  Direct numerical simulations of free round jets at a Reynolds number
($Re_{D}$) of $5000$, based on jet diameter ($D$) and jet-exit bulk velocity
($U_{e}$), are performed to study jet turbulence characteristics at
supercritical pressures. The jet consists of $\mathrm{N_{2}}$ that is injected
into $\mathrm{N_{2}}$ at same temperature. To understand turbulent mixing, a
passive scalar is transported with the flow at unity Schmidt number. Two sets
of inflow conditions that model jets issuing from either a smooth contraction
nozzle (laminar inflow) or a long pipe nozzle (turbulent inflow) are
considered. By changing one parameter at a time, the simulations examine the
jet-flow sensitivity to the thermodynamic condition (characterized in terms of
the compressibility factor ($Z$) and the normalized isothermal
compressibility), inflow condition, and ambient pressure ($p_{\infty}$)
spanning perfect- to real-gas conditions. The inflow affects flow statistics in
the near-field (containing the potential core closure and the transition
region) as well as further downstream (containing fully-developed flow with
self-similar statistics) at both atmospheric and supercritical $p_{\infty}$.
The sensitivity to inflow is larger in the transition region, where the
laminar-inflow jets exhibit dominant coherent structures that produce higher
mean strain rates and higher turbulent kinetic energy than in turbulent-inflow
jets. Decreasing $Z$ at a fixed supercritical $p_{\infty}$ enhances pressure
and density fluctuations (normalized by local mean pressure and density,
respectively), but the effect on velocity fluctuations depends also on local
flow dynamics. When $Z$ is reduced, large mean strain rates in the transition
region of laminar-inflow jets significantly enhance velocity fluctuations
(normalized by local mean velocity) and scalar mixing, whereas the effects are
minimal in jets from turbulent inflow.
