2.3.1 Under-Relaxation

Since the fiber equations are non-linear, the change of the solution variable $\phi$ has to be controlled. This is achieved by under-relaxation. The new value of $\phi$ in each cell depends to some degree upon the old value $\phi_{old}$ and the change in $\phi$, $\Delta \phi$. It is computed for a given under-relaxation factor $\alpha$ as follows: