Dynamics and Modeling of the Near-Field Pressure Fluctuations of Turbulent Subsonic Jets

Analytical models for installed jet noise often rely on the near-field pressure fluctuation spectrum from either isolated jet experiments or large eddy simulations (LES). To realize predictions independent of prior experimental data or LES results, it is necessary to develop an accurate near-field pressure fluctuation model. It is well-known that near-field pressure fluctuations often take the form of large-scale structures that are connected with instability waves. Despite this, a fully predictive model of these fluctuations remains elusive. In this paper, LES simulations are employed to scrutinize the dynamics of the near-field fluctuations in a natural subsonic jet in the hope of developing a model for the near-field pressure spectrum. First, the Spectral Proper Orthogonal Decomposition (SPOD) is used to dissect the near-field pressure field to examine its modal characteristics. Second, the Parabolized Stability Equations (PSE) is used to calculate the near-field instability waves. Using an amplitude spectrum inferred using data at x/Dj = 4, the Power Spectral Density (PSD) for upstream and downstream locations is predicted using PSE. Results show that the predicted spectra capture the essential features of those obtained in LES, but the predicted amplitudes are either higher or lower for positions upstream and downstream of the reference position, respectively. Further study is needed to examine whether the discrepancies are due to the acoustic signature in the LES data.