase alternative energies generation, the study of the flow downstream one or several wind turbines has captured the attention of the turbulence community for its complexity; it involves turbulent wakes, interactions between them and their coupling with the background turbulent flow. The main limiting factors to reproduce operating conditions in applications rely in the difficulties generating realistic conditions in experimental facilities. This seminar is divided in two parts. In the first one, we will focus specifically on the spatial development of the turbulent wake under different inflows. In a series of experiments performed at Oldenburg’s large wind tunnel, the near and far wake of a scaled wind turbine have been studied. The spatial evolution has been characterized for different Reynolds numbers, tip speed ratios and background flows (laminar and turbulent). The resulting database shows that, while the flow always evolves according to the classical Richardson-Kolmogorov scalings, operating conditions have a strong influence in the properties of the wake. For instance, they are related to the velocity deficit scalings and the streamwise extent of the production region of turbulence. In the second part we will discuss further efforts to achieve realistic conditions in experimental facilities. First, experiments using scaled rotors in a wind tunnel under rainfall conditions will be discussed. It has been found that rainfall influences the persistence of the wake, and that the rain droplets tend so segregate in clusters and voids within the far wake. Further, to assess mesoscale phenomena in the atmosphere, a rotating platform was used to assess the influence of Coriolis force in the large-scale wake generated by a wind farm. Such effects are becoming relevant as wind farms become larger, and the global wake they generate is characterised by Rossby numbers low enough to make the Coriolis force significant. Our experiments confirm that, while the wake of a single rotor does not experiences any deflection, the merged wind farm wake is deflected due to the effect of rotation.