Influences of freestream referenced friction Reynolds number on hypersonic boundary layer turbulence

Compressible turbulent boundary layers are known to have high Reynolds number features despite friction Reynolds numbers that are often amenable to DNS. This is the result of fluid property variations across the layer which can, at least partially, be accounted for using a semi-local inner length and velocity scale. These scales define a new friction Reynolds number at the edge of the layer based on freestream fluid properties, Re*, that can be approximately 5x10⁵ for a Mach 12 adiabatic boundary layer, providing an opportunity to explore the high Reynolds number features of these boundary layers. We utilize a broad DNS database covering a wide range of Mach numbers and heat transfer and approximately constant friction Reynolds number to demonstrate the near-wall streamwise stress peak grows in an approximately log-linear fashion with Re*, in a similar manner to that seen for incompressible flows, and that deviations from the incompressible curve appear to be the result of heat transfer. Similarly, we explore the magnitude of stress terms involving viscosity and density fluctuations, that similarly grow with Re*. These results suggest that highly compressible boundary layers may aid the determination of very high Reynolds number trends in the near-wall stress peak.