Numerical and experimental methods for wake analysis in complex terrains

The assessment of power output quality of wind farms operating in complex terrains is extremely challenging. Complex terrains actually provide a very difficult testing ground, which stresses to the limit techniques that are well established for the offshore case. Actual turbine performances are the result of the intertwining of complex wind flow, wake effects and control system response to these non-trivial phenomena. On these grounds, the present work aims at a numerical and experimental investigation of a wind farm sited in Italy on a very complex terrain. This test case is particularly valuable because the terrain is very steep, with high slopes (up to 60%) even nearby the turbines; also the layout is complex and large. In the present work, a subcluster of turbines is analysed: they represent an interesting testing ground of wake interactions and terrain effects, because of the slopes in their proximity, of the inter-turbine distance of around 2.5 rotor diameters, and of the orientation of the cluster with respect to the most frequent wind direction distribution. For these reasons, a numerical and experimental analysis of this subcluster is performed, through Computational Fluid Dynamics (CFD) techniques on one side, and SCADA data analysis on the other side. Particular attention is devoted to the effects of complex flow on machine capability of optimally following meandering wind direction. It is shown that modeling the free flow is not enough to capture the trend of wind intensity and direction and, in particular, the combination of wakes and terrain effects is fundamental, in order to encode the main features of the directional behaviour of the turbines.