Aeroelastic responses of airfoil under dynamic stall forced to oscillate by cyclic pitch input

Abstract

The helicopter blades are involved in a nonlinear aeroelastic system in which unsteady dynamic stall flow and flexible structure are coupled. This problem can be reduced to a 2D aeroelastic problem in which airfoil is forced to oscillate through cyclic pitch input. In this paper, a modified dynamic stall model which focuses on revising the modeling of aerodynamic loads in reattachment stage is adopted in the numerical simulation to predict the aeroelastic responses of the above-mentioned system. The results are validated against experiments, after which parametric studies are performed to investigate the complex aeroelastic responses. In single Degree-Of-Freedom (DOF) system, secondary resonances happen in specific ratios of natural frequency to forcing frequency, which are found in both experiments and simulation. Moreover, as free-stream velocity varies, the aerodynamic stiffness has an apparent influence on the pitching natural frequency of system with low structural frequency. The varying pitching natural frequency brings about different aeroelastic responses including 1/2 subharmonic resonances and quenching phenomenon in single-DOF system as well as internal resonances and combination resonances in 2-DOF system. The aeroelastic system with low structural frequency is further investigated through multiple scale method, which provides an illustration for the mechanism of those phenomena found in the aeroelastic simulation adopting a semi-empirical aerodynamic model.