Under-Relaxation reduces solution oscillations and helps to keep the computation stable. After each iteration, at each cell, a new value for variable  in cell  is then updated using following equation:

(11.18)

where  is under-relaxation factor. The choice    corresponds to no under-relaxation. The choice    is under-relaxation. This may slow down speed of convergence but increases the stability of the computation, i.e. it decreases the possibility of divergence or oscillations in the solution. For more details see e.g. [3].