UC Irvine

The present study is related to the field of imaging of aeroacoustic noise sources. Traditional techniques include the use of phased microphone arrays and acoustic beamforming of the signals signals using algorithms such as the Delay-And-Sum (DAS). Over the last years, there has been an increasing interest in methods in which some of the sensors traverse in prescribed paths and motion. Some of the challenges of this approach include the treatment of the non-stationarity of the signal due to the motion of the microphone(s).

An objective of this work is to review the methodology presented by D. Papamoschou, P. Shah and myself in the AIAA Journal "Inverse Acoustic Methodology for Continuous-Scan Phased Arrays" since it provides the building grounds for the thesis. The methodology accounts for the direct estimation of the spatio-spectral distribution of an acoustic source from microphone measurements that include fixed and continuously scanning sensors. The non-stationarity of the signal is addressed by means of the Wigner-Ville spectrum. Suppression of the non-stationary effects involves the division of the signal into blocks and the application of a frequency-dependent window within each block. The direct estimation approach involves the inversion of an integral that relates the modeled pressure field, the measured pressure field and the response of the array. A Bayesian-estimation that allows for efficient inversion of the integrals and performs similarly to the conjugate gradient method is reviewed.

The coherence-based noise source distribution is studied in this work and the influence of the signal segmentation on its spatial resolution is analyzed. This thesis provides specific guidelines related to the signal processing. The signal is divided into blocks meeting a desired mathematical condition. A minimum and maximum size for the resulting blocks is proposed in this work, as well as a minimum and maximum block overlap. A safe region for the signal segmentation is presented as well.

This work presents a methodology to synchronize the signals from the microphones (scanning or not) with the position of the scanning sensor. It also shows the methods to check the accuracy of the position scanning sensor.

The methodology is applied to acoustic fields emitted by impinging jets approximating a point source and an overexpanded supersonic jet. Noise source maps that included the scanning sensor and a dense block distribution have increased spatial resolution and reduced sidelobes. The ability of the continuous scan paradigm to provide high-definition noise source maps with a lower sensor count is confirmed in this work as well. The effect of the proposed signal segmentation on sparse arrays is discussed.