Vortical structures and turbulence characteristics in an isothermal channel flow at supercritical pressure

Due to effect of the variations in the thermo-physical properties, the role of vortical structures in wall-bounded turbulence characteristics at supercritical pressure becomes particularly complex. In this work, we investigate the dynamics of vortical structures in an isothermal turbulent channel flow at supercritical pressure by direct numerical simulation. We find that the streaks near the cold (respectively, hot) wall are strengthened (respectively, weakened), which are associated with the enhanced (respectively, decreased) frequency of bursting events induced by the higher (respectively, lower) mean values of density and viscosity near the cold (respectively, hot) wall. Additionally, the typical hairpin vortex is modulated due to the effect of the baroclinic field and the bursting events near the wall, and accompanied by the generation of the sheet-like, tube-like vortical structures and the wavepackets. Consequently, both the volume fractions of vortex sheets and vortex tubes increase in comparison with the incompressible flow, especially near the cold wall. Further, we confirm that, for the variable property cases, the mechanism underlying the increased contribution of vortex sheets to the enstrophy production in the viscous sublayer is the stronger vortex stretching than the incompressible counterpart. The turbulence anisotropy is found to be increased due to the increase in the mean shear and the overlap between the large energy-containing eddies and the dissipative small scales. For the downward flow with mixed convection, buoyancy mainly weakens the anisotropy in the viscous sublayer near the cold wall due to the larger mean density.