if(divSigmaExpMethod == "standard")
{
    divSigmaExp = fvc::div
      (
       mu*gradU.T() + lambda*(I*tr(gradU)) - (mu + lambda)*gradU,
       "div(sigma)"
       )
      - gradThreeKalphaDeltaT;
}
else if(divSigmaExpMethod == "surface")
{
     divSigmaExp = fvc::div
       (
    muf*(mesh.Sf() & fvc::interpolate(gradU.T()))
    + lambdaf*(mesh.Sf() & I*fvc::interpolate(tr(gradU)))
    - (muf + lambdaf)*(mesh.Sf() & fvc::interpolate(gradU))
    - threeKalphaDeltaTf
    );
}
else if(divSigmaExpMethod == "decompose")
{
    snGradU = fvc::snGrad(U);

    surfaceTensorField shearGradU =
        ((I - n*n)&fvc::interpolate(gradU));

    divSigmaExp = fvc::div
    (
        (
            mesh.magSf()*
            (
                - (muf + lambdaf)*(snGradU & (I - n*n))
                + lambdaf*tr(shearGradU & (I - n*n))*n
                + muf*(shearGradU & n)
            )
        )
      - threeKalphaDeltaTf
    );
}
/* else if(divSigmaExpMethod == "expLaplacian")
   {
     divSigmaExp =
       - fvc::laplacian(mu + lambda, U, "laplacian(DU,U)")
       + fvc::div
       (
    mu*gradU.T()
    + lambda*(I*tr(gradU)),
    "div(sigma)"
    );
    }*/
else
{
    FatalErrorIn(args.executable())
        << "divSigmaExp method " << divSigmaExpMethod << " not found!" << endl;
}