Tonal Mach wave radiation of a counter-rotating open rotor

The paper presents a numerical and theoretical study of aft-radiated interaction noise of a generic counter-rotating open rotor (CROR). While CROR configurations promise substantial gains in propulsive efficiency, their distinct and intense tonal acoustic signature remains a critical concern for community noise acceptance, driving the need for accurate prediction tools explored in this study. It is demonstrated that in the aft quadrant low-order combination tones may have the form of waves with conical wavefronts associated with long-lived wake disturbances. Physical analysis reveals that these disturbances originate when interaction noise excites weakly damped eigenmodes in the rotor wake, generating, in turn, Mach-wave radiation in the flight stream. The key methodological contribution of this work is the development of a hybrid approach that combines computational fluid dynamics (CFD), computational aeroacoustics (CAA), and analytical methods. We propose an improved Ffowcs Williams–Hawkings (FWH) method that integrates a short, computationally feasible FWH surface with a semi-analytical cylindrical extension. This extension models the downstream evolution of wake eigenmodes using a linear perturbation approach, calibrated for amplitude and phase at a short downstream distance from the rotor. The proposed methodology demonstrates prediction accuracy for the first combination tone comparable to that of a full CFD-informed long FWH surface, which would require a mesh with approximately twice the number of cells in the external domain.