# Diffusion term

(Difference between revisions)
 Revision as of 22:41, 14 September 2005 (view source)Zxaar (Talk | contribs)← Older edit Revision as of 22:56, 14 September 2005 (view source)Zxaar (Talk | contribs) Newer edit → Line 13: Line 13: [[Image:Nm_descretisation_diffusionterms_01.jpg]]
[[Image:Nm_descretisation_diffusionterms_01.jpg]]
'''Figure 1.1'''
'''Figure 1.1'''
- :$\vec r_{0}$ and $\vec r_{1}$ are position vector of centroids of cells cell 0 and cell 1 respectively.
+ $\vec r_{0}$ and $\vec r_{1}$ are position vector of centroids of cells cell 0 and cell 1 respectively.
${\rm{d\vec s}} = \vec r_{1} - \vec r_{0}$ ${\rm{d\vec s}} = \vec r_{1} - \vec r_{0}$ -

+ + We wish to approaximate $D_f = \Gamma _f \nabla \phi _f \bullet {\rm{\vec A}}$ at the face. + ===2. Approach 1 === ===2. Approach 1 === Line 23: Line 25: \vec \alpha {\rm{ = }}\frac{{{\rm{\vec A}}}}{{{\rm{\vec A}} \bullet {\rm{d\vec s}}}} \vec \alpha {\rm{ = }}\frac{{{\rm{\vec A}}}}{{{\rm{\vec A}} \bullet {\rm{d\vec s}}}} [/itex] [/itex] + + giving us the expression:
+ :$+ D_f = \Gamma _f \nabla \phi _f \bullet {\rm{\vec A = }}\Gamma _{\rm{f}} \left[ {\left( {\phi _1 - \phi _0 } \right)\vec \alpha \bullet {\rm{\vec A + }}\bar \nabla \phi \bullet {\rm{\vec A - }}\left( {\bar \nabla \phi \bullet {\rm{d\vec s}}} \right)\vec \alpha \bullet {\rm{\vec A}}} \right] +$
+ where $\bar \nabla \phi _f$ and $\Gamma _f$ are suitable face averages.

## Discretisation of Diffusive Term

### 1. Description

A control volume in mesh is made up of set of faces enclosing it. The figure 1.1 shows a typical situation. Where A represent the magnitude of area of the face. And n represents the normal unit vector of the face under consideration.

Figure 1.1
$\vec r_{0}$ and $\vec r_{1}$ are position vector of centroids of cells cell 0 and cell 1 respectively.
${\rm{d\vec s}} = \vec r_{1} - \vec r_{0}$

We wish to approaximate $D_f = \Gamma _f \nabla \phi _f \bullet {\rm{\vec A}}$ at the face.

### 2. Approach 1

We define vector $\vec \alpha {\rm{ = }}\frac{{{\rm{\vec A}}}}{{{\rm{\vec A}} \bullet {\rm{d\vec s}}}}$

giving us the expression:

$D_f = \Gamma _f \nabla \phi _f \bullet {\rm{\vec A = }}\Gamma _{\rm{f}} \left[ {\left( {\phi _1 - \phi _0 } \right)\vec \alpha \bullet {\rm{\vec A + }}\bar \nabla \phi \bullet {\rm{\vec A - }}\left( {\bar \nabla \phi \bullet {\rm{d\vec s}}} \right)\vec \alpha \bullet {\rm{\vec A}}} \right]$

where $\bar \nabla \phi _f$ and $\Gamma _f$ are suitable face averages.