Direct numerical simulation of the supersonic turbulent boundary layer of supercritical carbon dioxide

This paper presents a study of the turbulent boundary layer of supercritical carbon dioxide (sCO2) over an adiabatic flat plate using direct numerical simulation (DNS). As a non-ideal fluid, sCO2's non-ideal behaviors in the turbulent boundary layer are studied by comparing it with a perfect gas air case. Both the mean flow and the turbulent behaviors are investigated. In addition, the skin friction coefficient (Cf) and the dissipation coefficient (Cd) are analyzed due to their significance in engineering applications. The mean flow results reveal that sCO2 has lower temperature variation within the boundary layer than air due to its large Eckert number. By revising Walz's equation, it was found that Walz's equation in its classic form (using temperature ratios) fails to predict the temperature profile for sCO2, but its enthalpy form can accurately predict the enthalpy distribution. The viscosity of sCO2 displays liquid-like behavior inside the boundary layer. From the turbulent fluctuation behavior view, sCO2 boundary layer exhibits lower temperature fluctuations than air. Higher velocity fluctuation intensities are introduced due to local Reynolds number variation. Morkovin's hypothesis is still valid in sCO2 flow and no major differences are observed in the turbulent kinetic energy budget and velocity fluctuation intensities between sCO2 and air. Additionally, the van Driest II transformation for Cf relations is inapplicable for non-ideal compressible fluids, and the property ratio method is suggested as a promising alternative. Although the dissipation coefficient Cd is at a similar level for both air and sCO2 in this study, its components behave differently within the boundary layer due to the property variations.