A Study On The Effect Of Surface Structure On Aerodynamic Behavior Of Dragonfly Wing

Dragonflies demonstrate excellent flight performances,such as their mobility and ability to fly stably with low energy consumption.These flying properties have been actively studied and contributed to an airfoil design optimization.As biomimetic examples based on dragonfly,micro air vehicles(MAVs)have attracted much attention in recent decades.MAVs are small unmanned aerial vehicles that can be used in various fields such as inspections of structures and planetary exploration.The design of MAVs needs to overcome a stronger viscous force due to scale effect and other factors.Furthermore,a steep turn,sudden start and stop are required for the flight performance.To fulfill these requirements above,the studies on the mechanism of how dragonflies fly assume key roles.The remarkable properties of dragonfly wings could be divided into two types:aerodynamic and structural.As an example of a structural property,it is revealed that polygonal structures of veins help dragonfly wings flexibility of deformation.In addition,fine roughness representing wing superhydrophobicity could protects the wing membranes against water adhesion.However,these structural properties are studied independently of their aerodynamic properties.The studies on the wing properties from both prospective,structural and aerodynamic,would help to optimize biomimetic designs more than before.The objective of this research then is to investigate the effects of location-dependent surface design parameters,such as the polygonal patterns and roughness variation on aerodynamics of the dragonfly wings.To study the polygonal structural effects of dragonfly wing veins on aerodynamics experimentally,four major polygonal structures—pure square,pure shifted square,pure pentagon and a combination of square and pentagon were designed and 3D printed.The skin friction coefficient C_f generated by each model was measured using wind tunnel experiments and computational fluid dynamics(CFD)simulations.Furthermore,the effects of roughness variation of dragonfly wing membrane on aerodynamics were studied by simulations.Roughness variation of dragonfly wing membrane was observed using a 3D digital microscope,and inspired simplified models.CFD analyses were carried out with various angles of attack.The experimental and simulation results of the study on the polygonal structural effects indicate that:(1)Under laminar and transitional flow conditions,models with pentagonal structures manifest larger C_f values than models with square structures,especially a model only with pentagonal structures represent the largest C_f values:(2)A combination of square and pentagon generates the largest C_f values under turbulent flow condition.Furthermore,microscopic observations and the simulation results are summarized as below:(1)The posterior area of the membrane is covered with pentagon vein structure and always show larger R_aand R_z than the anterior area:(2)The model having larger roughness in an anterior area in the chord-wise direction generate a bigger vortex in the rear at?=8,which increases C_dvalues:(3)C_l value shows a positive correlation with an angle of attack,and its value profile is related to a roughness distribution.Along with these key findings,this thesis may allegedly benefit further flight performance with keeping the wing flexible and superhydrophobic,then contribute to optimize aerodynamic properties of bionic designs while keeping high structural properties.