Combinatorial Biomimetic Design Of Fluid Dynamic Shape And Analysis Of Aerodynamic Characteristics For Cross-media UAV

Cross-media UAV is a new concept of amphibious unmanned vehicle,which can cruise in the air and water and can freely cross the water-air interface.The development of crossmedia UAV needs to solve the key technologies of four operation modes: Dry Flight,Water Entry,Submerged Movement and Water Exit.The vehicle needs good water-entry and waterexit performance and less fluid resistance.In this paper,two species of kingfisher and diving beetle are selected as bionic models,and the head of kingfisher and the body of diving beetle are combined with the idea of combinatorial biomimetic design.The fluid dynamic shape of the cross-media vehicle is designed and its aerodynamic performance is analyzed,taking into account the multiple operation modes of air flight,underwater diving and water entry.The points cloud of the shape of kingfisher and diving beetle is obtained by threedimensional scanner.The characteristic curves of the kingfisher's head and diving beetle's body are intercepted,and the characteristic curves of kingfisher's head and diving beetle's body are smoothed by fitting method.In Matlab,Fourier series is selected as the fitting model,and the mathematical expression of the smoothed characteristic curve is obtained.The fourthorder Bessel curve and the third-order Bessel curve are selected as cross-section curves of the Kingfisher's head and diving beetle's body respectively.The three-dimensional models of the Kingfisher's head and diving beetle's body are generated in Catia,and then based on the parameters of Kingfisher body length,the diving beetle's body was enlarged 2.5 times to combine with the head of kingfisher,and the middle part is smoothly connected with the neck.The fuselage of the vehicle adopts a combined bionic model,which is combined with the head of Kingfisher and the body of diving beetle.The tail of the vehicle adopts a V-shaped tail configuration.The wing adopts a morphing wing with swept-back angle generated by NACA4415 airfoil.The wing is divided into two parts: the inner wing and the outer wing.The outer wing can sweep back by a rotating mechanism at a maximum sweep angle of 20 degrees.By means of wind tunnel test and numerical simulation,the lift-drag characteristics of the wing at different sweep angles are researched.The results show that the lift and drag of the morphing wing designed in this paper will decrease when the outer wing sweeps back 20 degrees,so as to meet the demand of water entry.The Hypermesh is used to mesh the aerodynamic shape of the cross-media UAV designed in this paper.The SAS model is used to simulate the aerodynamic shape of the cross-media UAV.The aerodynamic characteristics and flow field characteristics of the cross-media UAV are analyzed.When flying at low speed,the lift coefficient and drag coefficient of the vehicle are not affected by Reynolds number.After wing morphing,both lift coefficient and drag coefficient are decreased,but their variation tendency with angel attack is not changed compared with before morphing.The aerodynamic performances of the vehecle is researched,which the body of the diving beetle occupies different proportion of the vehicle The results showed that the aerodynamic drag of the vehicle decreased when the body of the diving beetle was elongated.Aerodynamic characteristics of the sweep morphing vehicle in diving flight are analyzed.The results show that when the velocity is 20 m/s,the angle of attack is 20 degrees and the angle of diving is 75 degrees,the vehicle remains stable in diving flight.