[samiahmed]

Hello
The following might be obvious but I cannot really understand how it works.

In fvOptions to create a scalarSemiImplicitSource we have to define some keywords:

volumeMode as either specific or absolute.
In the documentation they state that when choosing absolute, then the units are <quantity> and if specific is used then the units are <quantity>/m^3

Could you please give more details on how this works?

Here is an example that confuses me,
Let's consider the steady heat transfer problem in 1D rod given by the equation:


Where is the thermal diffusivity with units . is the temperature field in kelvins , and is the source term with units .
at we have and at , . The solution is then given by:



For instance, let , , and . and
In OpenFOAM fvOptions:

mySource
{
        type    scalarSemiImplicitSource;
        active  true;

        scalarSemiImplicitSourceCoeffs
        {
                selectionMode   all;
                //cellZone      heatSourceRegion;
                volumeMode              specific;
                injectionRateSuSp
                {
                        T       (100 0);
                }
        }

}

In OpenFOAM I noticed that in order to get the correct results (compared to the analytical solution) I must use volumeMode as specific instead of absolute (using absolute gives me wrong results).

Could you please why this works? because the units of S are not by units of volume!!!!

I don't know exactly what OpenFOAM changes in the equation when I use (specific/absolute)

[clapointe]

Specific vs absolute is used in a variety of places, and converting from one to the other is done by dividing/multiplying by volume. Without even looking at the source code, it is straightforward to see how the magnitude of the source would change. Two (arbitrary) examples :

1 : Volume of 1 cubic meter, with source of 100 K. The absolute source value is 100K and specific source value 100K/m^3

2 : Volume of 0.25 cubic meter, with source of 100K. The absolute source value is still 100K, but specific value is now 400k/m^3.

So the magnitude of the source will change to get the same effect depending on whether absolute or specific is selected.

So for your case I suspect that 100K/m^3 ended up working out, but an absolute value of 100K did not.

Caelan

[samiahmed]

Thank you very much for your reply. I think I got it.