LAUSR

Previous studies indicate that vortex pairs over slender wings can produce time-averaged asymmetries even before the occurrence of vortex breakdown, but the sources for the asymmetries are still not clear. The present investigation uses numerical simulations and experimental measurements to explore the physical nature for the symmetry breaking of vortices. The results indicate that the time-averaged asymmetries come from a spatial growth of local disturbances (spatial instability) instead of a temporal instability. In numerical simulations, the time-averaged flows around a perfectly symmetric wing exhibit symmetric vortex pairs in which any asymmetric initial perturbation will decay and eventually converge to symmetric flows. However, if a geometric micro-perturbation is asymmetrically placed on the apex, asymmetric vortex pairs can be produced; downstream sectional perturbation energy grows spatially, and the growth rates increase with increasing angle of attack. The spatial perturbation growth mainly comes from antisymmetric modes, whereas symmetric modes have smaller growth rates. The experimental results indicate that artificial apex perturbations can overwhelm natural irregularities on the apex and dominate the orientations of asymmetric vortices. Unsteady aspects of the flows were also studied before vortex breakdown, with the emphasis put on oscillatory global modes by applying dynamic mode decomposition to the entire flow field. It was found that the most energetic modes are related to vortex shedding around trailing edges and instabilities of downstream shear layers, which has significant contributions to fluctuations of aerodynamic forces and moments. The vortex shedding exhibits strong three dimensionality characterized by oblique shedding and a beating phenomenon.