if(divDSigmaExpMethod == "standard")
{
    divDSigmaExp = fvc::div
    (
        mu*gradDU.T() + lambda*(I*tr(gradDU)) - (mu + lambda)*gradDU,
        "div(sigma)"
    );
}
else if(divDSigmaExpMethod == "surface")
{
    divDSigmaExp = fvc::div
    (
        mesh.magSf()*
        (
            muf*(n & fvc::interpolate(gradDU.T()))
          + lambdaf*tr(fvc::interpolate(gradDU))*n
          - (muf + lambdaf)*(n & fvc::interpolate(gradDU))
        )
    );

     // divDSigmaExp = fvc::div
     // (
     //     muf*(mesh.Sf() & fvc::interpolate(gradDU.T()))
     //   + lambdaf*(mesh.Sf() & I*fvc::interpolate(tr(gradDU)))
     //   - (muf + lambdaf)*(mesh.Sf() & fvc::interpolate(gradDU))
     //  );
}
else if(divDSigmaExpMethod == "decompose")
{
    surfaceTensorField shearGradDU((I - n*n) & fvc::interpolate(gradDU));

    divDSigmaExp = fvc::div
    (
        mesh.magSf()*
        (
          - (muf + lambdaf)*(fvc::snGrad(DU) & (I - n*n))
          + lambdaf*tr(shearGradDU & (I - n*n))*n
          + muf*(shearGradDU & n)
        )
    );

     // divDSigmaExp = fvc::div
     // (
     //     mesh.magSf()*
     //     (
     //       - (muf + lambdaf)*(fvc::snGrad(DU)&(I - n*n))
     //       + lambdaf*tr(shearGradDU&(I - n*n))*n
     //       + muf*(shearGradDU&n)
     //     )
     // );
}
else if(divDSigmaExpMethod == "laplacian")
{
    divDSigmaExp =
      - fvc::laplacian(mu + lambda, DU, "laplacian(DDU,DU)")
      + fvc::div(mu*gradDU.T() + lambda*(I*tr(gradDU)), "div(sigma)");
}
else
{
    FatalErrorIn(args.executable())
        << "divDSigmaExp method " << divDSigmaExpMethod << " not found!"
        << abort(FatalError);
}