Aerodynamic characteristics of flow past tandem wing

Numerical simulations are conducted to investigate the effects of tandem wing on vortex shedding, three-dimensional instabilities, and aerodynamic force coefficients at Re=1000. The angle of attack of the tail wing, denoted by αtw, is varied between 0° and 14°, whereas the main wing is fixed at αmw=14°. For a single wing, the wake transitions from a two-dimensional to a three-dimensional state via mode C instability, accompanied by hairpin vortices. The tandem configuration suppresses the formation of hairpin vortex structures, and the wake exhibits quasi-periodic and periodic three-dimensional modes with spanwise wavelengths nearly three times than that of mode C instability. The tandem configuration increases the frequency of streamwise vortex generation. Unlike a single wing, where each vortex shedding cycle produces a single streamwise vortex, pairs of streamwise vortices of opposite polarity are generated in tandem wing wake. The three-dimensionality becomes weaker with a decrease in αtw, yielding two-dimensional vortex shedding and a steady-state wake for αtw<2°. The lift and drag coefficients of the main wing remain fairly constant with αtw. The tail wing experiences reduction in the lift and drag due to the main wing wake. The tail wing exhibits a drag inversion phenomenon. The pressure and the suction on the tail wing undergo a reduction due to the wake interaction, and the maximum pressure is seen near the mid-chord region of the tail wing. A separation bubble is formed on the upper surface of the tail wing for αtw≥6°. The weak vortex shedding due to the tandem configuration causes a significant reduction in the unsteadiness.