//- set gradU.zz() for plane stress
	  if(rheology.planeStress())
	    {
	      forAll(gradU.internalField(), celli)
		{
		  gradU.internalField()[celli].zz() =
		    (-C.internalField()[celli].xxzz()*epsilon.internalField()[celli].xx()
		     - C.internalField()[celli].yyzz()*epsilon.internalField()[celli].yy())
		    /
		    C.internalField()[celli].zzzz();
		}
	      gradU.correctBoundaryConditions();
	    }