Leading-Edge Vortex Structure over Multiple Revolutions of a Rotating Wing

Abstract

T HE flow produced by a bio-inspired flapping wing is unsteady, three-dimensional, and dominated by separated flow and strong vortices. To better understand which structures in this complicated flow are most responsible for the production of lift and drag, several canonical problems have been designed to model portions of a natural wing stroke. These include both twoand threedimensional transient, reciprocating, and quasi-steady variations of pitching, plunging, translating, and rotatingwings. The rotatingwing model is designed to represent the translational phase of an insect wing stroke. In this model, the wing rotates about its root in a propeller-likemotion at a fixed angle of attack. The spanwisevelocity and pressure gradients that exist on an insect wing due to forward/aft sweep about the wing root are preserved, along with the threedimensionality induced by the root and tip vortices. The result is a relatively simple flowfield that preserves the most important characteristics of the translational phase of an entomological wing stroke. Some of the earliest rotating-wing experiments were performed by Usherwood and Ellington on hawkmoth wings at Reynolds numbers O 10 [1,2]. In these experiments, the wing’s lift coefficient was found to decrease as Reynolds number increased from 10,000 to 50,000, and it was postulated that this change in lift production was due to the formation of aweaker leading-edge vortex (LEV) at higher Reynolds numbers. Later, Ozen and Rockwell used particle image velocimetry (PIV) to characterize the steady-state flow structure on a low-aspect-ratio rotating plate at fixed angles of attack between 30 and 75 deg. They observed a stable LEV for a range of Reynolds numbers between 3600 and 14,500 [3]. However, other experiments focusing on the start of a rotating wing accelerating to Reynolds numbers between 10,000 and 60,000 revealed an LEV that formed and shed early in the wing stroke, resulting in a high-lift transient, after which lift dropped to about half of themaximum value [4–6]. At lower Reynolds numbersO 1000 , flow visualizations and PIV have demonstrated spanwise flow on a rotatingwing, and an attached LEV during wing acceleration that later burst over the outboard half of the wing during deceleration [7–9]. The objective of the work presented here is to identify the formation, structure, and possible separation of the leading-edge vortex at the beginning of thewing stroke (i.e., within the first 90 deg of wing rotation) as well as after long convective times (i.e., for wing strokes greater than 90 degrees including multiple revolutions). To this end, flow visualization is performed for threewing revolutions to analyze the vortex structure and the location of the burst point. In addition, unsteady lift and drag measurements are acquired for two revolutions to relate the flow structure to the aerodynamic forces produced by the wing.