Abstract
AbstractAn experimental study of the near wake up to four rotor diameters behind a model wind turbine rotor with two different wing tip configurations is performed. A straight‐cut wing tip and a downstream‐facing winglet shape are compared on the same two‐bladed rotor operated at its design tip speed ratio. Phase‐averaged measurements of the velocity vector are synchronized with the rotor position, visualizing the downstream location of tip vortex interaction for the two blade tip configurations. The mean streamwise velocity is found not to be strongly affected by the presence of winglet tip extensions, suggesting an insignificant effect of winglets on the time‐averaged inflow conditions of a possible downstream wind turbine. An analysis of the phase‐averaged vorticity, however, reveals a significantly earlier tip vortex interaction and breakup for the wingletted rotor. In contradistinction, the tip vortices formed behind the reference configuration are assessed to be more stable and start merging into larger turbulent structures significantly further downstream. These results indicate that an optimized winglet design can not only contribute to a higher energy extraction in a rotor's tip region but also can positively affect the wake's mean kinetic energy recovery by stimulating a faster tip vortex interaction.
Highlights
Winglets are small extensions at the tip of any kind of lift-generating wing of finite length
The near wake behind the model wind turbine is scanned in the xz-plane, acquiring all three components of the velocity vector
The mean streamwise velocity in the near wake region up to x∕D = 4.0 is found not to be significantly affected by the presence of the winglets, a previous study by Hansen and Mühle[9] assessed a 7.8%-8.9% higher power coefficient of the wingletted rotor compared with the reference setup. This result indicates that winglets do not negatively affect the mean kinetic energy in the wake flow as a one-dimensional energy conservation law would suggest
Summary
Winglets are small extensions at the tip of any kind of lift-generating wing of finite length. Best known from their widespread application in modern aviation, winglets are recognized to reduce induced drag in the tip regions of aircraft wings. The formation of a tip vortex occurs at any kind of tip shape of a loaded wing due to a secondary flow from the wing's pressure to its suction side around the tip. The spanwise flow in the tip region reduces the local angle of attack and induces additional drag near the tip (Giuni and Green[4]). Winglets cannot suppress the formation of a tip vortex, they might be able to modify the strength and shape of the tip vortex. When applying winglets on a wind turbine rotor, positive effects on the rotor's performance as well as the breakup mechanisms of the tip vortices in the wake are desirable
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