High-Temperature Transport Collision Integrals for O₂+N Collisions.

Area of Science:

• * Molecular dynamics simulations
• * Quantum chemistry
• * Aerodynamics

Background:

• * Accurate calculation of transport properties is crucial for modeling high-temperature gas flows.
• * Understanding the influence of molecular vibrations on collision integrals is essential for non-equilibrium conditions.

Purpose of the Study:

• * To compute collision integrals for O2+N collisions using ab initio potential energy surfaces.
• * To investigate the impact of O2 vibrational excitation on transport properties under non-equilibrium conditions.
• * To evaluate the validity of equilibrium assumptions and ground-state approximations for collision integrals.

Main Methods:

• * Quasi-classical trajectory molecular dynamics simulations.
• * High-accuracy ab initio potential energy surfaces for relevant electronic states.
• * State-to-state kinetic theory with Wang-Chang and Uhlenbeck formulation for inelastic contributions.

Main Results:

• * Thermally averaged diffusion and viscosity collision integrals show deviations up to 12% and 17% under high vibrational temperatures (20,000 K).
• * Deviations are less than 5% for moderate non-equilibrium (|T - Tv| ≤ 5000 K), supporting the equilibrium assumption.
• * Approximating vibrational state-specific collision integrals using ground-state values introduces minor errors, simplifying calculations.

Conclusions:

• * The equilibrium assumption is adequate for moderate vibrational non-equilibrium in O2+N collisions.
• * Ground-state approximation for collision integrals offers sufficient accuracy for practical applications, simplifying hypersonic flow simulations.
• * Fitted mean collision integrals provide reparameterized data for widely used collision models.