Benefits comparison of vortex instability and aerodynamic performance from different split winglet configurations

Abstract In this paper, the benefits of vortex instability and aerodynamic performance from three split winglet configurations with different split angles are compared using Stereo Particle Image Velocimetry technology and bi-global linear stability analysis (LSA) method. Results show that the vortex pair system generated by split winglet configurations consisted of vortex-d and vortex-u twists and spirals with each other downstream the wing, in which the streamwise velocity and tangential velocity of vortex-d are larger than that of vortex-u for three winglet configurations. Concerning the eigenvalue spectrum of them, the branch information obtained from bi-global LSA is similar, mainly containing two different branch types: discrete branch and continuous branch. When increasing the split angle of winglets, the role of vortex-d in the instability will gradually be enhanced, which can be validated by the results of perturbation modes and stability curves. Among all perturbation modes corresponding to discrete branches, primary modes resolved from the primary branch of vortex-u (Mode P u ) and vortex-d (Mode P d ) are the dominant perturbation modes in vortex attenuation. Further, by comparing the penetration depth of Mode P u and Mode P d for three winglet configurations, it is found that the penetration depth of optimal perturbation modes with higher growth rates is within a relatively narrow range under all flow conditions, which can affect the induced drag performance of the wing. In particular, when the split angle increases from 45° to 75°, the induced drag density in the vicinity of vortex-u decreases significantly due to the increase of growth rate for Mode P u . However, the change of split angle from 45° to 75° causes a non-monotonic variation of induced drag coefficient, and the Cd i of SP60 is the smallest, which is consistent with the comparison results of vortex pair circulation. This result indicates an optimal split angle for the design of split winglets among [45°, 75°], though it remains to be further optimized design and verification in future work.