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High-speed vehicles experience a highly challenging environment in which thefree-stream Mach number and surface temperature greatly influence aerodynamicdrag and heat transfer. The interplay of these two parameters strongly affectsthe near-wall dynamics of high-speed turbulent boundary layers in a non-trivialway, breaking similarity arguments on velocity and temperature fields,typically derived for adiabatic cases. In this work, we present directnumerical simulations of flat-plate zero-pressure-gradient turbulent boundarylayers spanning three free-stream Mach numbers [2,4,6] and four walltemperature conditions (from adiabatic to very cold walls), emphasising thechoice of the diabatic parameter $\mathit{\Theta}$ (Zhang, Bi, Hussain & She,J. Fluid Mech., vol. 739, pp. 392-420) to recover a similar flow organisationat different Mach numbers. We link qualitative observations on flow patterns tofirst- and second-order statistics to explain the strong decoupling oftemperature-velocity fluctuations that occurs at reduced wall temperatures andhigh Mach numbers. For these cases, we find that the mean temperature gradientin the near-wall region can reach such a strong intensity that it promotes theformation of a secondary peak of thermal production in the viscous sublayer,which is in direct contrast with the monotonic behaviour of adiabatic profiles.We propose different physical mechanisms induced by wall-cooling andcompressibility that result in apparently similar flow features, such as ahigher peak in the streamwise velocity turbulence intensity, and distinct ones,such as the separation of turbulent scales.

Assessment of heat transfer and Mach number effects on high-speed turbulent boundary layers