Experimental study of flow over two oscillatory pitching airfoils in tandem configuration

The aerodynamic interaction between two NACA 4421 tandem oscillating airfoils has been investigated experimentally at a Low Reynolds number flow of 8.6 x 10³ in a closed loop water tunnel through force and PIV measurements. The inphase (φ{symbol} = 0°) and out-of phase (φ{symbol} = 180°) oscillatory pitching motions at stagger distances of St = 2 & 2.5 and at various pitch amplitudes were studied. It was discovered that the in-phase oscillatory pitching motion resulted in minimal lift improvements as compared to the single oscillating and out-of-phase cases. Conversely, it generated lower drag forces, thus making it ideal for thrust production as compared to the out-of-phase cases. With decreasing stagger distance, the upstream airfoil was able to produce better thrust forces but lower lift forces. However, the opposite trend was observed with the downstream airfoils. The increase in lift performance for the downstream airfoils at decreased stagger distance of St = 2 were credited to a single CW vortex interaction onto the LEV at the end of its upstroke at t/T = 0.25, thus resulting in a stronger LEV formation across the suction side. In contrast, the out-of-phase oscillatory pitching motion have shown that it is more suitable for hovering applications rather than thrust production as it produced huge improvements in lift forces but at the cost of generating high drag forces especially by the downstream airfoils. As the stagger distance was reduced, the downstream airfoil was able to generate higher lift forces due to the stronger downwashed vortex interactions which were also observed by Lee's work on out-of-phase tandem airfoils. At St = 2, the downstream airfoil was able to generate the best lift performance with a C_Lmax of 1.45 which relates to an increment of 158.9% when compared to the single oscillating case. This lift enhancement was attributed to the interaction with the down-washed vortices during their upstroke at t/T =0.75. These interactions resulted in the elongation of the vortex structure on the suction side and a stronger LEV, thus extensively increasing the lift production. This vortex structure elongation on the downstream airfoils was also seen in Broering & Lian's work.¹⁹ Undesirably, at St = 2, the downstream airfoil also produced the highest drag forces that was linked to the increased intensity of the down-washed vortex interactions in the middle of its down stroke at α_da,d = 0°, t/T ≈ 1.0. These interactions caused a thickening of the upper surface boundary layer and a more volatile wake structure that resulted in a significant increase in drag forces. The lift and drag performances for the upstream airfoils were relatively unaffected by the stagger distances. Nevertheless, the upstream airfoils were able to produce higher lift forces than the single oscillating airfoil through their ability to retain the suction lift peak and delay dynamic stall effects. This similar observation was evidently made by Lee as well.