AN APPROXIMATE THEORETICAL METHOD FOR DETERMINING KEY ENERGY PARAMETERS IN THE MODIFIED CLL MODEL

In the very low Earth orbit (VLEO) environment, free molecular flow characteristics make aerodynamic drag prediction highly dependent on accurate gas-surface interaction modeling. Due to the simple expression of coupled momentum and energy accommodation coefficients, the traditional Cercignani–Lampis–Lord (CLL) model exhibits inherent limitations in characterizing the non-equilibrium scattering of gas molecules from solid surfaces under high-energy incoming flows in VLEO. These deficiencies significantly restrict the model’s applicability for high-fidelity aerodynamic analysis requirements. To address this issue, this paper derives a generalized expression for the energy coefficient in the CLL model based on the temperature equivalence principle, which combines approximate solutions for tangential and normal reflection temperatures of the CLL model with the theoretical reflection temperature of free molecular flow. This leads to an improved CLL model that relies only on two undetermined parameters: the tangential and normal momentum accommodation coefficients. To verify the improved model, molecular dynamics (MD) simulations are employed to model the gas-surface interactions between nitrogen and a smooth platinum surface, covering an incident temperature range of 300-2300 K and incident velocity ranges of 0-10000 m/s. The results indicate that macroscopic kinetic energy input in any direction can alter scattering behavior in other directions. This cross-directional coupling causes local failure of the standard CLL model, which features independent tangential and normal scattering kernels. By introducing the influence of global information into the definition of the new energy coefficient, the improved CLL model yields predictions of scattering velocity distributions that are consistent with MD simulations under conditions such as macroscopic kinetic energy inputs within a certain range and large gas-solid temperature differences. Although the accuracy of the improved CLL model begins to decline when the velocity in single direction exceeds 5000 m/s, its predicted scattering velocity distribution remains significantly superior to that of the traditional CLL model.