An experimental investigation on the effect of in-flow distortions of propeller noise

Abstract

Propellers usually operate in non-uniform flow conditions, leading to significant levels of noise generation due to interaction with the leading edges of the propeller blades. Under these flow conditions, the noise generated can be substantially different from the rotor-alone self-noise generated under uniform mean flow conditions. This paper presents an experimental investigation aimed at studying the far-field noise due to a two-bladed propeller immersed in a controlled non-uniform flow with known characteristics. This paper presents the results of a parametric study with different degrees of flow non-uniformity generated from different geometric obstructions located in the jet nozzle, while maintaining constant thrust. Measurements of the radiated noise show that the incoming mean flow non-uniformities under investigation in this study cause a significant increase in tonal amplitudes of the blade passing frequencies of harmonic orders greater than 2. At the first two blade passing frequencies the tones are dominated by the rotor-alone ‘Gutin-type’ tones resulting from the steady blade loading.The measured noise spectra are compared with predictions from a simple frequency-domain flat plate model. The analytical model will be shown to capture the main features of the interaction tonal noise spectrum and directivity.