| The trans-medium aircraft is a new type of unmanned platform which can cruise in the air and underwater and cross the water-air interface freely.In order to ensure the four typical operation functions of air flying,water entry,submerged movement,and water exit,the design of trans-medium aircraft should take into account the differences in the physical properties of air and water,and integrate the design advantages of aircraft and vehicle.In this paper,combined bionics is used to design the aerohydrodynamic layout of the trans-medium aircraft.We analyzed the aerodynamic and hydrodynamic performance and explained the variation characteristics of the axial force and torque during the oblique water exit of the aircraft,which provides design basis for aircraft water exit.We selected kingfisher and diving beetle as bionic models,designed the aerohydrodynamic fusion fluid dynamic layout of aircraft.Taking the wing sweep angle as a variable,four different aircraft at different angles of attack were parametrically designed to analyze the aerodynamic and flow field characteristics.The hydrodynamic characteristics of the optimal design scheme are analyzed with the lift drag ratio as the parameter,and the wing sweep angle of 40 ° is determined as the fluid dynamic shape design scheme of the trans-medium aircraft.The water exit process of trans-medium aircraft is mainly affected by buoyancy and resistance,which decrease during the whole process.The resistance is mainly the frictional resistance of the aircraft surface and the drag force of attached water.The design scheme of the aircraft with a sweep angle of 40° was selected,and the size of the aircraft model was reduced by four times.9 groups of working conditions(three angles of 45°,60°,75° and three velocities of 1,2,3 m/s)were tested for water exit to analyze the axial force changes of the aircraft.The scaled-down model was made by3 D printing and was weighted according to gravity equal to underwater static buoyancy.The underwater state of the aircraft can be approximated as zero angle of attack operation before the water exit,and the axial force difference between different water exit angles is not large.As the water exit angle increases,the axial force of the aircraft becomes larger caused by the increase of the axial component of gravity,and the amount of change increases relatively.From the perspective of the water exit velocity,with the increase of the water exit velocity,the water exit time decreases,the friction resistance increases,and the axial force becomes larger and changes more drastically.There is no obvious water splash during the aircraft’s water exit,which has a good overall streamline.After the water exit,the tail plume forms a "water curtain" phenomenon,and the "water curtain" starts to break from the middle of both sides until it completely disappears.Using dynamic grid technology to simulate the water exit process of the original model of the aircraft and compile UDF to control the movement of the aircraft.By comparing the experimental and simulated axial forces at three velocities of 60° water exit angle,the trend of the change of the axial forces is approximately the same,which verifies the accuracy of the simulation.The numerical simulation of four water exit angles from 30° ～ 75° shows that the pitching moment of the aircraft increases greatly with the increase of water exit velocity,and the moment has obvious response to the change of velocity.At low velocity,as the water exit angle increases,the torque decreases,and the maximum torque of 75° is 25% lower than that of 30°.At medium and high velocities,the pitch torques of 4m/s and 6m/s change little with the change of water exit angle,and the increase of the water exit velocity weakens the influence of the water exit angle on the pitch torque. |
