中文摘要：

The aerodynamic force and flow structure of NACA 0012 airfoilperforming an unsteady motion at low Reynolds number (Re=100) are calculatedby solving Navier-Stokes equations.The motion consists of three parts:the firsttranslation,rotation and the second translation in the direction opposite to the first.The rotation and the second translation in this motion are expected to represent therotation and translation of the wing-section of a hovering insect.The flow structure isused in combination with the theory of vorticity dynamics to explain the generation ofunsteady aerodynamic force in the motion.During the rotation,due to the creationof strong vortices in short time,large aerodynamic force is produced and the force isalmost normal to the airfoil chord.During the second translation,large lift coefficientcan be maintained for certain time period and (?)_L,the lift coefficient averaged overfour chord lengths of travel,is larger than 2 (the corresponding steady-state liftcoefficient is only 0.9).The large lift coefficient is due to two effects.The first isthe delayed shedding of the stall vortex.The second is that the vortices createdduring the airfoil rotation and in the near wake left by previous translation form ashort 'vortex street' in front of the airfoil and the 'vortex street' induces a 'wind';against this 'wind' the airfoil translates,increasing its relative speed.The aboveresults provide insights to the understanding of the mechanism of high-lift generationby a hovering insect.

英文摘要：

The aerodynamic force and flow structure of NACA 0012 airfoil performing an unsteady motion at low Reynolds number (Re = 100) are calculated by solving Navier-Stokes equations. The motion consists of three parts: the first translation, rotation and the second translation in the direction opposite to the first. The rotation and the second translation in this motion are expected to represent the rotation and translation of the wing-section of a hovering insect. The flow structure is used in combination with the theory of vorticity dynamics to explain the generation of unsteady aerodynamic force in the motion. During the rotation, due to the creation of strong vortices in short time, large aerodynamic force is produced and the force is almost normal to the airfoil chord. During the second translation, large lift coefficient can be maintained for certain time period and (C) over bar (L), the lift coefficient averaged over four chord lengths of travel, is larger than 2 (the corresponding steady-state lift coefficient is only 0.9). The large lift coefficient is due to two effects. The first is the delayed shedding of the stall vortex. The second is that the vortices created during the airfoil rotation and in the near wake left by previous translation form a short 'vortex street' in front of the airfoil and the 'vortex street' induces a 'wind'; against this 'wind' the airfoil translates, increasing its relative speed. The above results provide insights to the understanding of the mechanism of high-lift generation by a hovering insect.