# How dose FLUENT define face normal?

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August 31, 2017, 05:31
How dose FLUENT define face normal?
#1
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Chi Zhang
Join Date: Aug 2017
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Hi everyone! I am confused by two illustration in UDF Manual.Please see picture in attachment. UDF Manual says that ANSYS Fluent assigns adjacent cells to an interior face (c0 and c1) according to the following convention: the cell out of which a face area normal is pointing is designated as cell C0, while the cell in to which a face area normal is pointing is cell c1. But in the description of F_FLUX macro, it says The sign of F_FLUX that is computed by the ANSYS Fluent solver is positive if the flow direction is the same as the face area normal direction, and is negative if the flow direction and the face area normal directions are opposite. In other words, the flux is positive if the flow is out of the domain, and is negative if the flow is in to the domain. We now assume a flow from left to right and it flows through cell C0 and C1. If the illustrations are both true, flux through 0-1 face will be positive both in cell C0 and C1 because flow direction is the same with face normal. However, in cell C1, flow is in to the cell , so it has to be negtive.
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 September 1, 2017, 04:14 #2 Senior Member   Join Date: Nov 2013 Posts: 1,965 Rep Power: 25 The part "In other words, the flux is positive if the flow is out of the domain, and is negative if the flow is in to the domain." is talking about flow in/out the domain, so there boundary faces are relevant, not interior faces. The picture that you attached is for interior faces.

September 1, 2017, 04:48
#3
Member

Chi Zhang
Join Date: Aug 2017
Posts: 32
Rep Power: 8
Quote:
 Originally Posted by pakk The part "In other words, the flux is positive if the flow is out of the domain, and is negative if the flow is in to the domain." is talking about flow in/out the domain, so there boundary faces are relevant, not interior faces. The picture that you attached is for interior faces.
Thank you pakk!