Study of asymmetric hovering in flapping flight

Study of the unsteady aerodynamics of flapping airfoils has received a major boost in the recent past for its application in the field of MAV design. The present study investigates the unsteady flow-field over a flapping airfoil with asymmetric kinematics. Different asymmetry mechanisms are investigated for their lift enhancement capabilities during hover. Three asymmetries have been tried: amplitude, frequency and unsteady free-stream. Average aerodynamic loads are compared for all the cases. Flow field vortices are also compared. For the amplitude asymmetry case, pitch angles are varied between the strokes. Both normal and sinusoidal hover are considered. The normal hover case shows a significant increase in the average lift value compared to its sinusoidal counterpart. Subsequently, the frequency asymmetry mechanism has shown a significant increase in load for the sinusoidal hover case. Frequency asymmetry kinematics is achieved by using a faster stroke followed by a slower stroke. Depending on the level of asymmetry, a large amount of lift can be generated during the faster stroke. A third mechanism of sinusoidally varying oncoming flow with mean zero is also investigated. The flow-field is simulated with a Lagrangian particle based technique. In this algorithm, field property vorticity is represented by discrete particles. The flow-field is simulated at a Reynolds number around 3000 which is much higher than similar studies reported earlier. The Reynolds number range considered here is applicable to various man-made flapping devices like Micro Aerial Vehicles (MAVs). Crucial flow features like the dynamic stall vortex and its interaction with the trailing edge structures are studied for different asymmetry factors in the flow-field.