• wakes
• wind turbines
• Stability

The wake produced by an horizontal axis wind turbine is the result of complex dynamics and interaction processes between different vorticity structures, which can be hardly characterized through experiments or numerical simulations. The main vorticity structures observed within a wind turbine wake are the helicoidal tip vortices shed from each blade tip and the hub vortex located at the wake center. The latter is characterized by a so-called meandering or wandering instability, which was ascribed from previous works to shedding of vorticity from the rotor disc acting as for bluff bodies. In this work a linear stability analysis of the wake of a wind turbine is performed on the time-averaged flow obtained from wind tunnel velocity measurements of a three-bladed wind turbine model. This study shows that the hub vortex is convectively unstable, producing a counter-winding helicoidal structure. Further, wind tunnel experiments confirm the characteristics of the helicoidal mode predicted through the linear instability analysis.