Turbulence and fluid-front area production in binary-species, supercritical, transitional mixing layers

Abstract

Databases of transitional states obtained from direct numerical simulations of temporal, supercritical mixing layers for two species systems, O2–H2 and C7H16–N2, are analyzed to elucidate species-specific turbulence aspects and features of fluid disintegration. Although the evolution of all layers is characterized by the formation of high-density-gradient magnitude (HDGM) regions, due to the specified, smaller initial density stratification, the C7H16–N2 layers display higher growth and increased global molecular mixing as well as larger turbulence levels than comparable O2–H2 layers. However, smaller density gradients and lower mass-fraction gradients at the transitional state for the O2–H2 system indicate that on a local basis, the layer exhibits an enhanced mixing, this being attributed to the increased mixture solubility and to mixture near-ideality. These thermodynamic features are found responsible for a larger irreversible entropy production (dissipation) in the O2–H2 compared to the C7H16–N2 layers. The largest O2–H2 dissipation is primarily concentrated in HDGM regions that are distortions of the initial density stratification boundary, whereas the largest C7H16–N2 dissipation is located in HDGM regions resulting from the mixing of the two fluids. To understand fluid disintegration, the area production of a fluid front perpendicular to the mass fraction gradient is calculated in a coordinate system moving with the relative velocity between the front and the flow. On a cross-stream local basis, the C7H16–N2 layers produce more area, and area production increases with smaller perturbation wavelengths combined with larger initial Reynolds numbers. The most active area-producing layer also exhibits the largest probability of having perpendicular vorticity and mass-fraction-gradient vectors. Analysis of the terms in the area production equation shows a large pressure-gradient-term root mean square contribution for the C7H16–N2 layers, due to the coincidence of regions with large magnitudes of pressure gradient with HDGM regions. Such coincidence is attributed to real-gas behavior, which is species-system specific, as the alignment of the pressure gradient and density gradient is similar for both species systems. The alignment of the mass fraction gradient with the strain rate is also species-system dependent. Independent of species system and of the initial conditions, the vorticity is preferentially aligned with the intermediate strain-rate eigendirection, indicating that eddy-viscosity-type models are not adequate for turbulent supercritical mixing.