Interplay of the leading-edge vortex and the tip vortex of a low-aspect-ratio thin wing
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RESEARCH ARTICLE
Interplay of the leading‑edge vortex and the tip vortex of a low‑aspect‑ratio thin wing Lei Dong1 · Kwing‑So Choi1 · Xuerui Mao1 Received: 20 May 2020 / Revised: 18 July 2020 / Accepted: 24 July 2020 © The Author(s) 2020
Abstract Three-dimensional vortical structures and their interaction over a low-aspect-ratio thin wing have been studied via particle image velocimetry at the chord Reynolds number of 105 . The maximum lift of this thin wing is found at an angle of attack of 42◦ . The flow separates at the leading-edge and reattaches to the wing surface, forming a strong leading-edge vortex which plays an important role on the total lift. The results show that the induced velocity of the tip vortex increases with the angle of attack, which helps reattach the separated flow and maintains the leading-edge vortex. Turbulent mixing indicated by the high Reynolds stress can be observed near the leading-edge due to an intense interaction between the leading-edge vortex and the tip vortex; however, the reattachment point of the leading-edge vortex moves upstream closer to the wing tip. Graphic abstract
1 Introduction Low aspect-ratio (AR) wings have been designed for microair-vehicles (MAVs) to meet their special requirements. Comparing to conventional wings, they have a high stall angle that improves the manoeuvrability of the aircraft. Flow structures around low AR wings are quite different from high AR wings or two-dimensional aerofoils because of the short span length, resulting in an intense interplay between the leading-edge vortex (LEV), the trailing-edge vortex * Kwing‑So Choi kwing‑[email protected] 1
Faculty of Engineering, University of Nottingham, Nottingham, UK
(TEV) and the tip vortex (TV). The imbalance of pressure distribution makes the flow at the wing tip-edge move from the pressure side toward the suction side, after the incoming flow is separated at the tip-edge. A similar geometrical wing can be found in some natural creatures, such as flies, insects and bats. To investigate the aerodynamic mechanism behind this, the flow visualization and particle image velocimetry (PIV) techniques were used to observe the flow field around those creatures and four types of vortices including the LEV, the TEV, the TV and the root vortex were identified, contributing collectively to the lift production (Willert and Gharib 1997; Warrick et al. 2005b; Bomphrey et al. 2009a; Carr et al. 2013b; Von Ellenrieder et al. 2003a). There were some common
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characteristics between the MAVs and the creatures. Normally, wing flapping comprises of three kinds of kinematics motions—rotation, pitching, and plunging (Wang 2005a). In the process of these motions, large-scale vortices could be formed, contributing to the lift production (McCroskey 1982b; Ohmi et al. 1991b; Yilmaz and Rockwell 2012). However, the formation and stability of the LEV are sensitive to parameters like the Reynolds number (Re), pitching angle, etc (Shyy and Liu 2007a). Baik et al.
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