The vortex formation of an unsteady translating plate with a rotating tip

We perform experiments to study the effect of a rotating tip surface on the outboard flow of a finite-aspect-ratio, high-angle-of-attack translating wing. Prior work on unsteady, translating wings showed that for rectangular planforms the tip vortex (TV) aids in the outboard leading-edge vortex (LEV) attachment, but the two are distinct. However, aft sweep promotes a connection between the outboard LEV and TV and potentially delays inboard LEV shedding. Moreover, rotating wings with low Rossby number exhibit LEV attachment, greater TV coherence, and higher lift. This paper examines whether a variable wingtip (panel) combining sweep and outward rotation, superimposed on the main-wing translation, can modify the adjacent, outboard LEV via an interaction with its swept-edge vortex (SEV), and affect the prior TV and trailing-edge vortex (TEV). The goal is a temporary lift boost, e.g. for a maneuvering unmanned aerial vehicle (UAV). The panel’s ability to disrupt the outboard vortex formation during a streamwise gust-like motion (surge), via inward actuation to mitigate the lift increase, is also studied. Water towing-tank experiments are done with a wing of submerged aspect ratio 3.4, minus the panel, translating at 45◦ angle of attack and Reynolds number 10,000. Dye visualization via multiple injection ports, imaged using three orthogonal cameras, captures the 3D flow structure qualitatively. A starting flow and 50% streamwise gust after 21 chords are tested, with varying panel actuation timing, compared to a rectangular wing and static tip-sweep. In all cases, the main-wing speed divided by the forward tip speed, i.e. the advance ratio, is 0.95. For starting flows, outward tip-panel actuation at 0.1 chords traveled produces a new SEV, causes the TV and TEV to stretch outward and forward to follow the panel motion, and shifts the nearby-attached LEV outward; this should be beneficial for lift generation. For static tip sweep, the SEV appears to be less closely-attached. In both cases, the main LEV adjacent to the swept edge entrains SEV vorticity and temporarily weakens the SEV. Later tip actuation is less effective at modifying the flow structures, since this LEV interaction is exacerbated due to the larger LEV present when actuation starts. For the gust cases, inward panel actuation sheds the SEV and local TEV, and enhances LEV shedding similar to the rectangular wing. Also, it causes the aft TV flow to move inboard which is detrimental for lift. Actuation earlier, before the gust, has the greatest effect as it allows these phenomena to develop.