13th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 26-29 June, 2006 - 1 - Aerodynamic Characteristics of Flapping Motion in Hover D. Funda Kurtulus 1,4 , Laurent David 2 , Alain Farcy 3 , Nafiz Alemdaroglu 4 1: LEA, ENSMA, Poitiers, France, dfunda@ae.metu.edu.tr 2: LEA, University of Poitiers, Poitiers, France, Laurent.David@univ-poitiers.fr 3: LEA, ENSMA, Poitiers, France, farcy@ensma.fr 4: Department of Aerospace Engineering, METU, Ankara, Turkey, nafiz@metu.edu.tr Abstract The aim of the present study is to understand the aerodynamics phenomena and the vortex topology of the highly unsteady flapping motion by both numerical and experimental solutions. Instead of the use of real insect/bird wing geometries and motions which are highly complex and difficult to imitate by an exact modeling, a simplified model is used to understand the unsteady aerodynamics and vortex formation during the different phase of the flapping motion. The flow is assumed to be laminar with the Reynolds number of 1,000. The experimental results obtained by the laser sheet visualization and the Particle Image Velocimetry (PIV) techniques are used for the phenomenological analysis of the flow. The vortex dynamics is put in evidence and explained with the use of different tools. Vortex identification from PIV measurements is performed by considering velocity magnitude, streamlines, second invariant of velocity gradient (Q- criteria), vorticity contours and Eulerian accelerations. 1. Introduction The numerical and experimental studies of low Re number regime become very important due to the advances in micro-technologies enabling the development of Micro Air Vehicles (MAV’s). One of the main objectives of MAV applications, i.e. constant position surveillance, reveals the need to focus the researches on hover mode. There are three generation of MAV’s namely fixed wings, rotating wings (like helicopters) and wings based on micro technology (MEMS, flapping or vibrating wings). The definition employed in Defense Advanced Research Projects Agency (DARPA) program limits these craft to a size less than 15 cm in length, width or height. This physical size puts this class of vehicle at least an order of magnitude smaller than any UAV developed to date. The studies on flapping motion flight can be classified into two main parts as the zoological configurations and the simplified configurations. Zoological configuration studies are performed based on the study of the insects or birds. Comprehensive reviews of the biological flight mechanisms could be found in Nachtigall (1974); Rayner (1979, 1985); Ellington (1984); Norberg (1985); Azuma et al. (1985); Pennycuick (1988) and Dudley (1998). The simplified configurations are mostly the studies based on the aerodynamics of the flow. The models are simplified such that different profiles are used instead of the real insect/bird wing geometries (Shyy et al. 1999; Ramamurti and Sandberg 2002; Platzer and Jones 2006). Although most of the researches are performed with numerical simulations, it is highly difficult to solve the full 3D Navier–Stokes equations for unsteady flows around insect wings. Hamdani and Sun (2000) simulated a series of impulsive starts at different accelerations around a 2D insect wing. The mean streamwise velocity field of the wake of a NACA 0012 airfoil oscillating in plunge at zero freestream velocity and at a zero angle of incidence at the neutral position was calculated by Lai and Platzer (2001). The vortical flow patterns in the wake of a NACA 0012 airfoil pitching at small amplitudes are studied by Koochesfahani (1989) in a low-speed water channel by considering the effect of both sinusoidal and non-sinusoidal shape of the waveform. Dickinson (1994) experimentally observed that four important parameters of stroke reversal