# Source term linearization

(Difference between revisions)
 Revision as of 18:57, 7 December 2005 (view source)Tsaad (Talk | contribs)m (Source term moved to Source term linearization)← Older edit Latest revision as of 10:25, 22 February 2007 (view source) (→Picard's Method) (One intermediate revision not shown) Line 9: Line 9: where $S_C$ denotes the '''constant''' part of S and $S_P$ denotes the coefficient of $\phi_P$ (not the value of S at P). This allows us to place $S_P$ in the coefficients for $\phi_P$.
where $S_C$ denotes the '''constant''' part of S and $S_P$ denotes the coefficient of $\phi_P$ (not the value of S at P). This allows us to place $S_P$ in the coefficients for $\phi_P$.
- Let $\phi_P^*$ denote the value of $\phi_P$at the current itertaion. We now write a Taylor series expansion of S about $\phi_P^*$ as
+ Let $\phi_P^*$ denote the value of $\phi_P$at the current iteration. We now write a Taylor series expansion of S about $\phi_P^*$ as
$S = S^* + \left ( \frac {\partial S}{\partial \phi} \right ) ^* \left ( \phi_P - \phi_P^* \right )$
$S = S^* + \left ( \frac {\partial S}{\partial \phi} \right ) ^* \left ( \phi_P - \phi_P^* \right )$
therefore
therefore
Line 15: Line 15: $S_P = \left( \frac {\partial S}{\partial \phi} \right ) ^*$
$S_P = \left( \frac {\partial S}{\partial \phi} \right ) ^*$
where $\left ( \frac {\partial S}{\partial \phi} \right ) ^*$ is the gradient of S evaluated at $\phi_P^*$.
where $\left ( \frac {\partial S}{\partial \phi} \right ) ^*$ is the gradient of S evaluated at $\phi_P^*$.
-
+ + ==Example== As an illustrative example, consider $S = -T^3 + 10 \,$. Following Picard's method, we have
As an illustrative example, consider $S = -T^3 + 10 \,$. Following Picard's method, we have
$\left( \frac {\partial S}{\partial \phi} \right ) = -3T^2$
$\left( \frac {\partial S}{\partial \phi} \right ) = -3T^2$

# Introduction

In seeking the solution of the general transport equation for a scalar $\phi$, the main objective is to correctly handle the non-linearities by transforming them into linear form and then iteratively account for the non-linearity. The source term plays a central role in this respect when it is non-linear. For example, in radiation heat transfer, the source term in energy equation is expressed as fourth powers in the temperature.
When the source is constant and independent of the conserved scalar, the finite volume method assumes that the value of S prevails of the control volume and thus can be easily integrated. For a given control volume P, we obtain
$\int_{\Omega} S d\Omega = S\Omega \,$

## Picard's Method

Picard's method is the most popular method used in conjunction with the finite volume method. For a given control volume P, we start by writing the source term as
$S = S_C + S_PT_P \,$
where $S_C$ denotes the constant part of S and $S_P$ denotes the coefficient of $\phi_P$ (not the value of S at P). This allows us to place $S_P$ in the coefficients for $\phi_P$.

Let $\phi_P^*$ denote the value of $\phi_P$at the current iteration. We now write a Taylor series expansion of S about $\phi_P^*$ as
$S = S^* + \left ( \frac {\partial S}{\partial \phi} \right ) ^* \left ( \phi_P - \phi_P^* \right )$
therefore
$S_C = S^* - \left ( \frac {\partial S}{\partial \phi} \right ) ^* \phi_P^*$
$S_P = \left( \frac {\partial S}{\partial \phi} \right ) ^*$
where $\left ( \frac {\partial S}{\partial \phi} \right ) ^*$ is the gradient of S evaluated at $\phi_P^*$.

## Example

As an illustrative example, consider $S = -T^3 + 10 \,$. Following Picard's method, we have
$\left( \frac {\partial S}{\partial \phi} \right ) = -3T^2$
$S_C = -T_P^{*3} +10 + 3T_P^{*2}T_P^* = 2T_P^{*3} +10$
$S_P = -3T_P^{*2}$

## References

1. Patankar, S.V. (1980), Numerical Heat Transfer and Fluid Flow, ISBN 0070487405, Hemisphere Publishing Corporation, USA..
2. Murthy, Jayathi Y. (1998), "Numerical Methods in Heat, Mass, and Momentum Transfer", Draft Notes, Purdue University (download).
3. Darwish, Marwan (2003), "CFD Course Notes", Notes, American University of Beirut.