Experimental study of the turbulence ingestion noise of rotor blades

The ingestion of turbulence can cause additional noise sources of rotor blades, which should be considered for multirotor powered urban air mobility vehicles encountering atmospheric turbulence. In this work, a turbulence grid was installed in the exit of an open-jet anechoic wind tunnel to generate turbulent flows. The grid turbulence was characterized using hot-wire anemometry, showing that turbulence intensity decays with the streamwise locations downstream of the grid, following a power law of -5/7. The power spectral properties of the grid turbulence were also assessed, and it agrees well with the von Kármán turbulence spectrum in the inertial subrange. Then, the aerodynamic force and noise of a rotor with a diameter of 217.2 mm were measured under both clean and turbulent flows. Force measurements show that the thrust and torque coefficients decrease with the advance ratio J. Noise measurements show that the tonal noise at the blade pass frequency (BPF) is more significant at the upstream locations under high advance ratios, and high-order BPF harmonics can also be amplified. Moreover, the turbulence ingestion noise mainly dominates the broadband contents from 10 to 50 BPF harmonics. The broadband noise can be scaled by the Mach number scaling of M∞2Mc4, where M∞ is the freestream Mach number and Mc is the corresponding Mach number of the rotating speed at the blade tip.