Passive scalar sources can be used to simulate the propagation of species like smoke from a burning car in a garage or the diffusion of dust or pollutants in a tunnel. Another possible application of passive scalars is to model the mean age of a fluid.
Two types of passive scalar sources are currently supported.
This type of source is defined by a flux, which may be interpreted as a concentration per unit time. The user needs to specify the flux value and select the source region.
This type of source is defined by a flux per unit volume. Consequently, the actual flux is implicitly computed using the volume of the source region. As in the first case, the user needs to specify the specific flux value and select the region previously defined.
It is important to understand the correct interpretation of units used for passive scalar sources. Since passive scalars do not affect the dynamics of fluid flow, units for passive scalars are independent to the unit system of simulation. Thus one can define a passive scalar flux as 100 1/s and interpret it as a flux of 100 g/s or 100 kg/s as per one's convenience. However, these interpreted units must be kept consistent throughout the simulation setup (for values specified in initial conditions, boundary conditions etc). The scale of the results will directly correspond to the absolute value of input variables.