Radiation cooling effects on high-temperature nonequilibrium flow for reentry at hypervelocity

<h2>Summary</h2> Radiation from high-temperature (>10⁴ K) air would greatly affect the aerodynamic thermal environment of hypervelocity (∼10 km/s) reentry vehicles. Although its cooling effects have long been a subject of interest, its quantitative mechanism is still unclear. Here, we report a thermochemical nonequilibrium flow-radiation coupling method that resolves this interaction with the S6 discrete ordinates method and Anderson's two-step model. Validated against FIRE-II flight data, the scheme captures the strong redistribution of energy in the shock layer and at the wall. A series of standard sphere cases are simulated with a wide reentry flight profile covering a 50–85 km altitude, Mach 20–45, and a nose radius of 0.03–5.0 m. The results show that radiation cooling intensifies with decreasing altitude, rising Mach number, and larger noses, lowering peak heat loads by up to 70%. The study quantifies how radiation suppresses heating and ionization along critical flight corridors, providing actionable guidance for next-generation thermal-protection design.