Domain and Boundary Nodes for the Transitional Flow Interface
The Transitional Flow Interface has these domain and boundary nodes available from the Physics ribbon toolbar (Windows users), Physics context menu (Mac or Linux users), or by right-clicking the main physics interface node to access the context menu (all users).
In general, to add a node, go to the Physics toolbar, no matter what operating system you are using. Subnodes are available by clicking the parent node and selecting it from the Attributes menu.
Domain
•
Flow Properties
•
Initial Values
Boundary
Across the full range of Knudsen number, Kn:
•
Wall
•
Continuity on Interior Boundary
In the Navier–Stokes limit (Kn<0.1):
•
Inlet
•
Outlet
These boundary conditions are functionally equivalent to the Inlet and Outlet conditions for single-phase laminar flows (see The Single-Phase Flow, Laminar Flow Interface in the COMSOL Multiphysics Reference Manual). The mathematical form of the boundary conditions in the two different interfaces is, however, very different. See the Theory for the Transitional Flow Interface for more details.
In the Free Molecular Flow Limit (Kn>10):
•
High Vacuum Pump
•
Outgassing Wall
•
Reservoir
•
Diffuse Flux
•
Total Vacuum
Although the conditions on the incident and outgoing fluxes are equivalent, the method for computing the incident flux and for setting the outgoing flux differs significantly between the two interfaces. See Theory for the Free Molecular Flow Interface and Theory for the Transitional Flow Interface for more detail.
On all the boundaries of the Transitional Flow interface, a number of physical variables are defined. These include:
•
tran.G — incident flux
•
tran.J — emitted flux
•
tran.Nin — incident number density
•
tran.Nout — emitted number density
•
tran.n — total local number density (inherited from domains)
•
tran.Pin — pressure due to incident molecules
•
tran.Pout — pressure due to emitted molecules
•
tran.p — total pressure
The pressure is not defined in the domain as a scalar because it is a tensor quantity for a rarefied flow. Also note that if a boundary is parallel to one of the lattice velocities, the sum of the incident and emitted number densities can differ from the total local number density due to molecules traveling parallel to the boundary.