Aeroacoustic analysis of a high-speed open rotor

Owing to their inherent fuel efficiency, there is renewed interest in developing open rotor propulsion systems that are both efficient and quiet. The major contributor to the overall noise of an open rotor system is the propulsor noise, which is produced as a result of the interaction of the airstream with the counter-rotating blades. Prediction of the propulsor noise is, therefore, a necessary ingredient in any approach for designing low-noise open rotor systems that can meet community noise regulations and have acceptable cabin noise levels. To that end, there has been a resurgence of activities in the aeroacoustic modeling of open rotors in recent years. While direct numerical simulations are gaining traction, the bulk of existing prediction capability resides in hybrid approaches in which the aerodynamics of the open rotor system is computed via CFD and is used as input in some appropriate "linear" acoustic model for computing the open rotor noise. At NASA the focus has been on assessing the utility of hybrid approaches for accurately predicting the open rotor tone spectra with an emphasis on the understanding of the role of the various underlying mechanisms of noise generation and their relative importance at different operating conditions. Using high-fidelity aerodynamic simulations of a benchmark (non-proprietary) open rotor blade set, together with acoustic models based on a high-blade-count asymptotic approximation of the Ffowcs-Williams Hawkings equation, tone noise predictions for a number of configurations have been carried out. These aerodynamic and acoustic predictions have been compared with wind tunnel measurements of the benchmark open rotor blade set to establish the capabilities and the limitations of the hybrid approaches. The results suggest that while predicting the absolute spectral levels is difficult, the noise trends are reasonably well predicted by such hybrid approaches at a reasonable overall computational cost.