Radial mode analysis of broadband noise in flow ducts using azimuthal sensor array

Abstract

For the evaluation and improvement of noise reduction notions and the verification of broadband sound power measurement in flow ducts, it is interesting to obtain the detailed information of the in-duct acoustic mode spectrum and subsequent broadband noise sources separation. A new broadband noise mode analysis method, which uses full wall-mounted sensor array, was experimentally applied on broadband sound fields at three operation conditions, which were generated by means of high loaded single-stage axial flow fan test rig. Two axial sensor arrays were mounted wall-flush upstream of the fan. Measurements were made at operating conditions from 60% to 100% rotor design speed. On the whole, the new method behaves robustly. It delivers physically meaningful broadband mode amplitudes. Mode coherence functions are calculated between all pairs of propagating modes. This feature enables the detailed comparison of different sound fields with characteristically coupled mode pairs. For tonal noise, mode coherence results show that modes are correlated with the source and with each other, especially at blade passing frequencies. The experimental outcome proves the usefulness of the analysis technique for interpreting andunderstanding broadband sound propagation in turbomachinery flow ducts.