AEROACOUSTIC INVESTIGATION OF GRID TURBULENCE INTERACTING WITH ROTORS https://hdl.handle.net/1783.1/130147

The aerodynamic noise of rotors is an environmental constraint in the operation of multi-rotor powered urban air mobility (UAM) vehicles. It is essential to study the rotor noise in turbulent flows that are common in practical flights. In this work, a benchmarking turbulence grid is installed in the jet exit of an open-jet anechoic wind tunnel to generate turbulent flows. The grid-generated turbulence is characterized by the hot-wire anemometry at several downstream locations of the grid. The hot-wire results show that the turbulence intensity decays with the streamwise locations, following a power law of -5/7. The power spectral properties of the grid turbulence are also assessed, indicating a good agreement with the von Kármán turbulence spectrum. Then, the aerodynamic force and noise of a high-speed rotating rotor with a diameter of 217 mm are measured in both clean and turbulent flows. Force measurements show that the thrust and torque coefficients decrease with the advance ratio J, and the coefficient curves collapse well for all the rotational speeds. The rotor induces more torque and power under the grid turbulence. The noise measurements show that the broadband noise above 1,000 Hz can be significantly increased by turbulence, and a difference of more than 10 dB can be observed at J = 0.84. The noise directivity indicates that turbulence interaction noise is more significant at the rotor upstream. The turbulence interaction noise of rotors can impose a significant noise source to surroundings, the physics of which needs to be further investigated for noise reductions of future UAM vehicles.