openfoam/applications/solvers/multiphase/reactingEulerFoam/reactingTwoPhaseEulerFoam/write.H

    if (runTime.outputTime())
    {
        volVectorField Ur
        (
            IOobject
            (
                "Ur",
                runTime.timeName(),
                mesh,
                IOobject::NO_READ,
                IOobject::AUTO_WRITE
            ),
            U1 - U2
        );

        runTime.write();
    }