Using Multiple Fluids to Represent Spatially Variable Fluid Properties

One way to represent spatially variable properties of a Fluid is to simply create multiple fluids. For example, if the pH varied significantly between two locations, then the solubilities may also change significantly. To represent this, you could simply create a second fluid (e.g., High_pH_Water), and assign it a different set of solubilities. You could then use the Reference Fluid (e.g., Water) in Cells in one portion of your model, and High_pH_Water in the other Cells, as appropriate.

When you create a Fluid and assign it a set of solubilities (instead of partition coefficients), GoldSim automatically computes the partition coefficients as the ratio of the solubilities in the Fluid to the solubilities in the Reference Fluid.

Warning: If you choose to specify solubilities (rather than partition coefficients) for a Fluid, and the Reference Fluid has unlimited (negative) solubilities for some of the species, you must also specify unlimited (negative) solubilities for the same species (and only those species) in the Fluid. If there is an inconsistency in specifying the solubilities (i.e., unlimited solubilities in one fluid but defined solubilities in another), GoldSim will produce a fatal error message when you try to run the model.

In general, however, this approach is not generally recommended, as in this approach, you are effectively defining a partitition coefficient between the two fluids. Although this would be appropriate when considering, for example, two fluids like water and oil, it is likely not to be appropriate when considering two different waters (one with a slightly different chemistry than the other).

To better understand the implications of simulating spatially variable fluid properties in this manner, it is instructive to consider an example in which two Cells with different fluids (having different solubilities and hence a defined partition coefficient) are connected by a diffusive mass flux link. Let us further assume that only a small amount of species mass is present in one of the Cells (such that neither Cell can ever exceed its solubility limit). If we run such a simulation long enough, the system will eventually reach a steady state. At steady state (the time at which there is no net diffusive flux between the Cells), the concentration in the two Cells would be different (with the ratio being equal to the partition coefficient).