An investigation of the mechanisms of sound generation in initially laminar subsonic jets using the Goldstein acoustic analogy
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The details of the sound sources within two initially-laminar subsonic jets corresponding to different inflow conditions, one fully laminar and the other with nozzle boundary layer forcing, are studied using the Goldstein acoustic analogy. The statistics of the acoustic analogy equivalent sources are generated from Large Eddy Simulations and issues associated with the range of validity of these are explored. The predicted comparative importance of various source terms confirms the results of previous studies with the exception of one extra term that involves the longitudinal and the lateral source components. Agreement within 1dB over the frequency range from St=0.8 to St=6, which was shown to best correspond to the validity range of the acoustic source model for the LES data available, is found between noise predictions using the acoustic analogy model with those of a previously-conducted LES-ILEE control surface method for the nominally-laminar inflow jet case. The acoustic analogy is used to determine the source lengthscales which contribute to noise at different radiated angles to the jet, how they are distributed and how they change with change of inflow conditions. For the vortex pairing frequency, using a new acoustic source decomposition technique based on the correlation length scale, two types of noise sources are found. One source, which corresponds to the peak source amplitude, is located at the upstream end of the jet and is associated with the vortex pairing. The other one, which is the dominant source, is associated with jet mixing at the end of the jet potential core. It is also shown that boundary layer forcing leads to a reduction in the contribution to noise of the large source lengthscales in comparison with the fully laminar case.