Influence of a numerical boundary layer trips within LES of a subsonic jet on spatio-temporal correlations

Large eddy simulation is a useful tool for jet noise prediction, and in particular its ability to predict two-point two-time fourth order correlations to guide jet noise modelling is promising. However, many predictions suffer from poor development of the initial jet shear layer and consequent incorrect prediction of critical parameters such as jet potential core length. In this work a simple numerical trip is introduced into the simulation of the convergent part of the upstream nozzle of a subsonic circular free jet with a Reynolds number of 1 million. After an initial decay, realistic turbulence is sustained to the nozzle exit. This then excites a rapid growth in the free shear layer giving a fully turbulent shear layer within 0.5Dj of the nozzle exit plane. When compared to an untripped simulation this gives an accurate prediction of potential core length and realistic growth of turbulent fluctuations along the nozzle lip-line. Flow visualisation and spectra show the untripped case to produce unrealistic vortex ring structures whereas the tripped case has a rapid onset of three-dimensionality. Similar benefits are found when analysing the second and fourth order space-time correlations at the early x/Dj = 1.5 station. The tripped correlations show good agreement with experimental PIV data. A complete map of correlation data is available for jet noise modelling.