I am solving for a scalar variable which should be equivalent to the fluid age by defining a volumetric additional variable Age with units of seconds and then setting it to zero at my inlet with a volumetric source throughout my domain equal to 1.0.

I'd like to convert my range of scalar ages at the exit to something which resembles a standard exit age distribution, but am not sure how to do so.

Is there some way to accomplish this in CFD Post? - KSVIHLA

The method you use to compute the fluid age is based on the tautology that the substantial derivative of fluid age is 1.0.

For a constant density fluid, you can convert the distribution of scalar ages at the outlet to a distribution using the following approach:

1. In CFD-Post, define an expression for the time value for which you want to know the fraction of fluid at the outlet with an age less than
that time value.
2. Define a new variable in CFD-Post which is defined as step ((Tvalue - Age)/1.0 [s]) where Age is the name of the variable
you used to represent the fluid age and Tvalue is the expression used to set the time value for which you want to compute the
3. Compute the massFlowInt of your new step function variable and divide it by the total mass flow at the exit. As Tvalue ranges from 0
to the maximum value observed at the exit, this fraction will vary from 0 to 1 wnd should be identical to the cumulative age distribution or
F-curve commonly defined in the residence time distribution literature. The computation at various values of Tvalue can be conveniently
accomplished using a Post session file which increments Tvalue over the desired range and computes the desired statistic.

The F-curve can be converted to the standard exit age distribution E curve by numerical differentiation, but this process can introduce
noise into the statistic if the number of intervals are not chosen carefully and the data are not very precise.

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