AIAC-2005-019 NUMERICAL INVESTIGATION OF 3-D EFFECTS ON THRUST GENERATION OF FLAPPING AIRFOILS Mustafa Kaya * and Ismail H. Tuncer † Middle East Technical University Ankara, Turkey Agis Spentzos ‡ , George N. Barakos § and Ken Badcock ¶ University of Glasgow Glasgow, United Kingdom ABSTRACT 2-D and 3-D flows over flapping airfoils are computed, and thrust generation and propulsive efficiencies are com- pared. The flapping motion of the airfoils is described by a combined harmonic plunge and pitch motion. The flows are computed for flapping motions which produce maxi- mum thrust and propulsive efficiency, predicted by a gradi- ent based optimization algorithm using a 2-D flow solver. 2-D and 3-D flow computations are both performed in parallel. PVM and MPI message passing libraries are used. 3-D simulations resulted in slightly higher thrust and efficiency values than 2-D predictions. Both 2-D and 3-D numerical simulations show that high thrust may be obtained at the expense of reduced efficiency. For high propulsive efficiency, the large scale formations at the leading edge are prevented. INTRODUCTION Due to bird and insect flight, flapping wing propulsion has already been recognized to be more efficient than con- ventional propellers for very small scale vehicles, so-called micro-air vehicles (MAVs). MAVs with wing spans of 15 cm or less, and flight speed of 30 to 60 kph are of re- cent interest for military and civilian applications. Current studies in the research and development community are to find the most energy efficient airfoil adaptation and flapping wing motion technologies capable of providing the required aerodynamic performance for a MAV flight. Recent experimental and computational studies investi- gated the kinematics, dynamics and flow characteristics of flapping wings, and shed some light on the lift, drag and propulsive power considerations[10, 17]. Lai and Platzer[18] and Jones et al.[19] conducted water tunnel * GRA in the Dept. of Aerospace Engineering, Email: mkaya@ae.metu.edu.tr † Prof. in the Dept. of Aerospace Engineering, Email: tuncer@ae.metu.edu.tr ‡ GRA in the Dept. of Aerospace Engineering, Email: as- pentzo@aero.gla.ac.uk § Senior Lecturer in the Dept. of Aerospace Engineering, Email: gbarakos@aero.gla.ac.uk ¶ Reader in the Dept. of Aerospace Engineering, Email: gnaa36@aero.gla.ac.uk Figure 1: Flapping motion of an airfoil flow visualization experiments on flapping airfoils to an- alyze the wake characteristics of thrust producing flap- ping airfoils. The significance of the phase shift between plunging and pitching in maximizing the propulsive ef- ficiency is shown by Anderson et al.[20] in their exper- imental study. The experimental and numerical studies by Jones et al.[7, 9, 8], and Platzer and Jones[13] on flapping-wing propellers point at the gap between numer- ical 2-D flow solutions and the actual 3-D test flight con- ditions. In the comparison of 2-D and 3-D panel-code re- sults, Jones et al.[8] observe less 3-D thrust and efficiency values than 3-D ones. However, 3-D results rapidly ap- proach 2-D results as AR, the aspect ratio of the wing increases from AR =4 to AR = 100. They also observe a phase lag in 2-D and 3-D thrust variation along time. The wake structures and hydrodynamic performance of finite aspect-ratio flapping foils are explored by Dong et al.[2]. The results of their numerical simulations indicate that the wake topology of the relatively low aspect-ratio foils is significantly different from that observed for infi- nite/large aspect-ratio foils. The present authors have been involved in numerical investigation of unsteady aerodynamics, and have per- formed studies of flapping/moving airfoil/wing. Tuncer et al.[22, 21, 12] and Isogai et al.[14, 11] studied the effect of flow separation on thrust and propulsive efficiency of a single flapping airfoil in combined pitch and plunge os- cillations. Tuncer and Kaya[5, 6] optimized the harmonic flapping motion of a flapping airfoil for maximum thrust and propulsive efficiency. Barakos and Drikakis[15, 16] in- vestigated unsteady turbulent 3-D flow over moving bod- ies while Spentzos et al.[3, 4] performed CFD studies on 3-D dynamic stall. Although their studies were designed