• CFD
• dynamic stall
• flusso inclinato
• offshore
• skewed flow
• stallo dinamico
• turbine eoliche asse verticale
• VAWT
• vertical axis wind turbine

The present work demonstrates the potential of an unsteady RANS 3D approach to predict the effects of skewed winds on the performance of a Darrieus vertical axis wind turbine. In a model validation section we show a comparison between numerical and experimental results for a full scale turbine to prove the superior prediction ability of 3D CFD with respect to both 2D CFD and semi-empirical models based on the double multiple stream tubes method. Then we adopt the 3D URANS approach to investigate the reasons for the power increasing achieved by a straight-bladed small scale turbine in a wind tunnel when a tilt angle in the range 0°÷15° is set. The main advantages of this approach are a more realistic description of complex three-dimensional flows like as dynamic stall and the opportunity to check what locally happens on any blade portion during upwind and downwind paths. As a consequence not only the prediction of the turbine overall performance in terms of power coefficient is allowed but also a deep insight in the temporal and spatial evolution of the dominant physical mechanisms is achievable. Our principal finding is that the power gain in skewed flows is obtained during the downwind phase of the revolution as the ending part of the blade is less disturbed by the wake generated during the upwind phase.