The Free Molecular Flow (fmf) interface (), found under the Rarefied Flow branch () is intended for molecular flows interacting with objects that are moving slowly with respect to the speed of the molecules, such as vacuum systems. Diffuse reflection from all surfaces is assumed (this is reasonable in the majority of practical situations) with molecules from all directions effectively adsorbed onto the surface and subsequently reemitted according to Knudsen’s law (that is, with an intensity that varies as the cosine of the angle of emission to the normal to the surface).

When this physics interface is added, these default nodes are also added to the Model Builder: Molecular Flow, Wall, and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and volume forces. You can also right-click Free Molecular Flow to add physics from the context menu.

The Label is the default physics interface name.

The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.

The default Name (for the first physics interface in the model) is fmf.

Select an Integration method — Hemicube (the default) or Direct area integration. These methods define how the incoming molecular flux, G, is computed at each element.

Hemicube is the default. The more sophisticated and general hemicube method uses a z-buffered projection on the sides of a hemicube (with generalizations to 2D and 1D) to account for shadowing effects. Think of it as rendering digital images of the geometry in five different directions (in 3D; in 2D only three directions are needed), and counting the pixels in each mesh element to evaluate its view factor.

Its accuracy can be influenced by setting the integration resolution of the virtual snapshots. The number of z-buffer pixels on each side of the 3D hemicube equals the specified resolution squared. Thus the time required to evaluate the flux increases quadratically with resolution. In 2D, the number of z-buffer pixels is proportional to the resolution property, and thus the time is, as well.

For an axisymmetric geometry, Gm and Famb must be evaluated in a corresponding 3D geometry obtained by revolving the 2D boundaries about the axis. COMSOL creates this virtual 3D geometry by revolving the 2D boundary mesh into a 3D mesh. You can control the resolution in the azimuthal direction by setting the number of azimuthal sectors, which is the same as the number of elements to a full revolution. Try to balance this number against the mesh resolution in the rz-plane.

In order to solve a free molecular flow it is always necessary to solve for the incident molecular flux, G for each species. The Pressure, Number density, and Heat flux on the surfaces can be optionally computed by performing additional integrations. The Pressure and Number density are computed by default.

Enter the Number of species. The default is 1, but it is possible to add more. It is often convenient to rename the species to something more descriptive for cases when multiple species exist, for example, H2 or SiH4.

The Incident molecular fluxes G (SI unit: 1/(m2s)) dependent variable is always solved for. It represents the incoming flux at the surface (the outgoing flux is available as fmf.J). Other dependent variables might not be solved for, depending on the settings in the Compute section (see above).