On filtering coherent structures in jets and its impact on jet noise

Abstract

This paper examines the impact of removing coherent structures (CS) on jet noise. By applying discrete wavelet transform (DWT) and Fourier transform in azimuth, the jet flow is decomposed into wave packets (WP), coherent eddies (CE), and incoherent structures (IS). It is shown that CS which consist of WP and CE constitute most of the fluctuation energy, and that IS have small coherence length scale and account for almost all the fluctuation energy in the high Strouhal number range. When a similar decomposition is applied to the far field and CS are removed from the low azimuthal modes, a significant sound reduction is obtained compared with the original jet. When viewed in the frequency domain, removal of the CS reduces acoustic energy only at low Strouhal numbers (St, based on jet exit velocity and diameter). The values of the correlations between the CS in the axisymmetric modes of the near and far field are significantly higher than those reported for flow-acoustic correlations of the axisymmetric mode. This suggests a link connecting the near-field and far-field CS, and that most of the low St acoustic energy is reduced by removing WP. It is discussed that the sound reduction obtained by removing CS from the low azimuthal modes of the acoustic field is an over-estimation of what could be achieved for the jet-without WP in the velocity field.