A revisit of the tonal noise of small rotors

An experimental investigation of noise characteristics of overlapping propellers

Multiple propellers arranged in overlapping configurations can reduce the sizes of unmanned aerial vehicles for compact architectures. However, the resulting aerodynamic interaction between the propellers could lead to extra noise and calls for extra studies on acoustic physics. In this work, we conducted aeroacoustic tests of two overlapping propellers in an anechoic chamber, focussing on the influence of the axial separation distance and lateral tip spacing at different tip Mach numbers, ranging from 0.18-0.23 and Reynolds numbers ranging from 5.2 × 10⁴ to 6.7 × 10⁴. The results indicate that, compared with the coplanar configurations, the propeller thrusts of the overlapping propellers are deteriorated. The rear propeller under the impingement of the upstream wake flows is significantly affected, leading to the mean value of the thrust being reduced by up to 8.2% and the thrust fluctuation being increased. Consequently, the tonal noise is intensified by up to 10 dB at the low observation angles (referenced to the rotation axis). However, the broadband noise is slightly changed within 3 dB. Also, the lateral tip spacing has a primary impact on the noise characteristics, while the effect of the axial separation distance is secondary.

Efficient computation of broadband noise propagation using Gaussian beam tracing method

The Gaussian beam tracing method has advantages in computing long-distance noise propagation in outdoor environments due to its high efficiency. Usually, repeated computations are required if the target sound wave is broadband or contains multiple frequencies because the method was developed in the frequency domain. This paper presents an efficient computation of broadband noise propagation using Gaussian beam tracing. The approach is based on the fact that the ray behaviors and source properties can be decoupled at high frequencies, where the wave equation is asymptotically solved. Consequently, only a single computation is needed to determine the frequency-independent ray properties, including ray-centered coordinates and beam dynamics. Then sound waves at different frequencies can be simultaneously introduced in a single computation. By separating the processes of determining the ray properties and incorporating the broadband noise source in the numerical implementation, the computational cost can be largely saved. In this work, several benchmark problems are studied, showing that the efficiency could be increased in comparison to the multiple individual computations. A computation acceleration up to 35-fold could be achieved when 200 frequency components are applied. The method is also applied to study the impact of broadband noise due to multiple drone propellers in urban environments. Applications to stochastic broadband problems are also discussed.

On the efficient modeling of generic source directivity in Gaussian beam tracing

For large-distance sound propagation with complex obstacles, the Gaussian beam tracing (GBT) method is often applied. An omnidirectional source model is commonly implemented in GBT, which, however, neglects the influence of generic directivity patterns of practical acoustic problems. There have been efforts to synthesize or reproduce the target directivity pattern over an observation surface by using multiple distributed point sources. However, the efficiency and applicability of general applications still call for improvement. More specifically, rays from each of the point sources and their traces in the space should be computed and superposed to estimate the sound field, making the computational cost largely dependent on the complexity of the source directivity pattern. In this work, a complex-valued radiation function model is developed to realize the generic source directivity for GBT computation. One advantage of the method is that only one source is required such that computation cost can be greatly reduced. Rays are emitted from the source with direction-dependent amplitude and phase to realize the target directivity pattern. The development of the radiation function is associated with the GBT method. The verification cases show that this method can give good agreement with analytical or wave-based numerical solutions. Capabilities of modeling a complex source model of the spinning sound field to mimic the propeller noise are studied, and the result matches well with analytical solutions. Finally, a demonstration case of a four-propeller-powered drone in an urban region is conducted.