Dense-gas effects on the similarity behavior of turbulent boundary layer flows*

Supercritical carbon dioxide (sCO₂) serves as an efficient working fluid in compact energy systems. However, real-gas effects on the turbulent boundary layers (TBLs) that form in such systems are still under investigation. In this study, direct numerical simulations (DNS) of several sCO₂ zero-pressure-gradient TBLs with isothermal walls are performed. The DNS solves the compressible Navier-Stokes equations and utilizes real-gas equation-of-state and transport models, and the numerical framework is based on high-order compact finite-difference methods without solution filtering. In addition, the wall and freestream temperature conditions vary from subcritical to supercritical across cases. The results show large gradients in transport and thermodynamic properties, such as density, viscosity, and thermal conductivity, for correspondingly small changes in temperatures near the critical condition. The similarity behavior of wall-normal mean profiles and turbulence statistics under real-gas thermodynamic conditions is investigated. Nondimensional streamwise correlations of quantities, such as the skin-friction coefficient and Stanton number, are also studied. Variable-density and compressibility effects are compared with corresponding theories formulated for ideal gas TBLs.