Investigation On Two-dimensional Aeroelastic Dynamics Under Dynamic Stall

As the helicopter is flying forward with a high speed,the blades are involved in a nonlinear aero-elastic system where unsteady flow field and flexible structure are coupled because of its periodic oscillation.When in dynamic stall,the overshoot of lift caused by dynamic stall vortex,sudden drop of lift caused by massive separation and the complex variation of boundary layer in reattachment process at low angle add to the nonlinearity of aerodynamic loads on the blades,which contributes to the complexity of aero-elastic response of the blades and affects the performance of helicopters significantly.There are many researches on the effect of structural nonlinearity on the aero-elastic responses,but the influence of aerodynamic nonlinearity on the response is not clarified yet.Therefore,further investigations on it are needed.The most critical issues include the appropriate aerodynamic model and proper nonlinear analysis of aero-elastic response.In view of the above problems,this paper has carried on the in-depth study on the aeroelastic problem of the airfoil under dynamic stall:1.Based on the existing Beddoes-Leishman dynamic stall model,supplementary modeling is performed for the reattachment process of dynamic stall.According to the characteristics of the boundary layer of the reattachment process with two different stages,the Kirchhoff equation that quantize the influence of separation point on the aerodynamic load in the original Beddoes-Leishman model is modified to obtain a more accurate dynamic stall aerodynamic model than the original model.The model was validated.At the same time,based on the neural network,the mapping relationship between the airfoil structure displacement and the airfoil aerodynamic load is obtained,and the dynamic stall neural network reduction model is established as a comparison supplement;2.The experimental study of the dynamic stall aero-elastic system is carried out,and the results of the aero-elastic response predicted by the theoretical model are compared and verified.At the same time,nonlinear analysis of the response of the aero-elastic system with damping is performed by phase space reconstruction and high-order spectral analysis.3.Adopting the improved aerodynamic model,the corresponding aero-elastic dynamics equation is coupled to calculate the aero-elastic problems,and the effects of natural frequency,Mach number and coupling degree on the response characteristics of the aero-elastic system are discussed.Among them,for the single-degree-of-freedom system,the secondary resonance phenomenon under the specific natural frequency-driving frequency ratio at higher natural frequencies is explored,and the counter-rotating vortex interaction instability at the trailing edge is proposed as the mechanism of inducing the resonances by studying the numerical simulation of the flow field.In addition,the influence of the aerodynamic stiffness of the system on the aeroelastic response at lower natural frequencies is explored.For the two-degree-of-freedom system,the nonlinear influence of the coupling degree on the aeroelastic response and the internal resonance phenomenon in the forced oscillation system are explored.