Large eddy simulation to extract fourth order space time turbulence correlations in jets including microjet injection

Jet noise is still a major component of overall aircraft noise emission at take-off, and its reduction is important to sustain the continuing growth of air transport. Computationally expensive Large Eddy Simulations can be used to assess the four-order spatio-temporal correlations so as to provide input and guidance to cheaper jet noise models. Large Eddy Simulations are presented for an isothermal Mach 0.75 jet at a Reynolds number of 1 million with and without microjet injection. The imposition of a numerical boundary layer trip inside the jet nozzle ensures that the shear layer is fully turbulent immediately downstream of the nozzle lip. The eight high-pressure microjets penetrate the shear layer producing streamwise vorticity on the inside of the jet. This dissipates before the end of the potential core and there is no effect on potential core length. The peak turbulence intensity within the shear layer is reduced, with the greatest reduction at locations aligned with the microjet injection points. The shapes of the fourth order correlation envelopes are little changed by the microjets, but there is a significant difference in the absolute magnitudes. Compared to a clean jet all significant correlation terms are reduced, with the reduction still occurring at x/Dj=6.5 where the effect of the microjets on the mean flow has dissipated. This reduction could be used to calibrate a jet noise model in order to take account of the microjets.