Effects of continuously increasing pitching and plunging amplitudes on the aerodynamic forces of flapping airfoils

Jun Hu,Jiaolong Zhang,Haibin Xuan,Yong Yu

doi:10.1063/1.5111808

Abstract

Recently, numerous studies have been conducted to clarify the effects of the increases in pitching and plunging amplitudes of flapping wings on thrust and lift generation. In the present study, the effects of continuously increasing pitching and plunging amplitudes on the aerodynamic performances of a two-dimensional (2D) flapping wing are investigated computationally. Continuously increasing pitching and plunging amplitudes have significant effects on the rate of leading-edge vortex (LEV) development and the time of LEV separation; as a result, the aerodynamic performance is influenced. Lift and thrust are gradually improved with increasing pitching and plunging amplitudes; however, higher amplitudes induce the production of drag forces. Furthermore, to compare the contributions of the pitching and plunging amplitudes, we conducted simulations with pure pitching or plunging amplitude increases while keeping the other factors constant. With the increase in pitching amplitude, the vortex on the upper surface becomes weaker during the downstroke and leads to the production of a vortex on the lower surface. During the upstroke, the effect of the increase in pitching amplitude on the vortex has a symmetric influence against the downstroke. The change in pitching amplitude has little effect on the lift and thrust but leads to the production of drag forces. When the plunging amplitude increases, the LEV and the second kind of vortex, the trailing-edge vortex (TEV), becomes stronger, which will cause a concurrent increase in lift and thrust. The increase in plunging amplitude greatly improves lift and can also enhance thrust.

Highlights

Insects were the first animals to start flying on Earth,1 and there are approximately 1000000 described species of flying insects.2–4 Many insects have unique flying capabilities; insects can move forward, flap up and down, plunge and sweep
When the plunging amplitude increases, the leading-edge vortex (LEV) and the second kind of vortex, the trailing-edge vortex (TEV), becomes stronger, which will cause a concurrent increase in lift and thrust
In the past few years, numerous experimental and numerical studies have been conducted on high-lift mechanisms, as noted in the reviews on flapping wing aerodynamics by Sane,10 and these mechanisms include the Wagner effect, clap-and-fling, delayed stall, leading-edge vortex (LEV), the Kramer effect, and wing-wake interaction

Summary

Introduction

Insects were the first animals to start flying on Earth, and there are approximately 1000000 described species of flying insects. Many insects have unique flying capabilities; insects can move forward, flap up and down, plunge and sweep. Many insects have unique flying capabilities; insects can move forward, flap up and down, plunge and sweep. Wing flapping, which consists of upstroke, pronation, downstroke and supination, has been regarded as an efficient specific mechanism to overcome the small-scale aerodynamic limitation of flying-wing performance.. Researchers have been studying the aerodynamics of flapping flight since the last century, and it has been shown that the conventional aerodynamic theory, which is based on fixed-wing aircrafts and steady-state flow conditions, cannot explain the generation of large lift by the wings of small insects.. Arranz and Flores stated that the flapping kinematics, Reynolds number, reduced frequency, and wing geometry and rigidity could greatly modify the aerodynamic forces

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