if(divSigmaExpMethod == "standard")
  {
    //- calculating the full gradient has good convergence and no high freq oscillations
    divSigmaExp = fvc::div(C && epsilon) - fvc::div(K & gradU);
  }
 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")
   {
     //- causes high freq oscillations and slow convergence
     divSigmaExp = fvc::div(sigma) - fvc::laplacian(K,U);
   }
 else
   {
     FatalError << "divSigmaExp method " << divSigmaExpMethod << " not found!" << endl;
   }