Measurement and analysis of phased array data on haystacked tones from a source located in a free jet

Abstract

Significant spectral broadening of tone noise by jet shear-layer turbulence can occur if the tone frequency and the path length through the shear layer are sufficiently large. A typical example of spectral broadening or ‘tone haystacking’ are turbine aero-engine tones which exhibit little or no haystacking when measured inside the engine but are sometimes so strongly haystacked in the far-field that the original tone cannot be observed in the measured spectrum. This haystacking of turbine tones requires an engineering method that can predict both the far-field haystacking and the associated reduction in incident tone level. A previous published method, the so-called ‘beta’ correlation has displayed encouraging agreement with a number of experimental datasets but significant disagreement with others. In this paper another set of experimental data, which includes phased array data, is analysed. The objective is to improve our understanding of the tone haystacking mechanisms with the aid of beamforming and to compare with a prediction from weak scattering theory, in the form of a simple Doppler shift relationship between frequency and angle scattering, assuming a frozen turbulence model. The experimental data acquired by QinetiQ and CLEAN-SC beamformer results described herein begin to provide validation of that prediction, which may lead to resolution of the discrepancies previously observed between the beta correlation and the data acquired under a GARTEUR project.