A study on large coherent structures and noise emission in a turbulent round jet

Abstract

A moderate Reynolds number, and high subsonic turbulent round jet is investigated by large eddy simulation. The detailed results (e.g. mean flow properties, turbulence intensities, etc.) are validated against the experimental data, and special attention is paid to study motions of coherent structures and their contributions to far-field noise. Eulerian methods (e.g. Q-criteria and λ2 criteria) are utilized for visualizing coherent structures directly for instantaneous flow fields, and Lagrangian coherent structures accounting for integral effect are shown via calculating fields of finite time Lyapunov exponents based on bidimensional velocity fields. All visualizations demonstrate that intrusion of three-dimensional vortical structures into jet core occurs intermittently at the end of the potential core, resulting from the breakdown of helical vortex rings in the shear layer. Intermittencies in the shear layer and on the centerline are studied quantitatively, and distinctively different distributions of probability density function are observed. Moreover, the physical sound sources are obtained through a filtering operation of defined sources in Lighthill’s analogy, and their distributions verify that intrusion of vortical structures into the core region serves as important sound sources, in particular for noise at aft angles. The facts that intermittent behaviors are caused by motions of coherent structures and correlated with noise generation imply that to establish reasonable sound sources in active noise production region based on intermittent coherent structures is one of the key issues for far-field noise prediction.