cell shift

[zsszabo86]

Hello!
So this is what I am reading about TRANSPORT simulations in PHREEQC:
"At each shift, advection is simulated by moving solution cells-1 to cell cells, solution cells-2 to cell cells-1 , and so on, until solution 0 is moved to cell 1 (upwind scheme)."
So my questions is:
Is there any volumetric mixing of the two solutions before the reaction with the rock would be simulated? or i understand correctly, the whole volume of solution-2 in cells-2 just goes to cells-1 where there is no volume of solution-1 left. Can I set on any way the mixing ratio?
Many thanks in advance!
Zsuzsanna

[Pak]

I am not sure, if i get your question right, though I will try to answer.

I think that advection does not introduce any volumetrix mixing, though after advection, (and if there are heterogeneous reactions) the volume of your solution might change since the chemical reactions take place. Afterwords, and only if you have diffusion, there is a mixing of concentrations at each cell, which might also change the volumetric content of the solution.

Phreeqc uses a sequential splitting that follows this sequence: advection + chemistry + diffusion + chemistry.

Look at the manual of equations, there it is well explained.

I hope it is useful.

[dlparkhurst]

Dispersion and diffusion mix the contents of adjacent cells. The amount of mixing is determined by the diffusion coefficient and the dispersivity. If dispersivity is large, then, the dispersive mixing is divided into multuple "mixrun" steps. Half of these steps are done before advection and half after advection. Chemical calculations are done after each dispersive and advective step.

You can define MIX definitions that apply to one or more cells. MIX n will affect cell n. Look at the manual for version 2 and the examples in version 2 and 3 for more information.