Turbulent plane jets are prototypical free shear flows of practical interest in propulsion,
combustion and environmental flows. While considerable experimental research has
been performed on planar jets, very few computational studies exist. To the authors'
knowledge, this is the first computational study of spatially evolving three-dimensional
planar turbulent jets utilizing direct numerical simulation. Jet growth rates as well
as the mean velocity, mean scalar and Reynolds stress profiles compare well with
experimental data. Coherency spectra, vorticity visualization and autospectra are
obtained to identify inferred structures. The development of the initial shear layer
instability, as well as the evolution into the jet column mode downstream is captured
The large- and small-scale anisotropies in the jet are discussed in detail. It is
shown that, while the large scales in the flow field adjust slowly to variations in
the local mean velocity gradients, the small scales adjust rapidly. Near the centreline
of the jet, the small scales of turbulence are more isotropic. The mixing process
is studied through analysis of the probability density functions of a passive scalar.
Immediately after the rollup of vortical structures in the shear layers, the mixing
process is dominated by large-scale engulfing of fluid. However, small-scale mixing
dominates further downstream in the turbulent core of the self-similar region of the
jet and a change from non-marching to marching PDFs is observed. Near the jet
edges, the effects of large-scale engulfing of coflow fluid continue to influence the
PDFs and non-marching type behaviour is observed.