// aitken acceleration

// update delta
aitkenDelta.storePrevIter();
aitkenDelta = (U - U.prevIter()) / aitkenInitialRes;

// update relaxation factor
if(iCorr == 0)
  {
    aitkenTheta = 0.01;
//     if(mesh.relax(U.name()))
//       {
//     aitkenTheta = mesh.relaxationFactor(U.name());
//       }
  }
 else
   {
     vectorField b = aitkenDelta.internalField() - aitkenDelta.prevIter().internalField();
     // scalar sumMagB = gSum(mag(b));
     scalar sumMagB = gSum(magSqr(b));
     if(sumMagB < SMALL)
       {
     // Warning << "Aitken under-relaxation: denominator less then SMALL"
     // << endl;
     sumMagB += SMALL;
       }

     aitkenTheta = -aitkenTheta*
       gSum(aitkenDelta.prevIter().internalField() & b)
       /
       sumMagB;
   }

// correction to the latest U
U += aitkenTheta*aitkenDelta*aitkenInitialRes;