Research On Dive-flight Aerodynamic Characteristics And Morphing Flight Strategy Of Kingfisher For Entering Water

The cross-media vehicle is a new concept amphibious unmanned motion platform that can cruise in the air and water and can freely traverse the water-air interface.The process of the vehicle entering and exiting water involves some challenges such as the coupling between the vehicle and the fluid medium,the load abrupt change across the medium,the instantaneous instability as changing flight attitudes suddenly,and so on.How to reduce the water-entry impact and to prevent instability? And how to control the water-exit load abrupt change and to take off from water quickly? Researchers need to break through these bottleneck technologies.The research on the control strategy of flight attitudes before entering water is the precondition to break through the technology of entering water and the key to ensure its smooth entering water.This paper adopts the research methods of Engineering Bionics,and selects Common Kingfisher as the bionic prototype to analyze its dive-flight aerodynamic performance and flight strategy,then explores the variant control strategy of the cross-media vehicle before entering water.The specific work is as follows:Three kinds of typical flight attitudes of Kingfisher during the subduction period were chosen as the research object.Three-dimensional models of Kingfisher’s flight attitudes were then built based upon the point cloud data obtained by the reverse engineering approach.Compared the body length parameter of the Kingfisher’s living body with that of the Kingfisher’s models,the accuracy of surface processing is verified.Through the geometric measurement of the Kingfisher’s three flight attitude models,the parameters of the physical models were obtained such as barycenter,wing span,wing reference area,maximum section area,and so on.The experimental samples of the Kingfisher’s three flight attitude models were prepared based on SLA(Stereo Lithography Appearance)printing technique.Then by using a simple wind tunnel test device,36 sets of aerodynamic test data weremeasured at wind speeds of 6 m/s,11 m/s,16 m/s and 21 m/s.The lift and drag characteristics of the Kingfisher’s three flight attitude at four speeds were analyzed.Studies suggest that Kingfisher can change the aerodynamic force by adjusting the wing shape and the flight speed before entering water.The coarse-scale unstructured grid,the medium-scale unstructured grid and the fine-scale unstructured grid were divided accordingly for the computational domain of the wing-closing attitude model by using Hypermesh software.The influence of grid’s scale on the drag coefficient of Kingfisher was studied by simulation based on the LES model.Then the best grid scale(the medium-scale grid)was obtained.In this paper,the computational domains of three typical attitude models were all discretized using the medium-scale grid.Under the condition of 6 m/s,11 m/s,16 m/s,21 m/s,26m/s,the dive-flight aerodynamic performance of Kingfisher was simulated.Compared the simulation results with the experimental results,it is found that the tendency of the lift and the drag is basically the same.Therefore,the correctness of the numerical simulation method is verified.Then,the aerodynamic characteristics and the flow field characteristics of Kingfisher were analyzed.Results show that the frictional drag of three attitude models is approximately equal and the pressure drag of bird body is also almost equal,as well,the proportion of them to the total drag is smaller.And due to the different wing shape,the lift,the induced drag and the pressure drag of wing have a great difference.By using Hypermesh and ICEM software,the hybrid grid of four sub-areasthe was divided for the computational domain.By utilizing the relevant macros and functions of FLUENT,the dive-flight process of Kingfisher was simulated based on SST k-ω turbulence model under the condition of initial velocity of 11 m/s and 26 m/s and initial glide angle of 60 o.The relationship between flight speed,glide angle,and flight trajectory over time was analyzed in the entire dive-flight process.And also the changes of the aerodynamic force,flight speed,pitching moment and pitch angular velocity of Kingfisher over time was analyzed in different phases.Results show that Kingfisher adjusts the flight trajectory to get more lift to maintain the stability of the accelerated subduction by a head-up method in the wing-closing phase,Kingfisherproduces high drag and high lift to achieve rapid deceleration at high speed in the wing-folding phase,and Kingfisher changes the lift direction and adjusts the speed when approaching the water to obtain the best initial state of entering water in the wing-sweeping-back phase.According to the study of Kingfisher dive flight process,the variant control strategies of the cross-media vehicle’s wing folding and wing sweepback before entering water were explored.