Direct numerical simulation of high-pressure free jets

Understanding turbulent round-jet flow dynamics at supercritical pressures is crucial to controlling fuel-oxidizer mixing and combustion in numerous propulsion systems. To that end, direct numerical simulations (DNS) of round jets are performed to discern the differences between turbulence and mixing characteristics at atmospheric and supercritical conditions. Single-species isothermal jets, with Nitrogen (N₂) injected into N₂ at same temperature, are considered at Reynolds number (Re_{D}), based on jet diameter (D) and jet-exit velocity (U_e), of 5000. To understand mixing characteristics, a passive scalar with unity Schmidt number is transported with the flow. For supercritical conditions, the compressible flow equations with the Peng-Robinson equation of state are solved to examine the influence of thermodynamic compressibility, quantified by the compressibility factor (Z), on jet-flow dynamics. The results show that decreasing Z at a fixed supercritical ambient pressure (p_∞) enhances the pressure and density fluctuations (non-dimensionalized by the local mean pressure and density, respectively), but the effect on velocity fluctuations depends also on local flow dynamics characterized by mean strain rates.