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
