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24.2.7 Under-Relaxation

Because of the nonlinearity of the equation set being solved by FLUENT, it is necessary to control the change of $\phi$. This is typically achieved by under-relaxation, which reduces the change of $\phi$ produced during each iteration. In a simple form, the new value of the variable $\phi$ within a cell depends upon the old value, $\phi_{\rm old}$, the computed change in $\phi$, $\Delta \phi$, and the under-relaxation factor, $\alpha$, as follows:


\begin{displaymath} \phi = \phi_{\rm old} + \alpha \Delta \phi \end{displaymath} (24.2-12)


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