On the thrust performance of a 2D flapping foil in a forward flight condition

Past studies have shown that the thrust performance of a 2D airfoil undergoing simple harmonic motion in both pitch and heave in a forward flight condition is dependent on maximum effective angle of attack ($\alpha _{\mathrm{o}})$ and Strouhal number ($S_{T})$. For a given $\alpha _{\mathrm{o}}$, it is found that the thrust coefficient ($C_{T})$ increases with $S_{T}$ until it reaches a peak value at the critical Strouhal number ($S_{Tc})$; beyond which $C_{T}$ deteriorates considerably. In order to extend $S_{Tc}$ and therefore increase the max.$C_{T}$, the airfoil must oscillate at a higher $\alpha_{\mathrm{o}}$. Further, it is found that, regardless of $\alpha_{\mathrm{o}}$ thrust degeneration is accompanied by cessation of the induced effective angle of attack profile ($\alpha $(t)) to exhibit simple harmonic function of time. As to why non simple harmonic function of $\alpha $(t) is detrimental to thrust generation is not fully understood. In an attempt to better understand this phenomenon, both numerical simulations and comparative experiments are performed on a 2D flapping elliptic foil at Re of 5000. Our results show that the proximity of the leading edge vortex from the previous stroke to the oscillating foil plays a crucial role in the thrust generation. Detailed results will be discussed in the presentation.