# Tracer gas simulation with CFX4.3

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 August 12, 2003, 11:06 Tracer gas simulation with CFX4.3 #1 CFX 4.3 users Guest   Posts: n/a Hello all We are trying to determine age of air for different position using tracer gas experiment. We are using tracer gas pulse injection to our instalation and from the carbon dioxide decay at several points we are calculating the age of air. Now we would like to use analogous CFD simulation for this study. How we need to start?.By the way we have already the geometry file and the CO2 pulse data at the inlet, we need just a command file to determine the decay response at several positions in the room. if some one did it before please help. Thanks for your help!! cfx 4.3 user

 August 14, 2003, 08:17 Re: Tracer gas simulation with CFX4.3 #2 Jon Guest   Posts: n/a 1) Run a steady state run to convergence. 2) Turn off hydrodynamics, add a scalar, diffusivity=0.0, zero value at inlet. 3) Add a source term on all cells su = su + density 4) Run simulation restarted from steady state (hydro off) as a steady state. This will give you the age of the fluid throughout your domain. If you want a decaying species (first order decay), make your source term: su = su + density*decay, where decay is the solution to dc/tc = -kc t=0, c=co t=t0, c=alpha*co c=c0*exp(decay*t) decay=ln(alpha)/t0. Or if you can neglect diffusion, you can apply the above equation for c straight to the water age calc as a post process. Better check the maths (and the sign of decay), as its a while since i did this.

 August 14, 2003, 09:55 Re: Tracer gas simulation with CFX4.3 #3 cfx4.3 users Guest   Posts: n/a Hello Jon thanks for your response, I understood some of the things you have written but not all. 1) what kind of scalar should I add? 2)what is su? and which density, density of air? 3) what is this decay? I mean a is it some numerical value? thank you very much for your help again cfx4.3 user

 August 15, 2003, 05:38 Re: Tracer gas simulation with CFX4.3 #4 Jon Guest   Posts: n/a scalar is a user scalar. su, is a source term. You should set this using USRSRC. Density should be the density of the fluid you are using. Decay comes from the first order DE I wrote last time. You will have to know the half life (or some other decay time) and fit it into the equation I gave.