CFD Online Discussion Forums - Age of air with function object

Hi,

I have added a function object to calculate the Age Of Air in OF4.1. SimpleFoam converges, but I have some doubt about the interpretation of the results.

I have based the implementation on this thread

https://www.cfd-online.com/Forums/op...n-age-air.html

In my controlDict I have added the following function

Code:

AoA

{

    type    scalarTransport;

    functionObjectLibs ("libutilityFunctionObjects.so");

    enabled true;

    writeControl outputTime;

    log yes;

    nCorr 1;



    field AoA;

    active          true;

    autoSchemes     false;

    resetOnStartUp false;

    DT              1e-5;



    fvOptions

    {

        IncrementTime

        {

            type            scalarSemiImplicitSource;

            active          true;

            scalarSemiImplicitSourceCoeffs

            {

                selectionMode all;

                volumeMode  specific;

                injectionRateSuSp

                {

                    AoA       (1 0);

                }

            }

        }

    }

}

I have then added the following boundary condition

Code:

dimensions [ 0 0 1 0 0 0 0 ];



internalField uniform 0.0;   



boundaryField

{

  inlet

  {

    type fixedValue;

    value uniform 0;

  }

  outlet

  {

    type zeroGradient;

  }

.....

Modified fvSchemes and fvSolution accordingly with

Code:

div(phi,AoA) bounded Gauss upwind;

and

Code:

    AoA

    {

        solver           smoothSolver;

        smoother         GaussSeidel;

        tolerance        1e-08;

        relTol           0.1;

        nsweep                   1;

    }

The solution converges, but I am not really sure I get what it means. My understanding is:

We inject some kind of tracer at each point of the domain
We assign uniform zero as BC
The value of AoA tells us how long, in average, it will take for the air in that point to be replaced by new air coming from the boundary (outside)
This is why we assign uniform 0 at the boundary

Is this correct?
What does IncrementTime actually do?
Why AoA has unit of Time?
Why we inject 1 of tracer?