Modal Analysis of Flutter Mechanisms in Damaged Rotor Blades Under Supersonic Inflow

Fatigue damage in high-performance rotor blades under supersonic inlet flow poses complex aeroelastic challenges beyond existing theories, potentially leading to catastrophic flutter in military applications. This study is thus to explore the mechanisms of aeroelastic instability in such a configuration. Initially, a fluid–structure interaction numerical model is employed to simulate aeroelastic behavior. Subsequently, proper orthogonal decomposition (POD), dynamic mode decomposition (DMD), spectral proper orthogonal decomposition (SPOD), and bispectral mode decomposition (BMD) are employed for exploring underlying flutter mechanisms. Results indicate that a damaged blade, despite operating under conditions safe for a healthy blade, may develop limit cycle oscillation (LCO) or flutter due to pressure redistribution at the leading edge. POD reveals energy dispersion and weakened modal correlation in flutter. DMD shows that as vibration destabilizes, low- and midfrequency modes become unstable, with high-frequency clusters emerging in LCO and their stability reversing during flutter. SPOD reveals that stable vibrations transition to LCO with an energy shift from low- to midfrequency modes, while LCO to divergent flutter involves energy redistribution back to low frequencies and a downshift of high-frequency components. BMD demonstrates intensifying triadic nonlinear interactions, with flutter characterized by dense bispectral peaks, strong sum/difference–frequency coupling, and a frequency cascade initiating at a lower fundamental frequency compared to LCO, explaining the downshift of high-frequency components identified by SPOD. These physical insights enhance understanding of aeroelasticity under supersonic inlet flow, aiding flutter prediction and mitigation.