Noise Sources of Subsonic Round Jets Investigated Using Phased Microphone Arrays

Abstract

To investigate the noise-source properties of a subsonic round jet at low frequencies (0.25 ≤ St ≤ 0.8), two sets of phased-array data studied in the past, one from a mid-field acoustic array and the other from a near-field hydrodynamic array, are post-processed. The acoustic data are processed with a generalized-inverse beam-forming algorithm, capable of resolving multipole distributed sources with high resolution regardless of the coherency. The hydrodynamic data are used to construct disturbances associated with instability waves, and acoustic fields are calculated by imposing these disturbances as near-field boundary values. Source maps produced by L3/2 generalized-inverse beam-forming indicate that acoustic fields upstream of the peak angle are recoverable from detected source distributions, while downstream sound is not representable with superposition up to octupoles even if taking refraction into account. The boundary-value problem demonstrates that the jet-spreading effect, which is included via streamwise growth and decay of the eigenfunctions from linear stability analysis, is too weak to explain the downstream-sound generation. Direct projection from the hydrodynamic data indicates that the downstream sound is governed by disturbances beyond the potential-core end, whose decay rate is gentler and phase velocity is faster than the predictions based on linear stability analysis using PIV mean-flow data.