In addition to the inputs describing Pipe geometry, Pipes require three additional inputs that control the basic behavior or the Pipe:
Dispersivity: This is the longitudinal dispersivity of the pathway. It has dimensions of length. The longitudinal dispersivity acts to spread or disperse species around the mean travel time as they are transported through the Pipe. Different systems disperse solutes to different degrees. A good first approximation for one-dimensional transport through a relatively homogeneous aquifer might be that the dispersivity is 10% of the total length of the pathway. You must specify a dispersivity which is greater than or equal to zero.
Note that for computational reasons, a minimum dispersivity is applied by GoldSim. The minimum value is a function of the Solution precision (specified in the Contaminant Transport tab of the Options dialog). If the precision is low, the minimum dispersivity is 2.5% of the pathway length; if the precision is medium, the minimum dispersivity is 0.5% of the pathway length; and if the precision is high, the minimum dispersivity is 0.1% of the pathway length.
Hence, if you enter a pathway length of 2000 m, and a dispersivity of 0, and the Solution precision is set to high, GoldSim will actually assume a dispersivity of 20 m (and will write a warning to the Run Log). Because practically no real system would have a dispersivity of less than 0.1% of the pathway length, this should not pose any problems.
Warning: For pathways with
very low dispersivities, it will likely be necessary to adjust the solution
algorithm (i.e., solution precision) in order to obtain accurate results. If the
dispersivity is too low (e.g., signficantly less than 1% of the pathway length),
it may not be possible to obtain accurate results.
Infill Medium: This is the porous medium which fills the mobile zone of the pathway. It must be an existing Solid medium in the model. You can, however, specify that there is no porous medium filling the pathway (e.g., if you were simulating a channel or river) by leaving this field blank.
The porous medium affects the behavior of the Pipe in two ways: 1) it increases the flow velocity (by reducing the effective flow area); and 2) it can act to retard any species which partition onto it.
Fluid Saturation: This is the degree of saturation of the Pipe. It is dimensionless, and must be greater than 0 and less than or equal to 1. If you were simulating a saturated aquifer or a river, you would specify the fluid saturation as 1 (the default). If, however, you were simulating flow through an unsaturated porous medium (e.g., surface soils or a vadose zone), this would be specified as less than 1.
Decreasing the fluid saturation has the same effect as decreasing the porosity of the porous infill medium: 1) it increases the flow velocity (by reducing the effective flow area); and 2) it increases the degree of retardation due to partitioning onto the porous infill and coating media (by reducing the effective volume of fluid present in the pathway relative to these solids).
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