Low Reynolds Number Flow Dynamics of a Thin, Flat Airfoil with Elastically Mounted Trailing Edge

Abstract

Direct numerical simulations were performed to study the effect of an elastically mounted trailing edge actuator on the unsteady flow over a plunging, thin airfoil at Reynolds number of 14700 based on the chord length. The goal is to investigate potential benefits of flow-induced passive actuation of the trailing edge to the lift and drag characteristics of flapping MAV wings. The trailing edge, of 30% the chord length, is hinged to the wing using a torsion spring. It may undergo flow induced rotation resulting in dynamic variations in the airfoil shape. This trailing-edge spring assembly is modeled by simple, linear, torsion spring dynamics. A small parameter space varying the spring flexibility is explored to investigate its effect on the airfoil performance. Firstly, the second-order fictitious domain based finite volume approach by Apte et al. (J. Comp. Phys. 2009) was extended to model this fluid-structure interaction problem on a fixed, Cartesian mesh. Verification studies were conducted on a canonical test case of a spring-mounted cylinder to show good predictive capability. Secondly, flow over a plunging thin flat airfoil, at reduced frequency of 5.7 (10 Hz) and 5 • angle of attack was investigated with and without the actuation of the trailing edge. It was found that, spring natural frequencies higher than the plunging frequency result in the tailing edge actuation that leads to net increase in thrust. The phase difference between the leading and trailing edge motions was found to vary based on the spring flexibility and played a critical role in governing wing performance.

Characterization of Vortex Dynamics in the Near Wake of an Oscillating Flexible Foil

“…For example, Heathcote and Gursul [31] demonstrated using PIV and hydrodynamic force measurements that a heaving airfoil with a deformable trailing edge generates stronger trailing edge vortices (TEVs), which results in an enhanced thrust coefficient when compared to a purely rigid airfoil. The direct numerical simulation results by Apte and Base [9], where the trailing edge flexibility is modeled using spring dynamics and allowed to be influenced by the fluid-structure interactions, are in good agreement with the experimental results of Heathcote and Gursul. Apte and Base concluded that in order to maximize the propulsion efficiency, the natural frequency of the trailing edge should be similar in magnitude to the heaving frequency of the airfoil.…”

“…Growing aero/marine applications for energy harvesting and propulsion have advanced research into animal locomotion [1][2][3][4][5]. The flow physics of swimming and flying animals has received significant attention mostly in the context of developing bioinspired micro-air vehicles and oscillating flow energy harvesters [6][7][8][9][10]. Many animals, including fish, insects, and birds, exploit complex oscillatory motion for highly efficient maneuvering and aero/hydrodynamic performance.…”

“…In an attempt to further improve energy harvesting efficiencies, the use of flexible surfaces has also received some attention in recent years [9,[32][33][34]. The study by Siala and Liburdy [33] experimentally evaluated the time-resolved aerodynamic load measurements in a combined forward pitching and heaving foil with a passively oscillating trailing edge.…”

Energy harvesting of a heaving and forward pitching wing with a passively actuated trailing edge

“…The minor peaks occur when the pitching angle is decreasing and as the upstroke in heaving just begins. At this point in the cycle, it is speculated that a leading edge vortex (LEV) forms on the bottom surface of the wing due to the heaving motion reversal, as described by Apte et al (2012). The LEV creates a negative lift by decreasing the pressure at the bottom surface relative to that of the upper surface.…”

The Effects of a Passively Actuated Trailing Edge on the Aerodynamics of an Oscillating Wing

“…Improving the aerodynamic performance of small, thin wings enhances the propulsion efficiency used in surveillance and rescuing missions. The drawback with thin airfoils in low Reynolds number flows is that at large angles of attack, leading edge flow separation occurs resulting in a Kelvin-Helmholtz flow instability [1,2] . Flow separation leads to a sudden change in lift and drag, which can cause a rapid decrease in aerodynamic performance.…”