Analysis of forced subsonic jets using spectral proper orthogonal decomposition and resolvent analysis

Abstract

Various passive and active control strategies have been applied to turbulent jets and have achieved up to about a 5dB reduction in overall sound pressure level. However, the mechanisms by which forcing alters the turbulence and far-field sound are poorly understood. We investigate the effect of forcing by performing large-eddy simulations of turbulent axisymmetric jets subjected to periodic forcing at multiple frequencies and amplitudes. Spectral proper orthogonal decomposition is used to study the effect of the forcing on the linear spectrum. Low-frequency periodic forcing, with St_f = 0.3, while producing highly energetic tonal structures and noise, has a limited effect upon the underlying turbulent spectrum of the jet and the most energetic modes. High levels of forcing, 1% of the jet velocity, are required to achieve a small change to the turbulent mean flow and a minor shift in the turbulent spectrum. The changes in the overall spectrum and the shift in the modes are predicted well via the resolvent analysis performed on the new turbulent mean flow. This shows that the turbulent spectrum stems from the turbulent mean flow and not via interactions between phase-locked structures and the natural turbulence. High-frequency periodic forcing, with St_f = 1.5, is less effective at altering the mean flow field compared to the low-frequency forcing at the same amplitude, but results in a nonlinear interaction, potentially associated with vortex pairing, amplifying the turbulence spectrum at St ~ 0.75.