[Mavier]

Hello all.
Currently I am involved in a project that will use floating treatment wetlands (FTW) to treat storm water in the cities of Los Angeles and Houston.

I am in charge of creating the CFD models for these projects. I have used a paper from Downing-Kunz and Stacey to validate my model so that step of the process is taken care of.

I would like to take my simulations a step further and add a First Order decay equation to my porous domains (the FTWs).

The first idea for the source term would was to add the following equation:

Source Term= Co*exp(-K*t)

Where Co = is the initial tracer concentration (kg/m^3 s); K = is our decay constant (1/s) and t = time (s).

The immediate issue with using this equation is that the units are not the same as the source term. The decay equation unit is the unit of concentration (kg/m^3) and the source term unit is (kg/m^3 s).
So my idea was to change the equation for the following with the intention of respecting the source term unit and maintaining the first term a constant:

Source Term = (Co*K)*exp(-K*t)

This equation made possible for me to test the first simulations. Also, since this is a sink term, I put a (-) sign in my source term.
I ran the simulation with this new term and the second problem became apparent, with this term added as a sink I was having the problem that all my concentrations registered at the outlet were negative.

My attempt to fix this issue was to establish a limit to when the Sink Term is “triggered”. So I did a new equation for the Source Term:
Source Term = if(massAve(Tracer)@FTW_1<=0 [kg m^-3],0 [kg m^-3 s^-1],((Cconditioned1)))
Ctracer is defined as the previous Source Term:
Ctracer = (Co*K)*exp(-K*t)

My idea with the source term is to try to do the following, the first part states that if the average mass of Tracer in the FTW_1 is <= 0 (kg/m^3 s) then then the value for the source term will be 0(kg/m^3 s) if not then the source term is equal to (Co*K – Ctracer).
Also, to avoid negative values for the (Co*K – Ctracer) expression, I added the following the Cconditioned1 term:
Cconditioned1= if(Ctracer<=0 [kg m^-3 s^-1],0[kg m^-3 s^-1], Ctracer);
This made the concentration profile at the outlet improve, now there are periods of positive concentration (before, as I said, I was having only negative concentrations) however, when I look at the average concentrations at my FTW faces, I still see negative values (In the link there is a spreadsheet with the concentrations values that I obtained for the channel Outlet and the FTW faces)
This is my first time using the logical expressions as well as using source terms in CFX, so and imput as to what I am doing wrong will be greatly appreciated.

In the link below you can find my spreadsheet, my .cfx and .ccl files

https://www.4shared.com/rar/_9tcwqwv...er_tests.html?

Thanks
Manoel Xavier