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Andre V. G. Cavalieri
Institut Pprime CNRS - Universite de Poitiers - ENSMA 43, Route de I'Aerodrome 86036 POITIERS CEDEX, France; Laboratorio de Engenharia Aeronautica Instituto Tecnologico de Aeronautica Sao Jose dos Campos, SP, BRAZIL

Peter Jordan
Department of Fluid, Thermal and Combustion Sciences Pprime Institute, CNRS-Universite de Poitiers, ISAE-ENSMA 43 rue de I'Aerodrome, F-86036 Poitiers CEDEX, France

Yves Gervais
Département Fluides, Thermique et Combustion Institut Pprime CEAT, Poitiers, FRANCE

Daniel Rodriguez
Department of Mechanical Engineering California Institute of Technology Pasadena, CA

Timothy Colonius
Division of Engineering and Applied Science California Institute of Technology 1200 E California Blvd, Pasadena, CA, 91125


The acoustic field of Mach 0.4, 0.5 and 0.6 jets, measured using an azimuthal ring array and then decomposed into azimuthal Fourier modes, is found to comprise a superdirective axisymmetric component (exponential decay with radiation angle) for low Strouhal numbers. This is shown to be consistent with an axially non-compact, wave-like source, the marked directivity being a result of axial interference over a source extent that spans several jet diameters. The source is then modelled using parabolised stability equations (PSE) for the axisymmetric mode, the experimental mean velocity field (obtained from measurements with a traversing Pitot) being used as a base flow. The PSE results closely match the velocity data on the jet centerline. Calculation of the axisymmetric mode of the acoustic field using a source term constructed from the PSE modes leads to agreement to within 3dB of the experimental values at low axial angles for Strouhal numbers between 0.3 and 0.9, and for all three Mach numbers, suggesting that linear instability waves constitute the flow mechanism responsible for the said radiation, and that PSE is thus a pertinent reduced-order model that connects fluctuations at the nozzle inlet, via a wave-packet sound-source mechanism, to low-angle sound emission.