Mass Sources for Species Transport
There are two types of mass sources in the Transport of Diluted Species interface: point sources and line sources.
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Point Source
A point source is theoretically formed by assuming a mass injection/ejection, (SI unit: mol/(m3·s)), in a small volume δV and then letting the size of the volume tend to zero while keeping the total mass flux constant. Given a point source strength, (SI unit: mol/s), this can be expressed as
(5-5)
An alternative way to form a point source is to assume that mass is injected/extracted through the surface of a small object. Letting the object surface area tend to zero while keeping the mass flux constant results in the same point source. For this alternative approach, effects resulting from the physical object’s volume need to be neglected.
The weak contribution
is added at a point in the geometry. As can be seen from Equation 5-5, must tend to plus or minus infinity as δV tends to zero. This means that in theory the concentration also tends to plus or minus infinity.
Observe that “point” refers to the physical representation of the source. A point source can therefore only be added to points in 3D components and to points on the symmetry axis in 2D axisymmetry components. Other geometrical points in 2D components represent physical lines.
The finite element representation of Equation 5-5 corresponds to a finite concentration at a point with the effect of the point source spread out over a region around the point. The size of the region depends on the mesh and on the strength of the source. A finer mesh gives a smaller affected region but also a more extreme concentration value. It is important not to mesh too finely around a point source since this can result in unphysical concentration values. It can also have a negative effect on the condition number for the equation system.
Line Source
A line source can theoretically be formed by assuming a source of strength (SI unit: mol/(m3·s)), located within a tube with cross section δS and then letting δS tend to zero while keeping the total mass flux per unit length constant. Given a line source strength, (SI unit: mol/(m·s)), this can be expressed as
(5-6)
As in the point source case, an alternative approach is to assume that mass is injected/extracted through the surface of a small object. This results in the same mass source, but requires that effects resulting from the physical object’s volume are neglected.
The weak contribution
is added on lines in 3D or at points in 2D (which represent cut-through views of lines). Line sources can also be added on the axisymmetry line in 2D axisymmetry components. It cannot, however, be added on geometrical lines in 2D since those represent physical planes.
As with a point source, it is important not to mesh too finely around the line source.
For feature node information, see Line Mass Source and Point Mass Source in the COMSOL Multiphysics Reference Manual.