Boundary Conditions for Molecular Flows
The Free Molecular Flow Interface works by computing the incoming molecular flux for each species, G (always a dependent variable), at each element by integrating the outgoing flux, J (available as fmf.J), arriving from all boundaries within the line of sight. Similar integrals with different weightings on J are used to compute the number density, pressure, and net heat flux on the surfaces. These quantities do not normally affect the solution (unless, for example, the heat flux is coupled into a heat transfer computation that in turn effects the surface temperature). Each of the boundary conditions specifies the outgoing flux, J, usually in terms of the incoming flux, G.
Boundary conditions are available to describe a range of surfaces including:
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Walls (including outgassing walls, walls with adsorption and desorption, and walls on which deposition occurs)
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Pumps
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Incoming flux (specified directly by the user with an arbitrary expression or a constant value)
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Molecular effusion from a large, adjacent reservoir and total vacuum (in practice a large chamber with a pressure low enough that molecules entering the domain from the chamber can be neglected)
The Surface Temperature feature can be used to set the surface temperature on different surfaces if the system under consideration is nonisothermal.