Prediction of Tonal Aerodynamic Noise from Open Rotors

A numerical approach for predicting tonal aerodynamic noise from \open rotors" is presented. \Open rotor" refers to an engine architecture with a pair of counter-rotating propellers. Typical noise spectra from an open rotor consist of dominant tones, which arise due both to steady loading/thickness, and due to aerodynamic interaction between the two bladerows. The proposed prediction approach utilizes Reynolds Averaged Navier Stokes (RANS) Computational Fluid Dynamics (CFD) simulations to obtain near-eld description of the noise sources. The near-to-far-eld propagation is then carried out by solving the Ffowcs Williams-Hawkings equation. Since the interest of this paper is limited to tone noise, a linearized, frequency domain approach is adopted to solve the wake/vortex-blade interaction problem. This paper focuses primarily on the speed scaling of the aerodynamic tonal noise from open rotors. Even though there is no theoretical mode cut-o due to the absence of the duct in open rotors, the far-eld noise is a strong function of the azimuthal mode order. While the former has circumferential modes of high order, due to the relatively large number of blades ( 10 12), the latter typically has modes of small orders. The high mode orders have very low radiation eciency and exhibit very strong scaling with Mach number, while the low mode orders show a relatively weaker scaling. The prediction approach is able to capture the speed scaling (observed in experiment) of the overall aerodynamic noise very well.

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