For learning purposes, how do I create an scalar variable like temperature divided by 2 and outputs it on files generated?
Supposing that your variable is totally dependent of T, so you don't need to read it on "0" file.
On createFields.h add:
Info<< "Creating T2\n" << endl; volScalarField T2 ( IOobject ( "T2", runTime.timeName(), mesh, IOobject::NO_READ, IOobject::AUTO_WRITE ), mesh,dimensionedScalar("T2", dimensionSet(0,0,0,1,0,0,0),0.0 ) );
0.0 inside dimensioned scalar just initializes the variable.
Then you should add the equation to your runTimeLoop: