Effect of Reynolds Number, Tip Shape, and Stroke Deviation on Flapping Flight

Abstract

In the current study the effects of Reynolds number, tip shapes and stroke deviation on the performance of flapping flight are analyzed. Three Reynolds nu mbers, Re =100, 10,000, and 100,000 are evaluated. At Re=100, a single coherent Leading Edge Vortex (LEV) forms during the downstroke, while at the higher Reynolds numbers, a less coherent LEV forms, which sheds near the wing tip and convects downstream. At higher Reynolds numbers the location of the LEV remains close to the surface as it sheds and convects, which le ads to high values of lift and thrust coefficients. At all Reynolds numbers, a strong spanwise flow of the order of the flapping velocity is observed along the core of the LEV, which contradicts previous observations of spanwise flow only at high Reynolds numbers. Analysis of three different tip shapes, straight, ro unded, and tapered show that the tip vortex does not have a significant effect on the lift and thrust production. During the downstroke, a continuous LEV forms for the rounded tip, while a discontinuous vortex forms for straight and tapered tip shapes. However, the time of LEV separation and the location of the LEV at different phases of the flapping cycle are identical for all the tip shapes. Flapping flight with stroke deviation alters the force production significantly during the upstroke as it captures the LEV shed during the downstroke. It reduces the average value of lift coefficient for the case analyzed, while the thrust coefficient remains the same.