// aitken acceleration 
aitkenDelta.storePrevIter();

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

// update relaxation factor
if(iCorr == 0)
  {
    aitkenTheta = 0.01;
    if(mesh.relax(DU.name()))
      {
	aitkenTheta = mesh.relaxationFactor(DU.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 DU
DU += aitkenTheta*aitkenDelta*aitkenInitialRes;