MITIGATION OF THRUST DETERIORATION AT A HIGH FLAPPING FREQUENCY OF A 2D AIRFOIL IN FORWARDING FLIGHT CONDITIONS USING ASYMMETRIC FLAPPING STROKES

In this article, the detrimental time-averaged thrust performance of a two-dimensional elliptic symmetrical flapping airfoil at very high flapping frequencies is addressed, and an attempt is made to enhance the thrust performance by implementing two separate flapping frequencies during the downstroke and upstroke of a flapping cycle thus making it asymmetric. Three different pivot locations, three effective angle of attack amplitudes, and a specific range of St are considered for our investigation, keeping the Reynolds number fixed as 5000. In general, the downstroke flapping frequency is always selected with a value higher than the critical Strouhal number, but the upstroke flapping frequency is kept fixed as the critical Strouhal number. It is worth highlighting that the observations are periodic and swapped suitably when the flapping stroke frequency selection is reversed in the flapping cycle. Asymmetric flapping stroke configurations yield more time-averaged thrust and non-zero lift than symmetric flapping stroke at thrust degrading flapping frequency range. Interestingly, the leading edge side pivot point produces positive lift when the implementation of a faster downstroke and slower upstroke configuration. At the same time, it's the opposite for the center and trailing edge pivot points. To understand these enhanced aerodynamic performances at high flapping frequencies, we have further analyzed the transient thrust and lift force profiles and their associated flow structures for all the pivot locations.