Journal of Fluids and Structures 26 (2010) 193–204 The manipulation of trailing-edge vortices for an airfoil in plunging motion T. Prangemeier, D. Rival à , C. Tropea Institute of Fluid Mechanics and Aerodynamics, Technische Universita¨t Darmstadt, Darmstadt, Germany Received 4 February 2009; accepted 14 October 2009 Available online 22 January 2010 Abstract Trailing-edge vortex manipulation has been investigated using particle image velocimetry (PIV) for an airfoil undergoing harmonic plunging superimposed with a pitching motion near the bottom of the stroke. The so-called quick- pitch motion has been evaluated through a comparison with a benchmark pure-sinusoidal plunge motion for Re ¼ 30 000 and k ¼ 0:25. It has been shown that the trailing-edge vortex circulation can be reduced by more than 60% for all quick-pitch cases. The reduction in trailing-edge vortex circulation has been achieved without diminishing the strength of the leading-edge vortex, thus maintaining the lift augmentation achieved through dynamic stall. The improvement over the benchmark case is then confirmed through a statistical analysis. Finally, an analysis of the flow separation over the airfoil shows that the various quick-pitch motions facilitate earlier flow reattachment at the bottom of the stroke. & 2009 Elsevier Ltd. All rights reserved. Keywords: Unsteady aerodynamics; Dynamic stall; Trailing-edge vortex 1. Introduction Insects, birds and bats make use of a range of unsteady aerodynamic effects in order to fly at low Reynolds numbers where the laminar boundary layer is prone to separation, as discussed by Ellington (2006). Complex wing kinematics have been observed for both cruise flight as well as manoeuvering, see Azuma (2006). This study investigates the elimination of certain negative aspects of the dynamic-stall process by careful tuning of the airfoil kinematics. A Reynolds number of Re ¼ 30 000 and reduced frequency of k ¼ 0:25 corresponding to birds and large insects are used here. Thomas et al. (2004) visualized the flow around free-flying dragonflies demonstrating the existence of large quasi two- dimensional leading-edge vortices (LEVs) over their high aspect ratio forewings. The production of LEVs is a common mechanism used to augment lift in biological flight and has been studied in detail for pitching airfoils. Numerous experimental studies exist, particularly related to helicopter aerodynamics, and have been reviewed in detail by Leishman (2006). Carr et al. (1977) measured the lift, drag and moment coefficients for a pitching airfoil undergoing dynamic stall and found large hysteresis loops that characterize the lift coefficient curve. Lift augmentation was present, as were peaks in drag and negative aerodynamic moment. For a harmonically plunging and pitching foil, Read et al. ARTICLE IN PRESS www.elsevier.com/locate/jfs 0889-9746/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfluidstructs.2009.10.003 à Corresponding author. E-mail address: rival@mit.edu (D. Rival)