Mitigation of Thrust Deterioration at High Flapping Frequencies of a Two-dimensional Elliptic Flapping Airfoil Using Asymmetric Flapping Strokes in the Forward Flight*

In this study, the deteriorated time-averaged thrust performance of a two-dimensional flapping elliptic airfoil as seen in the forward flight at high flapping frequencies (or beyond critical Strouhal number, St_cr) is mitigated by adopting asymmetric St for the downstroke and upstroke of a flapping cycle. Here, three different pivot locations, three effective angle of attack amplitudes, and St ≥ St_cr are considered for investigation, keeping the Reynolds number fixed as 5000. Furthermore, we have selected the St for the downstroke with a value more than St_cr, but the upstroke St = St_cr. Note that the observations are periodic and reciprocate suitably when the stroke St selection is reversed in the flapping cycle. It is found that the asymmetric St configurations of the flapping strokes yield more time-averaged thrust and non-zero lift than the symmetric one at St ≥ St_cr. In addition, we noticed that the pivot point location near the airfoil leading edge produces a higher lift than the cases where the pivot point is at the center or near the trailing edge. We have further analyzed the transient thrust, lift force profiles, and near wake flow structures to understand better this improved aerodynamic performance with asymmetric St, which will be discussed in the presentation.