Project Home - Simulation Basics
When a particle impacts with a surface energy is transferred in the form of heat and stress, which leads to 2 main types of accommodation coefficients thermal and transverse momentum.
The thermal accommodation coefficient is the fraction of heat transferred between the surface and the molecule. If a particle of 900 K and it impacts the wall of 300 K the wall will heat up and the particle will cool down by a fraction of the 600 K difference. After enough impacts the system reaches a thermal equilibrium. The equation used to use the thermal accommodation coefficients, a, can be seen below.
New Temperature = Old Temperature + alpha (Wall Temperature - Old Temperature)
In the simulation the wall is assumed to be a heat sink meaning that the temperature never changes significantly.
The TMAC is the fraction of the momentum normal to the wall that is transferred to the wall in terms of stress. This stress is more commonly known as pressure. By creating a pressure on the wall some of the vertical momentum is lost. If the particle had a perfect reflection and all of the normal momentum was maintained then that would be a TMAC of 0. If the particle lost all of its normal momentum that would be an TMAC of 1.
The surface roughness, weight of the particle, and oxidation of the wall are big factors when determining the accommodation coefficients. A flash animation of how the simulation deals with the accommodation coefficients is available here.