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

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.