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한국항공대학교 항공 모빌리티 및 공력음향학 연구실
Summary
Background
Electric vertical take-off and landing (eVTOL) type multirotor configurations
Urban air mobility (UAM) is a promising opportunity for the aviation industry.
The noise impact is a critical consideration for the commercialization of UAM transport systems.
UAM noise assessment is challenging as the acoustic signatures are unique.
Rotor noise is a dominant noise source in distributed electric propulsion (DEP) systems
RPM-based control can be applied to DEP systems.
Rotor noise transformed to Frequency-modulated noise due to RPM variations: Frequency modulation (FM).
Time-frequency analysis (TFA) of the eVTOL noise
To analyze a strongly non-stationary signal, high-resolution TFA techniques are required.
Objective
I developed a comprehensive multirotor noise assessment (CONA) framework for the real-time noise prediction of RPM-controlled multirotor configurations. The framework was based on a six-degree-of-freedom flight simulation environment. The trajectory tracking control algorithm was used for flight control, and an aerodynamic analysis was performed using the hybrid blade element momentum (HBEM) model and the Beddoes wake model. Additional aerodynamic models and a time reconstruction module were applied as a preprocessor for noise analysis. The source-time dominant and convective Ffowcs Williams and Hawkings (FW-H) acoustic analogy was used for the tonal noise prediction. After real-time noise prediction and auralization, the characteristics of frequency-modulated noise were identified through high-resolution TFA.
Contribution
Development of the comprehensive multirotor noise assessment framework
Noise assessment in diverse environmental conditions
Introductions of high-resolution TFA for multirotor noise
[Noise sources in UAM configurations]
Method
[CONA framework]
Real-Time Prediction Framework
The CONA framework is composed of many modules, including those for flight control, aerodynamics, time reconstruction, noise prediction, and TFA. For the real-time flight simulation, the flight control and aerodynamics modules receive flight settings such as the mission profile, rotor configurations, and wind conditions. Both modules operate simultaneously to identify the trim conditions for tracking the mission profile of RPM-controlled rotors. For the noise prediction module, all the flight variables are reorganized in the time reconstruction module. Because the tonal noise of RPM-controlled rotors is a non-stationary signal, the TFA module performs a high-resolution analysis for the synthesized noise signal. The framework was constructed by selecting the optimal techniques required for each module.
In the verification and validation studies of the CONA framework, different rotor scales (UAV and UAM) and flight conditions (hovering and forward flight) were considered to examine the broad range of applications. For the aerodynamic analysis, HBEM and the linear inflow model were used. The Beddoes wake model was applied when the wake interaction effects were not negligible in noise prediction, especially in the multirotor verification studies. Moreover, the flight control module of the entire system was verified.
Results
Mission profile and flight control
A Mission profile includes vertical flight, acceleration, deceleration, and 10 m/s cruise flight
Flight vehicle: DJI F450 cross-type quadrotor
Noise analysis was conducted in the cruise flight (9 to 14 s)
Wind-on condition: mean flow of 3 m/s in the flight direction with velocity perturbation (Tail wind)
The linear inflow model was used to ensure computational efficiency.
RPM trim conditions for mission profile
During cruise flight (9 to 14 s), rotational speeds of the front and rear rotors were different for trim conditions.
In vertical ascending and descending, the rotational speeds differed for attitude control in the wind-on condition.
The RPM change in long periods for tracking the trajectory and in short periods for control occurred.
[Schematic of quadrotor trajectory]
[RPM signal along the mission profile (wind-on condition)]
Wind effects on frequency-modulated noise: Wind-off condition
Spectrogram: tonal noise occurred up to high BPF harmonics.
As the flight vehicle advanced, constructive or destructive interference periodically occurred: AM characteristics.
Because of Doppler shifting, noise frequency continuously changed in stationary RPM sets: FM characteristics.
[TFA at the wind-off condition (noise time signal, spectrogram, and IMSST)]
Wind effects on frequency-modulated noise: Wind-on condition
Spectrogram: the BPF harmonics were not clearly observed because RPM fluctuation occurred in short periods.
The phase effects were different because the RPM of individual rotors continuously fluctuated.
Aperiodic constructive or destructive interference occurred.
In 2nd BPF noise, the phase effects were less intense because of the effects of aerodynamic distribution.
[TFA at the wind-on condition (noise time signal, spectrogram, and IMSST)]
Conclusion
The CONA framework was developed for real-time prediction of frequency-modulated multirotor noise. RPM-controlled rotor configurations exhibit unique acoustic signatures because of the continuous variation in the rotational speed due to the flight control. The highly non-stationary time signal of rotor noise, especially in complex maneuvers and wind conditions, should be clarified for the noise assessment. The proposed framework can synthesize the frequency-modulated noise and analyze the tonal noise characteristics through the high-resolution TFA. In a realistic flight environment, the multirotor configurations exhibit FM and AM characteristics that can be attributed to rotational speed variations, acoustic wave interference, and Doppler shifting. The framework is developed by adopting optimal techniques in tonal noise prediction. The proposed framework can facilitate noise assessment in diverse flight environments for the perception-influenced design stage of multirotor configurations.
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