A coherence-matched linear source mechanism for subsonic jet noise

Abstract

We investigate source mechanisms for subsonic jet noise using experimentally obtained data-sets of high Reynolds number, Mach 0.4 and 0.6 turbulent jets. The focus is on the axisymmetric mode which dominates downstream sound radiation for low polar an- gles and the frequency range at which peak noise occurs. A Linearized Euler Equation (LEE) solver with an inflow boundary condition is used to generate single-frequency hydrodynamic instability waves and the resulting near-field fluctuations and far-field acoustics are compared with those from experiments and Linear Parabolized Stability Equations (LPSE) computations. It is found that near-field velocity fluctuations closely agree with experiments and LPSE up to the end of the potential core, downstream of which deviations occur but LEE results match experiments better than LPSE results. Both the near-field wave packets and the sound field are observed directly from LEE computations, but the far-field sound pressure levels obtained are more than an order of magnitude lower than experimental values despite close statistical agreement of the near hydrodynamic field upto the potential core region. We explore the possibility that this discrepancy is due to the mismatch between the decay of two-point coherence with increasing distance in experimental flow fluctuations and the perfect coherence in linear models. To match the near-field coherence, experimentally obtained coherence profiles are imposed on the two-point cross-spectral density (CSD) at cylindrical and conical surfaces which enclose near-field structures generated with LEE. The surface pressure is propagated to the far-field using boundary value formulations based on the linear wave equation. Coherence-matching yields far-field sound pressure levels which show improved agreement with experimental results, indicating that coherence-decay is the main missing component in linear models. The CSD on the enclosing surfaces reveals that applying a decaying coherence profile spreads the hydrodynamic component of the linear wave packet source on to acoustic wavenumbers, resulting in a more efficient acoustic source.

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