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Passive Scalar Sources

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.


  • Passive scalar sources are currently only available in the Incompressible Fluid Flow and Convective Heat Transfer analysis types.
  • The passive scalar source has to be assigned to a volume, defined through either a geometry primitive (cartesian box, sphere and cylinder) or a cell zone.
  • It is generally good practice to refine the mesh in the vicinity of the source, in order to better capture the dynamics of the flow and avoid convergence problems.

Creation of a passive scalar source

  • To create a passive scalar source, first add one or more passive scalar species to the simulation setup from the main simulation panel.
passive species in main simulation panel
Create one or more passive species from the main simulation panel. One species can be assigned to one ore more passive scalar sources.
  • Navigate to Model in the simulation tree to specify the diffusion coefficient of each passive scalar.
  • In the simulation tree navigate to Advanced Concepts and add a Passive Scalar Source.

Types a passive scalar sources

Two types of passive scalar sources are currently supported.

Passive scalar source

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.

Volumetric passive scalar source

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.

passive scalar source setup in SimScale
In the Figure above, a passive scalar source is defined using a sphere as geometry primitive (highlighted in blue).


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.

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