Journal of Fluids and Structures 76 (2018) 37–59 Contents lists available at ScienceDirect Journal of Fluids and Structures journal homepage: www.elsevier.com/locate/jfs Enhanced thrust performance of a two dimensional elliptic airfoil at high flapping frequency in a forward flight S.M. Dash a , K.B. Lua b, *, T.T. Lim a , K.S. Yeo a a Department of Mechanical Engineering, National University of Singapore, 117576, Singapore b Department of Mechanical Engineering, National Chiao Tung University, 30010, Taiwan article info Article history: Received 26 February 2017 Received in revised form 12 July 2017 Accepted 28 August 2017 Keywords: High frequency flapping aerodynamics Elliptic airfoil Water tunnel experiments FLUENT simulations Thrust augmentation Vortex dynamics abstract This study is motivated by our earlier investigation Lua et al. (2016) which shows that a two-dimensional elliptic airfoil undergoing sinusoidal flapping motion experiences thrust deterioration when the flapping frequency exceeds a certain critical value (or critical Strouhal number, St cr ). To alleviate this unfavorable thrust generation condition, we pro- pose two novel effective angle of attack profiles, namely smooth trapezoid (STEA) profile and elliptic trapezoid (ETEA) profile. These profiles are designed to ensure that the airfoil still experiences the same effective angle of attack amplitude as the sinusoidal flapping while concurrently reducing the detrimental effect of high rotation rate. The effectiveness of these two proposed profiles is confirmed by our numerical and experimental studies. In particular, our results show that at low to moderate flapping frequency, thrust generation is almost invariant to the type of angle of attack profiles applied, but at high flapping frequency (St > St cr ), the proposed profiles produce higher thrust than the sinusoidal flapping. The thrust augmentation can be attributed to the suppression of the adverse low pressure region in the vicinity of the airfoil, which is a consequence of the reduced rotation rate. Of the two proposed profiles, the ETEA with its steep acceleration phase produces a higher time-average thrust than the STEA near the stroke reversal. Also, in line with our previous finding, thrust augmentation for both STEA and ETEA is a function of the base length of the trapezoid, with a broader one producing a better thrust performance. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Understanding the aerodynamics of flapping airfoil is important in the design of various propulsive devices including bio– inspired autonomous aerial and underwater vehicles (Lua et al., 2016; Ellington, 1999; Shyy et al., 1999; Ansari et al., 2006; Rozhdestvensky and Ryzhov, 2003). Generally speaking, the flapping airfoils are energy efficient and offer superior low speed maneuverability than a fixed or rotary airfoil in small-scale bio-inspired vehicles (Pesavento and Wang, 2009; Maxworthy, 1981; Platzer et al., 2008; Shyy et al., 2007; Spedding and Maxworthy, 1986; Mueller, 2001; Hubel and Tropea, 2009). Typically, the propulsive or locomotive force in these bio–inspired vehicles is generated through interaction of a translating and rotating airfoil with its surrounding fluid. Previous numerical and experimental investigations (Ellington, 1999; Hubel and Tropea, 2009; Lewin and Haj-Hariri, 2003; Dickinson et al., 1999; Smith, 1996; Sun and Tang, 2002; Freymuth, 1988; Jones et al., 1998; Lua et al., 2007) have shown that thrust generation of an oscillating airfoil is associated with the creation of a reverse von-Karman vortex street (RKVS) whereby the vortices are arranged in such a way that the downstream time- averaged velocity profile yields a momentum surplus jet-like flow scenario. * Corresponding author. E-mail address: engp4324@nctu.edu.tw (K.B. Lua). https://doi.org/10.1016/j.jfluidstructs.2017.08.008 0889-9746/© 2017 Elsevier Ltd. All rights reserved.