AbstractThere are many practical situations when jets are emanating from non‐axis‐symmetric apertures, yet numerical simulations of such three‐dimensional jets are scarce and most of them have failed to reproduce some of the unique flow features. Examples of this type of jets are gas leaks from flanges. These can be treated as jets issuing from high aspect ratio rectangular orifices. The present work consists of a series of large eddy simulations typifying such jets using different inflow boundary conditions. Good agreement with available experiments was observed provided appropriate boundary conditions were present.A discrete method for formulating turbulence data with a known energy spectrum for an inflow condition is outlined and evaluated with three other inflow conditions–a steady uniform profile, a steady parabolic profile and pseudo‐random noise. The implementation of the new inlet condition results in a more realistic centreline velocity decay where the division between the end of the potential core region and the start of the characteristic decay region is clearly visible. Large velocity oscillations are also observed in the final quarter of the domain (15–20 slot widths downstream). Similar oscillations have been observed in real jets. Off‐centre mean velocity peaks are present along the major axis 10 slot widths downstream of the exit in all the simulations. The peaks are approximately 3% of the centreline velocity. The presence of the off‐centre peaks are proved to be independent of jet inflow boundary conditions and an explanation for the mechanism causing the off‐centre peaks is given. Copyright © 2002 John Wiley & Sons, Ltd.