Effects of Flapping Frequency on the Thrust Performance of Elliptical Airfoils in Transverse Offset Tandem Configurations*

In this study, the effects of non-dimensional flapping frequency (Strouhal number, St) and transverse offset on the aerodynamic thrust performance of a two-dimensional tandem elliptical flapping airfoil in forward flight condition is investigated at a Reynolds number of 5000. The front and rear airfoils are identical, with an aspect ratio of 8. They are positioned at 1 chord length (c) apart along the longitudenal direction, with a transverse offset of 0.5c. Here, St is varied from 0.2 to 0.7 to examine thrust performance at different effective angles of attack amplitudes (α₀) of 10°, 15°, and 20°. It is observed that as the St value initially increases, the time-averaged thrust coefficient for both the front and rear airfoils also increases until reaching a critical St value (St_cr). Beyond this St_cr, the thrust declines for all α₀ values. The rear airfoil's thrust performance surpasses the front airfoil for St < St_cr but experiences quick reductions in thrust at higher St (St > St_cr). Additionally, both inline and transverse offset tandem systems show a similar trend in time-averaged thrust. The improved thrust performance of the rear airfoil is due to the strong Leading-Edge Vortex (LEV) and the constructive interaction of the shear layer. In the presentation, a detailed flow physics will be discussed.