Review of the physics of enhancing vortex lift by unsteady excitation

Abstract

Based on the authors' recent studies, this review aims to provide a physical understanding on the crucial mechanisms for obtaining super lift by means of unsteady excitations—a subject of wide potential applications. In Section 2 previous experimental studies and theoretical modellings toward high vortex lift at post-stall angles of attack are critically reviewed. Some important lessons are drawn. Five crucial physical problems relevant to super vortex lift generated by applying unsteady excitations are recognized. They are rolled-up vortex layer instability and receptivity; wave-vortex interaction and resonance; nonlinear streaming; instability of vortices behind bluff bodies and their shedding; and vortex breakdown (for slender wings). In Section 3 the first three mechanisms are examined in detail, based on the present understanding recently gained in apparently nonrelated fields. Section 4 introduces a general theoretical framework suitable for handling the nonsteady vortex flows, especially the aspect of wave/vortex interactions, and comments on possible, suitable numerical methods. The article concludes with a summary of our own views and suggests some further work needed at both fundamental and applied levels. The crucial physics is essentially based on certain intrinsic features of vortex motion, and the suggested work aims at the exploitation of these features. It is expected that innovatorily designed wings with swept and sharp leading edges, equipped with devices for unsteady excitations, could yield the first break through of the ‘unsteady separation barrier’ and provide super lift at post-stall angles of attack. This would imply the birth of the third generation of aeronautical types of flow, namely weakly unsteady detached vortex flows. Its utilization should bring a new revolution to aircraft design.