foam-extend4.1-coherent-io/applications/solvers/solidMechanics/elasticAcpSolidFoam/calculateRelativeResidual.H

{
    scalar maxDU = gMax(mag(U.internalField() - U.oldTime().internalField()));

    relativeResidual =
        gMax
        (
            mag(U.internalField() - U.prevIter().internalField())
           /(maxDU + SMALL)
        );

    //resid = (U - U.prevIter())/(max(mag(U-U.oldTime()))+dimensionedScalar("SMALL", dimLength, SMALL));
}