Characterizing Fan Broadband Noise Using Engine Intra-Stage Measurements

The dominant broadband (BB) noise sources in the fan stage are the rotor self-noise at the trailing edges (TE) of the fan blades and the rotor-stator interaction noise at the leading edges (LE) of the stator vanes. The pressure measurements in the engine intra-stage, between the fan (rotor) TE and the outlet guide vanes (OGV) LE, are suitable towards analysing the modal pressure distributions and therefrom characterise the fan BB noise. The BB noise generation mechanisms at the rotor and the OGV are dipole in nature and the corresponding dipole source orientations (relative to the duct axial coordinate) consequently impact the modal pressure distribution characteristics. In the present work, we aim to empirically determine the dipole orientations of the fan BB noise sources by conducting a robust and systematic study using the intra-stage noise data of three different turbofans obtained from the test campaigns of the NASA Fan Noise Source Diagnostic Test (SDT), the TurboNoiseBB, and the Rolls-Royce LSF (Low speed fan for research) across various test conditions. The theoretical component of our investigation approximates the rotor by a rotating dipole and the OGV by a stationary dipole, with the singular dipoles positioned at 2/3rd the annular intra-stage gap, each uni-directionally radiating the downstream modes and the upstream modes, respectively. The effective uni-directional dipole source orientations are determined based on the numerically estimated relative modal pressure distribution characteristics matching the corresponding measured distributions. The findings, validated across the different engines and test conditions, have shown that the rotor can be effectively approximated by a rotating dipole of orientation 〖20〗ᵒ, and the OGV by a stationary dipole of orientation 〖128〗ᵒ relative to the duct axial coordinate. The research presented validates as well as corrects previous studies, and it concludes by suggesting potential avenues for future research in intra-stage BB noise.