Effect of Winglet Serration Geometry on the Wingtip Vortex
The effect of serrations applied to the top edge of a winglet on the wingtip vortex is explored experimentally using dye-flow visualisation and stereoscopic particle image velocimetry. The wing is towed in a water tank at a chord-based Reynolds number of 100 000 and four different serration edge patterns are compared to a baseline winglet featuring a single shedding edge. The chevron winglets tested feature a triangular and sinusoidal shedding edge of the same wavelength, a multi-scale geometry imposed on the underlying triangular pattern as well as a short-wavelength triangular shedding edge, where the wavelength is reduced by half. The serrations of the wingtip are found to shed two distinct vortices from the peak and troughs of the pattern that subsequently merge around 10 chord lengths downstream of the wing. The merging process is thought to be the dominant contributor to the observed change in vortex structure. The resulting vortex features a lower swirl velocity as well as a larger vortex core compared to the vortex shed by the baseline endplate. These effects persist until at least 155 and 70 chord lengths downstream of the wingtip when the wing is at an angle of attack of 3◦ and 12◦, respectively. Applying a sinusoidal or fractal pattern has a negligible effect on the vortex when compared to the underlying triangular wave pattern. In contrast, modifying the wavelength of the serrations has a significant effect on the vortex structure, where the short-wavelength triangular pattern displays the biggest reduction in swirl velocity and core radius increase.