Numerical Investigation of Classic Flutter Mechanism under Circumferentially Non-Uniform Inlet Condition

Over recent years, the aeroelastic problem of fan blades has become significantly serious due to the demand for high performance of aero-engines. To explore the aeroelastic issue of flutter, fan instability is represented by the aerodynamic damping. When the inlet condition is uniform, aerodynamic damping is mainly caused by the temporal factor, and the classic flutter usually does not occur due to the usage of mistuning. However, with the existence of circumferentially non-uniform components in the distorted inflow, aerodynamic damping is also affected by the spatial factor, which may alter the occurrence mechanism of classic flutter. To investigate this issue, the full-annulus 3D (three-dimensional) unsteady calculation is conducted on a transonic fan NASA Rotor 67 with a typical circumferential distortion inflow. As demonstrated by the energy method for the rotor blades at each nodal diameter, the coefficients of aerodynamic damping contain an interval value rather than a fixed datum. Some of the blades have positive damping, and others are negative. The positive and negative aerodynamic damping is determined by the relative position between the rotor blade and the distorted region in the circumferential direction. Furthermore, a fast calculation method based on the fundamental principle regarding flow periodicity is developed to obtain the aerodynamic damping at different nodal diameters under non-uniform inlet conditions.