Unsteady lift generation of corrugated wing by lambda vortex collapse

Dragonfly wing is not smooth but corrugated; its vertical cross-section consists of connected series of line segments. Some previous studies suggest that the aerodynamic performance of the corrugated wing is higher than that of the flat wing at the low Reynolds numbers (Re \simeq O(10^3)). However, the details are not fully investigated. In particular, the aerodynamic characteristics and flow property during unsteady wing motion has not been studied in detail because of the complicated flow characteristics.

In this study, we analyzed flow around two-dimensional corrugated wing model that started impulsively by direct numerical simulations. We focused on a period between the generation of the leading-edge vortices (LEVs) and subsequent interactions, and its detachment. For the flat wing, so-called lambda vortex with the sign opposite to LEV. On the other hand, for the corrugated wing, the lambda vortex collapsed and stuck in the valleys of the corrugated structure in a parameter range. The lambda vortex collapse changes the LEV dynamics to encourage the instantaneous lift enhancement of the corrugated wing. In the presentation, we will discuss the details.