Microphone arrays for imaging of aerospace noise sources

With the continuous growth in demand for air traffic and wind turbines, the noise emissions they generate are becoming an increasingly important issue. To reduce their noise levels, it is essential to obtain accurate information about all the sound sources present. Phased microphone arrays and acoustic imaging methods allow for the estimation of the location and strength of sound sources. Experiments with these devices are one of the main approaches in the current research in aeroacoustics, along with computational simulations or noise prediction models. This thesis presents a detailed literature review on the most common aerospace noise sources, challenges in aeroacoustic measurements, and the acoustic imaging methods typically used to overcome them. Practical recommendations are provided for selecting the appropriate imaging technique depending on the type of experiment. New integration techniques for distributed sound sources, such as leading– or trailing–edge noise, are proposed in this thesis and are proven to provide the best performance in retrieving the source levels, compared to other well–known methods. In addition, the high–resolution version of the deconvolution method CLEAN–SC, HR–CLEAN–SC, is explained and applied to wind–tunnel measurements. It is confirmed that this method can resolve sound sources at half the frequency associated with the Rayleigh resolution limit, while keeping the inherent advantages of CLEAN–SC. The most appropriate acoustic imaging methods (according to the recommendations from the literature study) were applied to aeroacoustic experiments and compared with other approaches, when possible. Since the landing gear is considered as the dominant airframe noise source in commercial aircraft, this source was analyzed using four different approaches: aircraft flyover measurements under operational conditions, full–scale wind–tunnel experiments, computational simulations and noise prediction models. Strong tonal noise at certain frequencies was observed and suggested the presence of open cavities. Noise prediction models do not account for this behavior and seem to provide erroneous estimates. Eliminating the contribution of the cavity will reduce the noise levels considerably. Trailing–edge noise is considered to be the dominant noise source for modern wind turbines. The performance of the two most promising noise reduction measures was investigated in wind–tunnel experiments. First, trailing–edge serrations featuring different geometries were studied and showed noise reductions of more than 10 dB. In case a serration–flow misalignment angle occurs, the performance of the serrations decreases and they even cause a noise increase after a crossover frequency. Similar results were found with computational simulations. Secondly, trailing–edge porous inserts showed noise reductions of approximately 10 dB at low frequencies and a noise increase after a crossover frequency. It is argued that the reasons for these phenomena were, respectively, the cross–flow between the pressure and suction sides of the airfoil and the increased roughness of the porous material with respect to the solid case. Lastly, the issue of the variability in aircraft noise levels was considered, since it is not properly taken into account by current best practice noise prediction models and hinders the enforcement of environmental laws. It was observed that variations in the fan rotational speed explain a large part of this variability. Two different approaches were proposed for estimating the fan rotational speed of aircraft flyovers based on audio recordings. Implementing these more accurate estimates of this parameter in the noise prediction model (rather than the default values as usual) considerably reduces the errors made and provide more accurate aircraft noise estimates. In conclusion, phased microphone arrays have confirmed their importance for aeroacoustic studies, such as measuring aircraft noise emissions under operational conditions and assessing the performance of noise reduction measures.