Coherent structures and wavepackets in subsonic transitional turbulent jets

A large eddy simulation(LES) is performed for two subsonic jets with a Reynolds number of Re = 105,which have different core temperatures, i.e., the cold and hot jet. The far-field overall sound pressure levels(OASPL)and noise spectra are well validated against previous experimental results. It is found that the OASPL is raised by heating at shallow angles. The most energetic coherent structures are extracted with specified frequencies using the filter based on the frequency domain variant of the snapshot method of proper orthogonal decomposition(POD). The m = 0, 1 modes have high coherence of near-field pressure for both jets, while the coherence of m = 0 modes is enhanced greatly by heating. Based on the coherent structures, spatial wavepackets are educed and the characteristics of growth, saturation and decay are analyzed and compared between the two jets in detail. The results show that heating would enhance the linear growth rate for high frequency components, and nonlinear growth rates for low frequency components in general, which are responsible for higher OASPL in the hot jet. The far-field sound generated by wavepackets is computed using the Kirchhoff extrapolation,which matches well with that of LES at shallow angles.This indicates that the wavepackets associated with coherent structures are dominant sound sources in forced transitional turbulent jets. Additionally, the present POD method is proven to be a robust tool to extract the salient features of the wavepackets in turbulent flows.