Efficient low-fidelity aeroacoustic permanence calculation of propellers

Abstract

A noise footprint prediction framework for propeller-driven aircraft which couples an aerodynamic model and several aeroacoustic models is presented in this study. The aerodynamic model is based on the blade element momentum theory, while the aeroacoustic models are based on a time-domain compact dipole/monopole Ffowcs-Williams and Hawking's acoustic analogy, a trailing edge noise model, and a noise hemisphere database approach including a straight-ray propagation model, respectively. In order to reduce the runtime, the frequency-domain acoustic formulation, derived by Hanson (1980), is implemented and validated against the compact dipole/monopole Ffowcs-Williams and Hawking's acoustic analogy. The framework evaluates the acoustic effects of variations in the design and operating conditions of a propeller in forward flight. Noise footprints, obtained with different propeller configurations having varying advance ratio and number of blades are compared. It is found that, for a given thrust, a drop in advance ratio alters the source directivity dramatically, which resulting in a variation of up to 30 dBA on the acoustic footprint. When the advance ratio is kept the same and the number of blades increases from 5 to 7, the variation becomes 16 dBA due to the change in the source directivity, but the maximum noise level remains the same. The latter condition reduces the loading for each blade, and consequently the associated noise. However, the total noise remains unchanged as a consequence of increasing thickness noise due to the lower advance ratio, high blade tip Mach number, and addition of extra blades.