Hi, I am trying to figure out
 

Hi,
I am trying to figure out what the appropriate boundary conditions are for a diffusive tracer at the outlet of a t mixer. I am using twoLiquidMixingFoam with gamma as the tracer. I know from Fick's law that I want zero gradient boundary conditions at the walls, and fixed value bondaries are fine at the inlet. However, neither of these make physical sense at the outlet. Is their a way to let the value float at this point?

Shawn

The "normal" way would be zero
 

The "normal" way would be zero gradient. The tracer is just convected out of the domain from upstream, anyway (upwind discretisation for the convective flux).

/Ola