Understanding the Wind Turbine Wake Breakdown Mechanism with CFD

This work explores the breakdown of the wake downstream of the MEXICO rotor and assesses the capability of CFD in predicting its correct physical mechanism. The wake is resolved on a fine mesh able to capture the vortices up to 8 radii downstream of the blades. At a wind speed of 15m/s, the main frequency present in the CFD signals for up to 4 radii was the blade passing frequency (21.4Hz), where the vortex cores fall on a perfect spiral. Between 4 and 5 radii downstream, higher frequency content was present, which indicates the onset of instabilities and resulted in vortex pairing. The effect of modeling a single-blade domain and a full rotor domain was studied, showing similar prediction of the location of the onset of instabilities, but an increased frequency content was captured in the latter case. Empirical and wake models were also explored, and compared with CFD and a combination of kinematic and field models was proposed. The encouraging results obtained suggest that the wake instability of wind turbines can be predicted with CFD methods, provided adequate mesh resolution is used.