Investigation of Rotating Coherent BPF Noise Sources via the Application of Beamforming and Proper Orthogonal Decomposition

Rotating coherent noise sources are major contributors to the noise of various turbomachinery applications having contra-rotating blade sets. The noise sources are associated with Blade Passing Frequencies (BPF) and interaction tones, which are comprised of the harmonics of the BPFs. The present study examines BPF noise sources through the combined application of two distinct methods in the investigation of the noise of a Counter-Rotating Open Rotor (CROR) aircraft engine. In a first step, in order to localize the noise sources of CROR, acoustic beamforming is performed on a data set of phased array microphone measurements. Then, Proper Orthogonal Decomposition (POD) is used to filter out the dominant features of the beamforming maps as a function of the frequency. Particular emphasis is placed on the POD post-processing in the common-base sense to quantify the impact of the BPF noise. It is found that the POD-based post processing can be utilized to make a connection between the relative energy contributions of BPF turbomachinery noise sources to their spatial noise patterns, furthermore, their impact to the overall noise can be quantified and visualized.