Extracting engine noise source levels from phased arrays with improved internal source models and evaluation against simulated and measured data

Abstract

Phased arrays are frequently used on outdoor engine test beds, in order to investigate engine source characteristics, identify the location of unexpected sources and to separate out, qualitatively, the contribution of the individual, directional broadband noise sources as a function of far-field angle and frequency (‘source breakdowns’), which cannot be achieved by spectral analysis alone. Quantitative source breakdowns are of much more useful but the required post-processing of the phased array data involves additional assumptions and modelling along with numerical issues which need to be addressed in order to achieve a robust process. A particular type of post-processing based on nonlinear least squares algorithm (NLS) is described called AFINDS which requires a model of the far-field coherence matrix (CSM) for each axially distributed jet exhaust noise source and for each transversely distributed ‘source’ at engine duct inlet/exhaust planes generated by the various internal sources. This paper describes new calculations of the far-field CSM radiated by a compact monopole source distributed over a source plane inside a pipe using a Wiener-Hopf based code and also inside a realistic turbofan intake, using a finite element potential flow code. Comparisons between the two, with and without flow are assessed and compared with previously developed approximate, analytic models. In addition the Wiener-Hopf code is used to generate the CSM for an exhaust flow with the same internal source, which is compared with that of a recently published DNS solution for a turbulent jet containing a nozzle-based source. Based on these comparisons, recommendations are made as to the most appropriate models for the AFINDS process. Finally, the validity and robustness of the AFINDS source breakdown process is demonstrated on simulated data, on DNS jet noise data and on phased array measurements taken from a small engine tested under static conditions.