Bionic Configuration Design And Water Entry Performance Of Aquatic UAV Based On Water Entry Strategy Of Kingfisher

Aquatic Unmanned Aerial Vehicle(Aquatic UAV),which has significant military and civil application prospects,is a new concept amphibious unmanned motion platform that can freely pass through the water-gas interface.The development of the Aquatic UAV needs to solve the key technologies involved in the four motion modes:air cruise,water entry,underwater diving,and water exit.Among these technologies,a series of problems will be easily caused by the large impact load in the process of the Aquatic UAV entering the water,such as structural damage,failure of internal components and ballistic instability.It is of great practical significance to study the configuration design and water entry control method of the Aquatic UAV for reducing the impact load,which can improve the stability and the safety of the vehicle entry water.Although the concept of Aquatic UAV has been put forward for more than a hundred years,a systematic design theory has not yet been formed due to the difficulty in design and manufacture.It is a common way to imitate the waterbirds or insects with the function of cross water-gas media to break through the key technologies of the Aquatic UAV.In this dissertation,we take Kingfisher as the bionic model,studied the influence of water entry strategy on the water entry characteristics of Kingfisher under typical body posture,and revealed the buffering mechanism of Kingfisher head and neck in water entry process via CFD(Computational fluid dynamics)numerical simulation and experimental analysis method.And based on above,we carried out the combined bionic design for the hydrodynamic configuration of the Aquatic UAV,and studied the water entry performance of the bionic Aquatic UAV.The research contents of the dissertation are as follows:(1)Based on the analysis of the predation process of Kingfisher water entry,the typical posture of Kingfisher water entry was determined:the wings swept back first,and then the head,body and tail kept in a straight line.And an accurate 3D model of the typical water entry posture of Kingfisher was obtained by using reverse engineering technology.To analyze the influence of different initial water-entry velocity and angle of Kingfisher on the impact acceleration,pitching moment and flow field distribution,the CFD numerical simulation method based on VOF model and dynamic grid technology was used.The Reynolds-averaged Navier-Stokes(RANS)equation was used to solve the whole flow field.And the mesh density was determined by studying the influence of the mesh cell number on the axial impact force.Numerical simulations results show that the pressure difference resistance plays a decisive role in the generation of Kingfisher impact acceleration.The axial impact acceleration and pitching moment increase with the increase of initial water-entry velocity and angle.The peak impact acceleration and peak pitching moment of Kingfisher diving into the water have quadratic relationship with the water-entry velocity and linear relationship with the initial water-entry angle.Reducing the water-entry angle of Kingfisher will reduce its diving depth.By analyzing the pressure field and velocity vector field at the time of peak impact acceleration,we found that increasing the initial water-entry velocity will significantly increase the disturbance of flow field around kingfisher,enhance the added mass effect of kingfisher,and lead to larger impact acceleration.At the water-entry angle of 90°and initial water-entry velocity of 8m/s,the Kingfisher experienced the maximum axial impact acceleration and peak torque,which were 18.4g and 0.867N·m respectively.(2)According to the requirements of free diving test for Kingfisher,we designed and built an unrestrained water entry test platform,which can realize the accurate adjustment of initial water-entry speed and initial water-entry angle of Kingfisher model.The test platform includes:high-speed water entry propulsion mechanism,which can adjust the water-entry speed of the model;angle adjustment mechanism,which can adjust the water-entry angle;release system,which can realize the accurate release of the model before entering the water;embedded inertial measurement system,which can collect Kingfisher kinematics data;high-speed dynamic image capture system,which can obtain video image data.Through the water entry test,we studied the variation of the impact acceleration,pitching angle of Kingfisher model and water body response with the initial water-entry velocity and water-entry angle.The experimental results show that the axial peak impact acceleration of Kingfisher model has a quadratic relationship with the initial water-entry velocity,and a linear relationship with the initial water-entry angle,which corresponds to the numerical simulation results.The minimum value of radial impact acceleration decreases significantly with the increase of water entry angle,even close to 0.Throughout the water entry process,the Kingfisher model shows the trend of head up first and then head down to dive,and the trend of head down diving is intensified after the deep closure of the cavity behind the tail.By comparing the axial peak force between the experimental results and the simulation results,the applicability of the CFD numerical simulation method is verified,and the maximum difference between the simulation value and the experimental value is 11.5%.