| Stall flutter is self-excited oscillation,occurring on the helicopter blades that are experiencing dynamic stall.It is not only one of the factors that hinder the improvement of advance speed and mobility of a helicopter,but also one of the factors that result in the fatigue and even broken of the blades on helicopters and wind turbines.Since the highly non-linear phenomenon of dynamic stall is involved in the process,the prediction and control of the stall flutter has always been a focus along with the development of rotors.ONERA and Beddose-Leishman Model are used in engineering to predict the load on the blades and also in researches to numerically investigate the response of an aero-elastic system.However,recent papers reported the underestimation of the leading-edge-induced air-load by B-L model when Mach number is less than 0.3,and this cast doubts on the result of the response of an aero-elastic system by utilizing B-L model.Beside the weak confidence in numerical researches,lack of experiments for systems with cyclic pitch as the input leads to poor understanding of the mechanism of dynamic-stall-involved aero-elastic problems.Based on the facts stated above,in order to deepen the understanding of the mechanism of an aero-elastic coupled system,several works have been carried out:1.Based on recent researches in dynamic stall,theories of aerodynamic forces and vorticity fields and CFD simulation of flow field around an airfoil undergoing dynamic stall,leading-edge-induced additional forces is modeled and the model is then combined as a vortex model into B-L model.Results show good agreement of the modeled forces and that acquired by experiment in 1978 in low Mach number,which provides the numerical research with a more reliable aerodynamic model.2.The revised dynamic stall model is then utilized to numerically research on the influence of different parameters,i.e.the natural frequency,damping ratio and driving frequency,on the response of the aero-elastic system,in which the cyclic pitch(instead of cyclic moment)is the power input.Bifurcation plots of these parameters are sketched.The result shows that with increasing damping ratio,the nonlinearity of the response of the difference angle(between pitch input and angle of attack of the airfoil)weakens.With increasing natural frequency(or increasing torsional stiffness),the non-linearity of the response strengthens and then weakens.And with increasing driving frequency,the response frequently get into and out of chaos(strong non-linearity)and then get completely chaotic.3.Investigation of how driving angles,driving frequency and natural frequency impact the characteristic of response is carried out on an established aero-elastic system with cyclic pitch as the input.The structural parameters of the system is measured.The result shows the great impact of driving frequency on the response,while the reason why such impact overrides the natural frequency and driving angle remains to be further discussed.The aerodynamic load of airfoil in elastic system is measured and roughly compared with rigid system.4.Simulation of the experiment system using the method established in the 2nd part is made and compared with the experiment.Result shows agreement of the simulated and experiment-obtained phase plot while difference in the Poincare Map.Further analysis suggests that the strong non-linearity of the aero-elastic response lies in the non-linear aerodynamic force during the period after LEV travels off the airfoil and before the flow reattaches.The understanding of the complexity of the air-load in dynamic-stall-involved aero-elastic response needs further information of the flow field,and suggestions for later researches is consequently proposed. |
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