Axisymmetry and azimuthal modes in jet noise

Abstract

This paper will show that the matrix of all noise spectra radiated from a jet in a stationary condition, axisymmetric in construction and boundary conditions, has a special invariant structure which simplifies measurements and enables the total composite noise field to be expressed as a sum of mutually incoherent partial fields. Each such partial field is described in terms of an azimuthal mode with a integer wave number and polarity. The consequences of this structure include a simple explanation of the nature of azimuthal coherence and a practical procedure for estimating the dominant partial fields of the jet from measurements with a limited azimuthal coverage. It should be noted that the conclusions are only drawn from the assumptions of stationarity and axisymmetry, such that both large and fine scale mixing noise and broadband shock associated noise are covered by this theory. This paper also introduces a concept of sound flares as a model for how random fluid dynamic events in a turbulent flow superpose stochastically to produce measured spectra and crossspectra in the linear hydrodynamic and acoustic fields. This concept may be applied to model both axisymmetric as well as noncircular nozzles.

Azimuthal coherence of the sound field in the vicinity of a high performance military aircraft

“…Considering the azimuthal modes, because they are mutually incoherent, a reduced azimuthal field coherence must be due to the mixing of two or more azimuthal modes. 6,8 Several studies have calculated both correlations and coherence in the acoustic field, and have found that azimuthal coherence decreases with increasing frequency. 8,12,13,14,15 Most of the studies involving azimuthal variation in the jet noise field deal with laboratoryscale jets in an anechoic environment, where measurement apertures cover an extended portion of the jet plume.…”

“…6,8 Several studies have calculated both correlations and coherence in the acoustic field, and have found that azimuthal coherence decreases with increasing frequency. 8,12,13,14,15 Most of the studies involving azimuthal variation in the jet noise field deal with laboratoryscale jets in an anechoic environment, where measurement apertures cover an extended portion of the jet plume. The current study takes data from a measurement of a high performance military aircraft 16 and calculates the azimuthal coherence as a function of downstream distance.…”

The separation of radiating and non-radiating near-field pressure fluctuations in supersonic jets

“…Successful applications have been reported both experimentally [12,34,38] and computationally [39,40].…”

Adaptation of the Azimuthal Decomposition Technique to Jet Noise Measuerements of an Aircraft Engine in the Ground Tests

“…In fact, this procedure corresponds to the one described in Ref [39]. Before going any further, note that the method described above allows reconstruction of full time histories of modal signals in the far-field making it possible to perform correlation analysis of azimuthal modes.…”