Thrust Enhancement For Two Staggered Pitching Airfoils

Birds,fish and many kind of insects use their flapping wings or bodies to generate propulsive forces in their motion.They may take hydrodynamic/aerodynamic advantages to save energy when they move in a group of members.By studying optimal solutions in nature,human can get inspiration for new inventions and designs.Better understanding of the motion principle of biological species will promote the study of man-made machines such as aircrafts and marine vessels.Thrust enhancement of the two pitching airfoil in staggered arrangement is investigated,where the upstream airfoil is forced pitching and downstream is free to pitch.When the frequency and amplitude of the forced pitching airfoil are fixed to St = 0.25 and AD = 0.6675 the maximum ?/?0 is 3.6 with J = 6.54 × 10-4(m* = 11.33).The effects of pitching amplitude(AD = 0.55 ~ 0.80)and pitching frequency(St/St0 = 1.30 ~ 2.05)are investigated.In order to increase the pitching frequency and/or pitching amplitude,higher amount of input power is required for either single forced pitching or staggered arrangement.For the two staggered airfoil system,10 ~ 24% more input power is needed for the forced pitching airfoil compared to single forced pithing airfoil at the same combination of frequency and amplitude.At lower pitching frequency and amplitude,streamwise force on the single forced pitching airfoil is drag.Drag will turn into thrust and keep increasing in amplitude with the increase in pitching frequency and amplitude.As for the two-airfoil system,drag force on the upstream airfoil turns into thrust at low pitching frequency and amplitude values compared to single airfoil.Thrust on the upstream airfoil is greatly enhanced at high pitching amplitudes and frequencies.Thrust on the downstream airfoil in the two staggered airfoil system is smaller than the upstream airfoil.Different thrust is mainly caused by the pressure gradient that the airfoil experienced.Streamwise force on the downstream airfoil is drag when the upstream airfoil pitches in low amplitude and low frequency or high amplitude and low frequency.At specific frequency,thrust coefficient first increases then decreases with the increase of pithing amplitude.Maximum thrust coefficient of the downstream airfoil is obtained at St/St0 = 1.93,AD = 0.65.For the total thrust of the two-airfoil system,there is only a small drag region at low frequency and amplitude.Maximum thrust coefficient of the two-airfoil system is obtained St/St0 = 2.05,AD = 0.75.Thrust efficiency for single forced pitching airfoil has a clear drag region at low pitching amplitudes and frequencies.Drag region is greatly decreased for the two airfoil system.Maximum efficiency for single airfoil is 9%,for the two-airfoil system it doubles to 18% when S_t/S_t0 = 1.80,A_D = 0.75.