Turbulent Flow Impact on the Acoustic and Aerodynamic Performance of Overlapping Propellers

Experiments were carried out to examine the aerodynamic and aeroacoustic characteristics of overlapped multi-propeller configurations under various inflow turbulence conditions. The study focused on two five-bladed propellers, each with 9 inches in diameter, 9 inches in pitch, and operating at a uniform rotational speed of 8000 rpm. The propellers were positioned with a fixed axial and lateral tip separation to analyze the effects of different levels of inflow turbulence on both the aerodynamic force coefficients and the noise levels in the far field. The experiments were conducted at a velocity range from 8 to 26 m/s, at fine increments corresponding to a range of advance ratios between 0.25 and 1. The testing approach was designed to provide a detailed understanding of the influence of inflow turbulence, generated by two turbulence grids, on the aerodynamics and acoustics of overlapped propellers. The results from measuring aerodynamic forces indicate that propellers operating in turbulent flow conditions experience aerodynamic performance reduction, particularly at advance ratios of J>0.46. The findings indicate that while the tonal components of propeller noise remain unaffected up to an advance ratio of J>0.46, beyond this point, tonal noise decreases due to turbulent interactions. The increase in broadband noise is independent of the advance ratio and directivity angles, suggesting a fundamental influence of turbulence on propeller noise characteristics. At lower advance ratios, the difference in noise levels between uniform and turbulent flows is about 2 dB, which widens to as much as 8 dB as advance ratios increase.