Direct numerical simulations of free round jets at a Reynolds number
(ReD) of 5000, based on jet diameter (D) and jet-exit bulk velocity
(Ue), are performed to study jet turbulence characteristics at
supercritical pressures. The jet consists of N2 that is injected
into N2 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∞)
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∞.
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∞ 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.Comment: In pres