Study On The Hydrodynamic Characteristics Of Lateral Free Flexible Filament In The Wakes Of Flapping Wing

Fluid-solid coupling is a common phenomenon in nature.For example,seaweed plants in the sea sway with sea water,birds flying in the sky,and fishes swim in the water.In order to explore the control mechanism,scientists had done a lot of research including experiments and numerical simulation.This paper used computer numerical simulation to explore the fluid-solid coupling phenomenon of the flexible filament.Immersed boundary method was adopted in the simulation.The physical model of this paper is active flapping wing and passive filament,in which the head of the filament is free only in the Y direction,and is fixed in the X direction,flapping wing and the filament are set with different parameters,they are density of the filament,distance between the head of the filament and tail of the flapping wing,and the length of the filament.Through numerical simulation,we found that as the density of the filament increased,that was,the mass became larger(in the case of a distance of 2),the drag coefficient of the filament became larger too,and almost linearly increased,this was because the inertial force of the filament increased with the raise of filament's density,and since the inertial force of the filament directly acted on the flow field,the various physical fields of the flow field would also change with the change of inertial force.Among various physical fields,the velocity field determined the drag coefficient of the filament and the flapping wing,and the ratio of the velocity of the flow field to the swinging speed of the filament also determines the motion pattern of the filament,under some parameters the movement pattern of the filament changed from mode one and mode three and four to mode two.This was because the ratio of the flow field's velocity to the filament's swinging speed got changed,and caused the head of the filament to hit the vortex core.While drag coefficient of the flapping wing hardly changed,and there was only a slight fluctuation,which meaned that the change in filament's density at the spacing of 2 did not affect the flow field around the flapping wing.When the length of the filament changed,the drag coefficient of the filament also changed.Specifically,as the distance and density were constant,the drag coefficient of the filament almost increased linearly with the increase of the filament's length.When the length of the filament changed,the contact range of the filament with the flow field also increased,so that the degree of influence filament had on the flow field also increased.At a spacing of 2,influence of filament's length have on the flow fieldaround the flapping wing was less than the flow field around the filament,so the drag coefficient of the flapping wing did not change with the increase of filament's length.When we explored the influence of the distance between the head of the filament and the tail of the flapping wing on the system,we found that the drag coefficient of the flapping wing gradually increased with the increase of the distance from 0.5 to 2.5,that's because when the distance is small,filament had larger effect on the tail vortex of the flapping wing,that was tail vortex of the flapping wing was inclined when the filament was close to the flapping wing,and the tilting caused the thrust of the flapping wing to decrease,and the thrust was equivalent to the resistance.As the distance increased,the drag coefficient of the filament gradually decreased.This was because the velocity of the tail vortex near the flapping wing was stronger,and the flow velocity imposed on the filament was also larger.Therefore,the drag coefficient was larger.And away from the flapping wing,energy of the tail vortex was weakened after a certain distance of transmission,so that the strength of the flow field imposed on the filament was reduced,and the drag coefficient of the filament was gradually became smaller as the distance increased.