Experimental study of reynolds number and gust influence on transient force and flow generated by a robotic hummingbird

Abstract

A two-degree-of-freedom, up-scaled, robotic hummingbird model with rigid wings was used to simulate the hovering, flapping kinematics of a hummingbird to understand the flow structure around the wing and the underlying mechanism for dynamic force generation. Digital Particle Image Velocimetry (DPIV) method was applied to visualize the flow field at the mid-section of the wing over a range of Reynolds numbers (850 < Re < 13,000). It was observed that in the major duration of the stroke the leading edge vortex (LEV) was kept attached to the wing's suction side over the entire range of Reynolds numbers. It was also found that the rapid change of angle of attack at the beginning of each half stroke generated shedding of LEV. Time history of the lift and drag forces were measured to relate the dynamic response with the flow field development. The magnitudes of lift and drag forces increased with Reynolds number, however, the overall lift coefficient decreased for Re < 6,000. Beyond this limitation, influence of Reynolds number on the mean lift coefficients was indiscernible. Additionally, it was verified that the effects of horizontal gust on both flow field and dynamic forces were equivalent to that of effective Reynolds numbers, e.g., the effect of the head-on gust during the down-stroke increased the lift and drag forces, similar to the influence of increased Reynolds number.