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### Recommended Iteration Criteria when having solute transport

Posted: Wed Aug 02, 2017 7:32 pm
Hi,

My question: when I have three different solutes to model (with different concentrations) and two of them have nonlinear adsorption parameters, what would be the recommended Iteration Criteria, i.e. Absolute Con. Tol. (and Relative Con. Tol.)? And does it have something to do with maximum time step?

Currently I am using 1E-6 for Absolute (the lowest input concentration is of order 1E-4) and 0.001 for Relative one but after a while I get convergence error saying modify the Iteration Criteria for nonlinear adsorption.

Thanks,

### Re: Recommended Iteration Criteria when having solute transp

Posted: Tue Aug 08, 2017 6:53 pm
The recommended value for Relative tolerance concentration as recommended in the manual is '0.001'. In the case when there are multiple solutes, as I re-read the help, the solution is to specify (very) small value for Absolute T.C. and let the Relative one do the job since the final tolerance is evaluated by the sum of these two criteria. (I hope I'm right about this )

I solved the convergence error by refining the mesh size on the surface (there is evaporation on it).

### Re: Recommended Iteration Criteria when having solute transp

Posted: Wed Aug 09, 2017 4:23 am
You are right.

if(abs(cNew(i)-cTemp(i)).gt.cTolA+cTolR*Conc(jS,i)) lConv=.false.
cTolA- absolute tolerance
cTolR- relative tolerance
Conc - concentration at previous time step
cTemp - concentration at previous iteration at the current time step
cNew - latest concentration

cTolA is there mainly so that the absolute tolerance is not zero (when conc is zero).

J.