THERMOCHEMICAL NONEQUILIBRIUM EFFECTS ON WALL SKIN FRICTION AND WALL HEAT FLUX IN TURBULENT BOUNDARY LAYERS

This study investigates the generation mechanisms of skin friction and heat flux in turbulent boundary layers at a freestream Mach number of 9.74, a unit Reynolds number of 4.9 × 10⁶ m⁻¹, and a wall-to-recovery temperature ratio of 0.135, under both calorically perfect gas and thermochemical non-equilibrium conditions using direct numerical simulation. The wall skin friction and heat flux decomposition method is employed to compare the contributions to the mean wall skin friction, mean heat flux, and mean vibrational heat flux. Results show that under both conditions, the mean skin friction is dominated by the turbulent kinetic energy production and viscous dissipation terms, with negligible influence from thermochemical non-equilibrium. The mean heat flux is jointly determined by four terms: heat conduction, turbulent heat flux, work done by Reynolds stress, and work done by viscous stress. The effects of thermochemical non-equilibrium are mainly manifested in the turbulent heat flux term, which not only converts part of the turbulent translational–rotational temperature flux into turbulent vibrational temperature flux, but also significantly enhances its contribution to the heat flux through turbulent transport of species. The mean wall vibrational heat flux exists only in the non-equilibrium case and is dominated by the vibrational energy source term. Furthermore, quadrant analyses of the shear Reynolds stress and wall-normal turbulent heat flux are performed at the peak locations of their respective integrands for turbulent kinetic energy production and turbulent heat flux. It is found that turbulent kinetic energy production is jointly dominated by ejection and sweep events. The turbulent heat flux is dominated by the sweep of high-temperature fluid in the near-wall region and by ejection in the outer region. The coherent structures associated with these dominant events are pairs of counter-rotating streamwise vortices, which are significantly correlated with the temperature and species fluctuation fields.