Comparison Between Postprocessing Methods Applied to Rotor-Stator-Interaction Tone-Noise Problems

The range of applicability and accuracy of the major postprocessing methods available today to extract tone-noise information from unsteady aerodynamic source data for the rotor-stator-interaction problem are systematically examined using four numerical examples of increasing geometric and flow complexity. The test cases span from the benchmark problem of a vortical gust impinging on an unloaded cascade of flat plates to a fan stage representative of a modern turbofan engine. A linearized Navier-Stokes computational fluid dynamics method is used to obtain the aerodynamic solutions for the tone-noise postprocessing. The underlying assumptions of each postprocessing method are examined, highlighting the shortcomings and the strengths of each method. In particular, the effects of the duct geometry and mean flow are investigated. An acoustic analogy integral method, and various wave-splitting techniques are considered. Furthermore, the influence of the nonuniformity and swirl that characterize the mean flow in a real fan stage on the sound power conservation is examined using a modal expression of the sound power for nonuniform swirling flows in the high-frequency limit. A comparison with the widely used expression of the sound power for a uniform flow is also carried out.