  The influence of high-enthalpy effects on hypersonic turbulent boundary
layers is investigated by means of direct numerical simulations (DNS). A
quasi-adiabatic flat-plate air flow at free-stream Mach number equal to 10 is
simulated up to fully-developed turbulent conditions using a five-species,
chemically-reacting model. A companion DNS based on a frozen-chemistry
assumption is also carried out, in order to isolate the effect of finite-rate
chemical reactions and assess their influence on turbulent quantities. In order
to reduce uncertainties associated with turbulence generation at the inlet of
the computational domain, both simulations are initiated in the laminar flow
region and the flow is let to evolve up to the fully turbulent regime. Modal
forcing by means of localized suction and blowing is used to trigger
laminar-to-turbulent transition. The high temperatures reached in the near wall
region including the viscous and buffer sublayers activate significant
dissociation of both oxygen and nitrogen. This modifies in turn the
thermodynamic and transport properties of the reacting mixture, affecting the
first-order statistics of thermodynamic quantities. Due to the endothermic
nature of the chemical reactions in the forward direction, temperature and
density fluctuations in the reacting layer are smaller than in the
frozen-chemistry flow. However, the first- and second-order statistics of the
velocity field are found to be little affected by the chemical reactions under
a scaling that accounts for the modified fluid properties. We also observed
that the Strong Reynolds Analogy (SRA) remains well respected despite the
severe hypersonic conditions and that the computed skin friction coefficient
distributions match well the results of the Renard-Deck decomposition extended
to compressible flows.
