On the Necessity of Species-specific Internal Energy Resolution in Hypersonic Aerothermoelasticity

Hypersonic flight yields extreme temperatures that drive high-temperature air into complex chemical and thermal processes. Using NASA’s Chemical Equilibrium with Applications (CEA) code for a five-species air model (N2, O2, NO, N, O), we first assume equilibrium to characterize gas behavior at hypersonic conditions. Notably, different species respond very differently, for example, at ∼5000 K and 1 atm, oxygen is almost fully dissociated while nitrogen remains ∼99% intact, indicating essentially uncoupled behavior[1]. In general, translational and rotational modes equilibrate much faster than internal energy excitation or chemical reactions, which relax on much longer timescales[2] . Conventional CFD often simplifies such flows by assuming instantaneous equilibration (effectively an infinite Damköhler number), so that the gas composition adjusts immediately to flow changes[3] . However, in a hypersonic aeroelastic regime, where vehicle structures can dynamically interact with unsteady aerodynamic loads, these differing relaxation times become critical. A lag in each gas species or energy mode reaching equilibrium means the flow cannot respond uniformly to rapid structural motions, introducing phase lags and nonlinear coupling. Such thermochemical nonequilibrium effects can induce unexpected, highly nonlinear behavior in aeroelastic responses, potentially destabilizing the structure or leading to catastrophic failures if ignored. Our findings highlight that relaxation times, rooted in molecular energy transition kinetics, must be accounted for in hypersonic aeroelastic modeling. Capturing these effects may require high-fidelity models (informed by quantum-level energy state considerations[2]) beyond the standard equilibrium approach. Ultimately, this work underlines the necessity of incorporating species-specific and mode-specific relaxation processes to reliably predict hypersonic aerothermoelastic phenomena.