Flow field assessment in a vertical axis wind turbine

In this work, flow field features in a vertical axis Savonius wind rotor were investigated by means of a specially developed CFD method validated using wind tunnel testing data. The Savonius rotor exhibits a strong variation of its angular velocity during the rotation. The solution of the second cardinal equation of dynamics is then needed to obtain a high-fidelity physical modelling of the device and an accurate performance prediction. The proposed methodology is based on the integration between a commercial CFD code and custom MatLab routines. The former is used for the flow field computations, while the latter are used for the time integration of the second cardinal equation of dynamics. Unsteady RANS equations closed with the v2-f turbulence model were solved to consider the turbulent flow behaviour. This procedure allows one to numerically evaluate the dimensionless parameters used in the performance characterization of wind turbines. Moreover, the flow field around the rotor can be evaluated, at every solved angular position (with respect to the incoming wind direction), taking into account the effect of the angular velocity variations of the rotor. An extended wind tunnel testing campaign was conducted on a rotor model in order to validate the computational procedure. A very good agreement between experimental and numerical data can be observed. The main features of the flow field at different operating points of the rotor are presented in this paper. An evaluation of the characteristic of the wake behind the rotor obtained by means of a spectral analysis of the velocity magnitude signal is reported too.