Low-order CFD simulation of a ducted wind turbine in realistic hilly environments

It is common to place wind turbines over smooth hills to exploit the speed up effect resulting from the hill slope. On the other hand, another way to increase the power output of small turbines consists in the adoption of a diffuser. Therefore, it would be interesting to investigate the union of these two techniques, however, since the computing resources required by traditional fully 3D CFD simulations would be prohibitive, other less expensive approaches are needed. Using CFD, the performance of a 50kW wind turbine positioned on the top of a hill and equipped with a symmetrical convergent-divergent diffuser was predicted. To save computation time, the fluid dynamic interaction with the rotating blades is mimicked by adopting the momentum source low-order approach, that in our case is based on the Virtual Blade Model (VBM). The diffuser behavior has been analyzed for three hill profiles characterized by the same height but different shapes that are representative of realistic environments. The results show that the diffuser not only allows a general massive increase in power but also that its use is especially advantageous, compared to the bare turbine, in cases of flows made yawed by the slope of the hill or the geometry of the cliff. This happens because the converging section of the diffuser is able to realign the skewed flows normally to the turbine rotor.