if(divSigmaExpMethod == "standard")
  {
    //- calculating the full gradient has good convergence and no high freq oscillations
    divSigmaExp =
      fvc::div(
           (C && symm(gradU))
           - (K & gradU),
           "div(sigma)"
           );
  }
 else if(divSigmaExpMethod == "surface")
   {
     //- this form seems to have the best convergence
     divSigmaExp =
       fvc::div(mesh.magSf()*
        (
         (n&(Cf && fvc::interpolate(symm(gradU))))
         - (n&(Kf & fvc::interpolate(gradU)))
         )
        );
   }
 else if(divSigmaExpMethod == "laplacian")
   {
     //- can cause high freq oscillations and slow convergence
     divSigmaExp =
       fvc::div(C && symm(epsilon), "div(sigma)")
       - fvc::laplacian(K,U, "laplacian(K,U)");
   }
 else
   {
     FatalError << "divSigmaExp method " << divSigmaExpMethod << " not found!" << endl;
   }