//- write force displacement to file

label topPatchID = mesh.boundaryMesh().findPatchID("top");
if(topPatchID == -1)
  {
    Warning << "Cannot find patch top for calculating force" << nl
	    << "Not writing to force file" << endl;
  }
else
  {
    //- calculate force in specified direction on topClamp patch
    vector direction(0, 1, 0);
    
    //- for small strain or moving mesh
    // scalar topForce = gSum(
    // 			direction &
    // 			(mesh.boundary()[topPatchID].Sf() & sigma.boundaryField()[topPatchID])
    // 			);
    
    //- for large strain total lagrangian
    tensorField F = I + gradU.boundaryField()[topPatchID];
    vectorField totalForce = mesh.Sf().boundaryField()[topPatchID] & (sigma.boundaryField()[topPatchID] & F);
    
    vector force = sum( totalForce );
    
    scalar topForce = force[vector::Y];;
    
    //- patchIntegrate utility integrates it this way but this is worng because the sigma tensor should
    //- be dotted with the surface normal to give the actual traction/force
    //- you cannot just take the component of the sigma tensor
    //scalar topForcePatchIntegrateMethod = gSum(
    //					    mesh.magSf().boundaryField()[topPatchID]
    //					    *sigma.boundaryField()[topPatchID].component(symmTensor::XY)
    //					    );
    
    scalar disp = max(U.boundaryField()[topPatchID].component(vector::Y));
    
    //- write to file
    if(Pstream::master())
      {
	OFstream& forceDispFile = *filePtr;
	forceDispFile << disp << "\t" << topForce << endl;
      }
  }