BIRCH, J.M.*; DICKINSON, M.D.: The influence of wing-wake interactions on aerodynamic force generation in insects

The aerodynamic forces that wings generate are dependent on both fluid velocity and angle of attack. A complicated relationship develops between the wing and these two parameters during the flapping of hovering flight. During hovering, each stroke generates a wake that may influence force generation in subsequent strokes. We quantified the influence of the previous strokes’ wake by comparing the time course of both flow fields and instantaneous forces during a series of strokes using a dynamically scaled robot. We used particle image velocimetry to gather a continuous time history of the flow around a wing starting from rest and continuing through four stroke cycles. By subtracting the instantaneous flow fields generated during the fourth stroke from identical fields generated in the first stroke cycle, we created an image of the wake uncontaminated by the immediate motion of the wing. We collected instantaneous aerodynamic forces in a similar manner; the difference between stroke 4 and stroke 1 represented the influence of the wake on force production. Results of both flow and force measurements indicate that the influence of the wake on force production can be divided into two distinct phases, an early augmentation (described previously as wake capture), and a later attenuation akin to the phenomenon of induced drag on conventional airfoils. The force augmentation correlates with increased fluid velocity and results in an average 64% increase in lift when compared to strokes starting from rest. Later in the stroke, a 7-8% attenuation in force is due to the decrease in angle of attack because of the downward movement of fluid from the previous strokes’ wake. This reversal of the wakes’ influence through time demonstrates the dynamic nature of wake/wing interactions during hovering flight.