RESEARCH

A comprehensive multirotor noise assessment framework is developed to predict the noise of rotational-speed-controlled rotor configurations in real-time. The key objectives are synthesizing the frequency-modulated multirotor noise and analyzing the frequency modulation (FM) characteristics. The framework includes modules associated with flight control, aerodynamics, time reconstruction, noise prediction, and time-frequency analysis (TFA).

The framework is verified through validation and verification studies for diverse rotor configurations and flight conditions. During the cruise flight of the multirotor, the tonal noise exhibits simultaneous frequency and amplitude modulations. In wind gust conditions, these modulations result from rotational speed variations, acoustic wave interference, and Doppler shift. The proposed framework can facilitate noise assessment in the perception-influenced design stage of multirotor configurations by clarifying the non-stationary noise signal in diverse flight environments.

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Multirotor configurations such as sUAV and UAM have been focused on today due to their high maneuverability. The multirotor aerodynamic and aeroacoustic characteristics differ from those of a single rotor. This study uses a numerical analysis based on the free wake vortex lattice method to quantify the wake interaction effect.  

Aerodynamic and aeroacoustic characteristics are significantly affected by the wake interaction. In the hovering flight, the unsteady loading changes periodically, and loading fluctuation increases as decreasing the spacing. It causes variation in the unsteady loading noise and the noise directivity pattern. In the forward flight, the type of formation induces the difference in loading noise. By comparing with a single rotor, multirotor has different directivity patterns according to the location of each rotor. The wake interaction effect becomes a significant factor for aeroacoustic analysis of multirotor configurations.

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The high-wake-resolution method is a novel combination of the truncated vortex tube model for initial and boundary conditions and the wave-number-extended finite-volume interpolation scheme. The wake resolution of this method is verified by comparison with other general computational fluid dynamics techniques. By using this method, the wake dynamics of coaxial rotors were analyzed with different inter-rotor spacing. 

The aerodynamic characteristics of coaxial rotors are compared with those of single-rotor systems to identify the effect of wake interaction on coaxial rotor blades. Variations of thrust and sectional thrust coefficients indicate that the wake of the upper rotor continuously influences the lower rotor. In addition, the blade-vortex interaction and the wake instability phenomena are observed near the lower rotor. Aerodynamics and wake dynamics are strongly correlated with inter-rotor spacing. The high-wake-resolution method can identify details of important features such as the wake trajectory, miss distance of each rotor, blade-vortex interaction phenomenon, and wake instability at different inter-rotor spacing.

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A multirotor drone generally controls its attitude and position through the variation of the angular speed of each rotor, which makes the sound signature from it considerably different from the single rotor. The sound spectrum of rotorcrafts is dominated by the blade passing frequency (BPF) components, determined by the angular speed of the rotor. However, the variation in the angular speed in the multirotor makes it challenging to predict the effects of the multirotor noise.

In this study, the effects of the variation in the angular speed of each rotor on the noise of a hovering multirotor were quantified and predicted through uncertainty quantification and single-rotor stochastic analysis. Multirotor experimental results indicated that the revolution per second (RPS) uncertainty of the multirotor is sensitive to maximum tilt angle and payload. The experimentally quantified angular speed variation is used as input to single rotor stochastic analysis, and the measured aerodynamic and acoustics results show good agreement with the analytically/numerically predicted results. When comparing multirotor noise, the single rotor noise with RPS uncertainty results exhibit similar spectral characteristics, especially near the BPF harmonics. Furthermore, the results reveal that the single rotor stochastic simulation with angular speed uncertainty can predict angular speed variation effects on the multirotor noise in the hovering condition. This study concludes that the sound characteristics of a hovering multirotor can be predicted by considering a single rotor case with RPS uncertainty.

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The complex noise source mechanism can be identified based on the high-order computational aeroacoustics (CAA) with appropriate modelings. I have analyzed flow properties around complex objects using immersed boundary method (IBM) and adaptive mesh refinement (AMR).

Optimized IBM for compressible flow is developed and applied to study the muzzle blast waves regarding noise source generation. The developed algorithm allows oscillation to be minimized by continuous discretization near the solid boundary. Based on IBM techniques, the muzzle blast simulation is performed with complex-shaped moving projectiles. The muzzle blast flow field is analyzed to understand the mechanism of noise source generation. A vortical interaction area is defined where the distinct location of the noise source is observed using acoustic perturbed equations. I concluded that the shape of the projectile is a major parameter in determining the moment of shock-vortex interaction, which is dominantly related to the noise generation mechanism.

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