Experimental investigation of ducted fan noise control by turbulence grids insertion between propellers and struts

The control of turbulence-ingested propeller noise remains a challenging topic for unmanned aerial vehicles and urban air mobility applications. In this work, passive noise control by inserting turbulence grids between a propeller and struts for a ducted fan system is experimentally studied. The 3-bladed, 8 in. diameter propeller is mounted in a short duct downstream of 7 struts, each featuring symmetrical airfoil shapes with a chord length of 30 mm. Two turbulence grid configurations, namely, fully shaded and tip-shaded, are investigated by comparing the spectra and directivity characteristics of the aeroacoustic noise and their aerodynamic performance. Beamforming technology and hot-wire anemometry are employed to explore the fluid mechanics. The results show that at same rotating speed, the fully shaded turbulence grid is more effective in reducing the tonal noise. The overall noise reduction capability for the two cases is similar, and an overall sound pressure level noise reduction of roughly 3 dB is achieved. Due to the blockage effect of the turbulence grid, the fully shaded case bears a 20% thrust loss, while the tip-shaded case has a marginal deficit of thrust. When the rotating speed is adjusted to produce the same thrust of the baseline, the fully shaded case shows an increase in the overall noise, while the tip-shaded case maintains the noise reduction capability. The beamforming results and turbulence intensity measurements show that the propeller tip region has a strong interaction of the turbulence ingestion flow and tip leakage flow where the broadband noise source is dominant.