Large-eddy simulation and modal analysis of low-frequency jet physics

Advances in high-performance computing have enabled high-fidelity simulations of turbulent jets. To alleviate the cost of such simulations, the jets are typically truncated in the streamwise direction just tens of nozzle diameters from the nozzle exit. These simulations can therefore only capture flow structures that are supported within their truncated domains. In contrast, coherent structures that oscillate at low frequencies exist over large spatial scales, which can exceed the boundaries of finite domains. They are thus inaccurately captured at best, completely discarded at worst. This is at odds with past experimental observations showing that low-frequency structures in fact possess significant energy. Linear stability analyses have also demonstrated the effects of domain truncation on the calculation of jet instabilities. To accurately simulate low-frequency jet dynamics, in this work we extend an experimentally validated large-eddy simulation of a subsonic turbulent round jet. We elongate the original domain length from 30 to 100 nozzle diameters, and build a long-time database for statistical analysis with spectral proper orthogonal decomposition, focusing on the eduction of low-frequency coherent structures.