The Unsteady Aerodynamic Performance Of Low Speed Unmanned Aerial Vehicles

A majority of UAV control systems are designed based on the single control point.The attitude of the UAV would deviate from the balance and vary with time,when encountering interferences from the ambience or control system.It would affect the aerodynamic efficiency.The unsteady aerodynamic performances of low speed unmanned aerial vehicles are researched in this thesis and the detailed work is summarized as follows:Firstly,the flowfields around the unsteady-motion UAVs are highly complicated,which accompany with the generation and shedding of vortex on the wings.The aerodynamic forces are changed violently with time.To simulate the unsteady aerodynamic characteristics of vehicles more accurately,the Navier-Strokes equations,spatial and temporal discretizations,solutions of the control equations,selections of turbulence models and mesh generations are investigated separately.The IDW-MSA algorithm is proposed to deal with the mesh deformation.The highly efficient solution method established is validated with four classic examples.The comparison shows that the calculation results are in good agreement with the experimental data.Secondly,the unsteady motion parameters are designed based on the flight data,which are the basis for further study.The variations of the attitude can be regared as the combinations of trigonometric functions.The complicated threedimensional motions can be simplified to the two-dimensional trigonometric motions.The influences of different parameters(reduced frequency,amplitude,phase angle etc.)on the aerodynamic characteristics and flowfields for pure pitch motions,pure plunge motions and coupled motions are studied systematically.Thirdly,to eliminate or alleviate the dynamic stall of the airfoil during the pitch motions with low frequencies,the approach of controlling the trailing edge is designed.The influences of flap actuation start time,maximum flap deflection,flap actuation duration and different types of flap motions are researched.During the pitch-down phase,flapping down the trailing edge can increase the camber of the airfoil,which raises the flow velocity near the airfoil to resist the adverse pressure gradient.The method can alleviate the dynamic stall near the maximum angle of attack efficiently.Fourthly,the Theodorsen's model and the Proper Orthogonal Decomposition mothod are applied to calculate the unsteady aerodynamic forces,as the CFD simulation is a great computational burden.The former is simple but has great error,the latter reqiures the system has the strong regularity.A reduced-order modeling is developed to predict the unsteady aerodynamic forces under light dynamic stall conditions at low-speed regimes.The filtered white Gaussian noise is selected as input signals for the computational fluid dynamics solver in order to generate training data.Because of the time history influences,the reduced-order modeling combines the Kriging function and recurrence framework together in this approach.The predictions via the proposed approach are in agreement with the results using a computational fluid dynamics solver over the designed ranges of amplitude and reduced frequency,which is suitable for engineering applications,such as fluidstructure interaction,and aircraft design optimizations.Fifthly,taking an unmanned aerial vehicle as the research object,the flight data is analyzed to find the variation laws of frequency characteristics and amplitude.Based on the CFD simulation method and ROM,the unsteady aerodynamic performances of the UAV are investigated.The aerodynamic performance under dynamic conditions is lower than that under static condition,which affects the flight range and endurance directly.