Understanding Wind-Turbine Wake Breakdown Using Computational Fluid Dynamics

This work explores the breakdown of the wake downstream of the Model Experiments in Controlled Conditions Project (known as the MEXICO project) wind-turbine rotor and assesses the capability of computational fluid dynamics in predicting its correct physical mechanism. The wake is resolved on a fine mesh able to capture the vortices up to eight rotor radii downstream of the blades. At a wind speed of 15m/s, the main frequency present in the computational fluid dynamics signals for up to four radii was the blade-passing frequency (21.4Hz), where the vortex cores fall on a perfect spiral. Between four and five radii downstream, higher-frequency content was present, which indicated the onset of instabilities and results in vortex pairing. The effect of modeling a 120deg azimuthally periodic domain and a 360deg three-bladed rotor domain was studied, showing similar predictions for the location of the onset of instabilities. An increased frequency content was captured in the latter case. Empirical and wake models were also explored, they were compared with computational fluid dynamics, and a combination of kinematic and field models was proposed. The obtained results are encouraging and suggest that the wake instability of wind turbines can be predicted with computational fluid dynamics methods, provided adequate mesh resolution is used.