Aerodynamic Layout Optimization Of High Altitude Long Endurance Flying Wing Unmanned Aerial Vehicle

With the gradual improvement of weaponry requirements of various countries,High Altitude and Long Endurance Unmanned Aerial Vehicle（HALE UAV）was paid more and more attention.Its characteristics of large flight space and long flight time enabled it to carry out combat reconnaissance,intelligence gathering and strike missions for a long time.Because of its unique configuration,the flying wing configuration had great advantages in aerodynamic characteristics,structural feature,and stealth performance,and was more suitable for the aerodynamic configuration of HALE UAV.In this thesis,firstly,the aerodynamic calculation software based on vortex lattice method were lapped with the optimization software mode FRONTIER to build a preliminary optimization platform for the layout design of flying wing.According to the overall design requirements,the multi-objective optimization of the aerodynamic layout of a certain type of high-altitude long-endurance flying wing UAV with high lift-drag ratio and light structural weight was carried out with the platform,and the correlation between the geometric parameters and aerodynamic characteristics of the flying wing configuration was briefly analyzed.Secondly,the reliability of SST k-ωturbulence model and γ-（?）_θt transition model in STAR-CCM+,a commercial CFD software,was verified by the DLR-F6 wing-body configuration,S&K plate and Aerospatiale-A airfoil calculation examples,and the appropriate grid distribution was determined.Based on the above research results,the original flying wing configuration and a group of optimized flying wing configuration were selected to build three-dimensional models,and the aerodynamic characteristics of these two configurations at different attack angles（-2°～10°）were calculated by the full turbulence model.The flow field and separation characteristics of these two configurations at different attack angles were analyzed.Then,the transition model was used to calculate the optimal flying wing configuration.According to the surface friction coefficient and intermittent factor,the process of transition from laminar flow state to turbulent flow state was analyzed.Meanwhile,the separation bubble generated by airflow reattached under the condition of high altitude and low Reynolds number was analyzed.The difference and relationship between the calculation results of full turbulence model and transition model were compared.Finally,the optimal configuration was fit out negative twist angle at wing tip and its aerodynamic characteristics under different attack angles（-2°～10°）were calculated.The effects of negative twist angle at wing tip on aerodynamic characteristics and surface separation characteristics of flying wing UAV were analyzed by comparing with the configuration without negative twist angle.The results showed that the lift force of the flying wing was mainly from the fuselage.With the increased of the attack angle,the separation phenomenon of the wing surface was gradually significant.When the airflow separation occurred on the fuselage surface,the aircraft stalls.Under the condition of high altitude and low Reynolds number,transition occurred on the surface of the aircraft.As the attack angle increases,the starting point of transition moved towards the leading edge.When the attack angle reached 2 and 4 deg,transition started at about 40%chord length,when the attack angle reached 6 and 8 deg,transition started at about 10%chord length,and the separation bubble generated when attack angle was 8 deg.When the attack angle reached 10 deg,the airflow transition directly at leading edge.In addition,the negative twist angle at wing tips could effectively delay the stall of the flying wing UAV and increased the stall attack angle.In this thesis,an optimization platform for high altitude long endurance flying wing UAV was built.The results of numerical simulation using transition model had certain reference value for the study of high-altitude transition problem.