(3)Based on the streamline configuration of kingfisher's head,the Kingfisher Nose Slender-Body,the Configuration Bionic Revolution-Body and the Ogive Nose Revolution-Body were designed and manufactured by using the method of configuration bionics.The influence of different drop heights on the axial impact acceleration and the evolution characteristics of cavitation were qualitatively investigated by vertical water entering experiment.The experimental results show that compared with the Ogive Nose Revolution-Body,the Kingfisher Nose Slender-Body and the Configuration Bionic Revolution-Body can eliminate the peak impact acceleration at the initial stage of water entry and reduce the resistance during diving process.However,due to the asymmetric contour of kingfisher's head,Kingfisher Nose Slender-Body will deflect after entering the water.In addition,the Ripples effect after the cavity pinch-off behind the tail is the cause of the axial impact acceleration oscillation,and the vibration frequency under different water-entry velocities is basically maintained at about 200 Hz.(4)Inspired by the neck contraction behavior of the kingfisher during the water entry,the Functional Bionic Revolution-Body was designed and manufactured by introducing a linear spring between the head and the main body of the revolution-body.According to whether the nose of the revolution body is compressible or not,the Nose Fixed Revolution-Body the Nose Compressible Revolution-Body were distinguished.By comparing the vertical water entry tests of two forms of the Functional Bionic Revolution-Body,the influence of initial water-entry velocity on impact acceleration and cavitation dynamics were quantitatively studied,and the relationship between the initial peak impact acceleration,the linear stiffness and maximum compression was derived.The experimental results show that the peak impact acceleration of the main body can be significantly suppressed by introducing a spring between the nose and the main body of revolution-body.And the maximum impact acceleration of the main body is only related to the maximum compression of the nose under the same spring stiffness.In addition,the use of spring increases the initial velocity required for the occurrence of cavity formation and pinch-off on the side of the main body events.However,it slightly affects the non-dimensional pinch-off times of the cavity on the side of main body and the tail of the revolution-body.At the velocity of 3.96m/s,the peak impact acceleration can be reduced by71%for the Nose Compressible Revolution-Body.(5)Based on the study of the water entry mechanism of Kingfisher prototype,the fuselage shape of the Aquatic UAV with kingfisher head and diving beetle body is designed by using the idea of combined bionic design.Combined with the general process of wing design,we determined the wing area of 0.49m~2,take-off weight of 28.031kg,installation angle and up reflection angle of 0.By using strake wings to connect the wing and fuselage,four aerodynamic/hydrodynamic configurations of the Aquatic UAV with sweep angles of 30°,40°,50°and 60°were designed.Through CFD numerical simulation,the aerodynamic characteristics of four kinds of swept angle vehicles are studied.Comparing and analyzing the lift coefficient,drag coefficient,lift drag ratio and pitching moment coefficient of the vehicle,the final design scheme of the Aquatic UAV with 40°swept angle was determined.(6)Under the guidance of the flow similarity theory,we designed and manufactured a quarter scale model of 40°swept Aquatic UAV,and carried out the experimental study on the water entry performance.The influence of the initial velocity,angle and center of mass on the water entry performance was analyzed.Taking diving depth of an Aquatic UAV length as a node,the water entry performance was evaluated by the indexes of rapidity,stability and safety.The experimental results show that the higher the initial speed,the faster the vehicle enters the water,but it is easy to lose stability and is more unsafe.The larger the initial water angle,the faster the vehicle enters the water,the safer it is in radial direction,but it is easy to lose stability.The more forward the center of mass position is,the faster the vehicle enters the water,and the more unstable it enters the water,but it has no significant impact on its safety.Therefore,the vehicle should choose the appropriate initial water-entry velocity,angle,and centroid position according to the actual conditions to control the water entry process.