| Birds and fishes in nature normally generate thrust by flapping their body and wings,and prone to an instinctive group movement due to energy saving.This discovery will provide important help on underwater research and application of energy,such as micro aerial vehicle(MAV)and robotic fish.In this paper,the thrust enhancement of two forced pitching airfoils in tandem arrangement under different working conditions is studied.The two forced pitching airfoils are in tandem arrangement.By changing the pitching amplitude A_D,pitching frequency St_D,stream wise distance S/c and phase-lag φ of two forced pitching airfoils,the thrust efficiency of the two airfoils system is changed.The present study is focus on the effects of Strouhal number and the dimensionless pitching amplitude(St_D = 0.3,A_D = 1.0 and St_D = 0.2,A_D = 0.7)on the thrust efficiency of the two airfoils system.It is found that the two forced pitching airfoils get a larger thrust efficiency when St_D = 0.2 and A_D = 0.7.When St_D = 0.2 and A_D = 0.7,the effects of stream wise distance and phase-lag of two forced pitching airfoils system on the thrust efficiency are investigated.When φ is constant and S/c changes from 0.109 to 1,thrust efficiency of the upstream airfoil decreases,thrust efficiency of the downstream airfoil increases and total thrust efficiency of two airfoils system increases slowly.When S/c is constant and φ changes from 0° to 180°,thrust efficiency of the upstream airfoil increases,and thrust efficiency of the downstream airfoil decreases with the increase of phase-lag.When φ = 90°,total thrust efficiency of the two airfoils system gets the maximum value.When S/c = 1 and φ = 90°,the two airfoils system get the maximum thrust efficiency.In general,for two airfoils system with a fixed pitching frequency and a fixed pitching amplitude,increasing the phase-lag φ of the two airfoils system will result in larger thrust efficiency compared increasing the stream wise distance S/c between the two airfoils system. |
