Fast prediction method for multirotor global tonal noise based on acoustic modal analysis

With the popularity of multirotor aircraft, the aerodynamic noise it generates has an increasingly serious impact on the environment. A fast method for globally predicting the tonal noise is proposed in this study. The goal of this method is to predict the global aerodynamic noise at a small computational cost, then providing a fast model for online noise prediction and control. Firstly, the global noise model of a single rotor in acoustic modal domain is established by using the near-field acoustic holography method. Then, the global noise model of a rotor with different configurations is obtained by using the mapping relationship of acoustic modal coefficients. Subsequently, the global noise of multirotor is predicted based on the configuration information of rotors by using the wave superposition principle. To validate the performance of this method, the predicted global noises of single rotor and multi rotors were compared with other numerical methods. It is revealed that the acoustic modal analysis can accurately predict global noise at a small computational cost. Moreover, a parametric study was conducted to evaluate the effect of different configurations on the global noise. The mechanism of phase synchronization method was analyzed comprehensively at the same time. Lastly, experiments were carried out in the anechoic chamber to validate the method proposed. When the separation distance between rotor shafts is greater than 2.8 R, the aerodynamic interference between the rotors can be ignored and the method can be well applied globally. Otherwise, the aerodynamic interference result in predictions that do not match the experiments at higher elevation angles, e.g. 60 degrees. Meanwhile, as the elevation angle decreases, the prediction error decreases rapidly.