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 (sCO(2)) over an adiabatic flat plate using direct numerical simulation (DNS). As a non-ideal fluid, sCO(2)'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 sCO(2) 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 sCO(2), but its enthalpy form can accurately predict the enthalpy distribution. The viscosity of sCO(2) displays liquid-like behavior inside the boundary layer. From the turbulent fluctuation behavior view, sCO(2) 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 sCO(2) flow and no major differences are observed in the turbulent kinetic energy budget and velocity fluctuation intensities between sCO(2) 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 sCO(2) in this study, its components behave differently within the boundary layer due to the property variations. (c) 2025 Author(s).