Comparative study between Cubic and Ellipsoidal Fokker-Planck kinetic models

Motivated by improving the performance of particle-based Monte-Carlo simulations in the transitional regime, Fokker–Planck kinetic models have been devised and studied as approximations of the Boltzmann collision operator. By generalizing the linear drift model, the cubic Fokker–Planck (cubic-FP) and ellipsoidal Fokker–Planck (ES-FP) have been proposed, in order to obtain the correct Prandtl number of 2/3 for a dilute monatomic gas. This study provides a close comparison between both models in low Mach and supersonic settings. While direct simulation Monte-Carlo (DSMC) here serves as the benchmark, overall close performance between cubic-FP, ES-FP, and DSMC is observed. Furthermore, while the ES-FP outperforms the cubic-FP model in the shock region of the supersonic flow around a cylinder, the latter shows a better accuracy in the near continuum regime. It is argued that the reason behind these discrepancies lies in the entropy law besides the transport properties.