Meshing
To solve molecular flow problems it is only necessary to define degrees of freedom on the surfaces of the flow domain. Consequently the Free Molecular Flow interface supports two methods for meshing:
•
The entire domain selected in the Free Molecular Flow interface can be meshed.
•
Exterior boundaries of the selected domain can be meshed.
The physics interface does not support a mixture of these two approaches, so, for example, volume meshing only a part of the flow domain and surface meshing the rest can lead to incorrect results.
COMSOL Multiphysics does not return an error message, but in some cases the results generated are incorrect (in a mixed approach, the method used to find the normals that point into the molecular flow domain can fail in certain circumstances).
By default the Free Molecular Flow interface uses constant elements for all the degrees of freedom. In general, using constant elements with a fine mesh provides better accuracy than using higher-order elements on a coarser mesh with the same number of degrees of freedom. It is important to check that solutions obtained do not change significantly (within the desired degree of accuracy) when the mesh is further refined.
The method used by COMSOL Multiphysics to integrate the flux arriving at a particular mesh element can be inaccurate near interior or convex corners. If the results in such regions are important, it can be necessary to use a highly refined mesh and a high integration resolution. See Integration Resolution for further information.