| dc.description.abstract | In order to optimize wind farm control, wake deflection strategies through yaw misalignment have recently been widely discussed. For this purpose, the characteristics of the mean and turbulent wake flow behind a yawed turbine have to be understood in detail. Especially, the interaction mechanisms of the tip vortices behind a yawed rotor and their influence on the deflected curled wake shape have not been investigated yet. An experimental study is performed in order to reveal the position and behaviour of the tip vortices in the near wake of a wind energy turbine model operating in steady yaw. The measurements are executed using a TFI Cobra Probe, which is phase-locked to the rotational speed of the turbine rotor. A non-yaw reference case is compared to a λ = 15° positive- and negative-yaw case.
The tip vortices shed on the upstream side of the yawed rotor are found to contain more turbulent kinetic energy and tend to interact and dissolve earlier than on the downstream side, while the vortices shed by the non-yawed rotor are found to be equal in the interaction behaviour on both sides This is matching the wake border behaviour, which is found to spread asymmetrical due to the influence of the wind turbine's tower and nacelle. Furthermore, the interacting vortices shed on the downstream turned rotor side experience a true wrapping process, while the ones on the downstream side rather merge. This is validated by studying the vortex pitch, which shows that the tip vortices on the downstream turned rotor side move closer together before the wrapping, while the distance between the vortices on the other side increases to a point, where the last vortex of one cycle interacts with the first vortex of the next cycle. | |