Reservoir
The Reservoir boundary condition is appropriate in the molecular flow limit (Knudsen number, Kn>10) when a boundary represents a stationary opening to a large adjacent vessel or region filled with gas at a specified number density or pressure. In this case the effusing flux obeys Knudsen’s law.
Reservoir
Select the Boundary condition: Reservoir pressure (the default) or Reservoir number density.
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For Reservoir pressure enter a value or expression for the Reservoir pressure p0 (SI unit: Pa).
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For Reservoir number density enter the Reservoir number density n0 (SI unit: 1/(m3)).
The Reservoir boundary condition represents an opening to an adjacent chamber whose extent is much larger than the molecular mean free path. The boundary condition assumes that the molecules in the reservoir have a Maxwellian distribution of velocities and that the size of the opening to the reservoir is much less than the mean free path. Molecules travel through the opening by a process known as molecular effusion. Effusion is a process that selects molecules with higher molecular speeds than average because they are traveling toward the hole faster than the other molecules. Effusing molecules therefore have a larger average velocity than the molecules in the adjacent reservoir. As a result the pressure or number density at the reservoir boundary can differ from the value set in the boundary condition — which determines the pressure that would be measured within the reservoir away from the opening.