Turbulence anisotropy in fully developed channel flow at supercritical pressure

Fluids at supercritical pressure (SCP) exhibit strong real-fluid effects near the pseudo-critical point. These effects could modulate turbulence anisotropy and pose challenges to existing Reynolds stress turbulence models and turbulent heat transfer correlations empirically calibrated for fluids at atmospheric pressure. This study performs direct numerical simulation of the fully developed channel flow, with an isothermally heated upper wall and a cooled lower wall. The working fluid is CO2 at a SCP of 8 MPa. The primary objective is to explore how real-fluid effects shape turbulence anisotropy at SCP. The findings reveal marked differences in turbulence anisotropy on the hot wall side, where transcritical process occurs, compared to the cold wall side or atmospheric pressure flows. In the viscous layer of the hot wall, anisotropy approaches a more one-component turbulence state than that in the vicinity of the cold wall, while in the log-law region, anisotropy deviates further from axisymmetric turbulence. Detailed Reynolds stress budget analyses show the velocity pressure-strain term plays a key role in redistributing turbulent kinetic energy and driving anisotropy. Real-fluid effects induced by density fluctuations near the hot wall intensify anisotropy and reverse its behavior at increasing wall-normal distances. Vorticity budget analyses further demonstrate that large density fluctuation on the hot wall side enhances torque and increases streamwise vorticity and anisotropy, while the thermal expansion effect reduces spanwise vorticity and anisotropy. Beyond this region, these effects are reversed.