Wall-temperature effects in a turbulent hypersonic boundary layer in chemical non-equilibrium

Hypersonic vehicles operate in extreme environments where intense aerodynamic heating, wall heat transfer, and thermochemical non-equilibrium are tightly coupled. In this regime, wall temperature plays a pivotal role in mediating the interaction between turbulence, thermal transport, and chemical activity, progressively undermining similarity properties derived for adiabatic or low-enthalpy flows. In this work, direct numerical simulations of a spatially evolving, zero-pressure-gradient hypersonic turbulent boundary layer under chemical non-equilibrium are performed to assess wall-temperature effects. The configuration corresponds to a Mach 10 flow characterised by a stagnation enthalpy sufficiently high to activate thermochemical processes. Three isothermal, non-catalytic wall conditions are considered, with wall temperatures in the range $T_w = 1400-2000\:\text{K}$. Wall cooling promotes an earlier transition and a monotonic increase in wall heat flux. Mean velocity profiles collapse satisfactorily under modern compressibility transformations, confirming their applicability to high-enthalpy boundary layers. The thermal field exhibits two distinct modifications with stronger wall cooling: a progressive reduction of the mean temperature profile and an outward shift of the wall-normal position of the temperature peak. These modifications strongly affect both turbulence statistics and chemical activity. By lowering the mean temperature profile, wall cooling suppresses endothermic chemical reactions and reduces the associated energy drain, allowing larger temperature fluctuations to develop. As a result, the concentration of dissociation products decreases by up to a factor of two between the hottest and coldest wall conditions, highlighting the key role of the wall temperature in shaping the thermal field and, in turn, in regulating the chemical activity within the boundary layer.