Node or Cell Values

Diger · May 15, 2016, 12:59

When plotting cell or node values they deviate like 10% from each other and I was wondering which values u could trust more. In particular I was modelling diffusion losses due to reactions of the species at the wall. Right now I'm plotting (X-Y plot) the concentration along the crosssection of the tube.
I know the difference of both. I just am not quite sure which values (maybe by experience) you could trust more in this situation.

LuckyTran · May 16, 2016, 10:42

They are different values. Node values are on nodes and cell values are on cell centers. Just take it as it is.

The cell values are generally the values actually being stored at cells and node values are generally interpolated (interpreted) from the cell values. But cell values are not node values. If I want temperature on a surface, I wouldn't go after cell values because a surface node is a node and not a cell center.

There are too many other things going on to be worrying about cell versus node values. Things like your discretization scheme for example.

Diger · May 17, 2016, 08:32

At the moment I'm using the SIMPLE scheme. Are you refering to that?
So depending on the situation one is better over the other!?
Probably there is not something like a general rule, but could you say that for certain geometrics this scheme is better over the other or what does it depend on?

LuckyTran · May 18, 2016, 13:59

SIMPLE is an predictor-corrector algorithm for solving the P-V coupling problem. It has nothing to do with interpolation and node & cell values.

Face values of various quantities are determined by your discretization schemes. In particular, they are highly dependent on your gradient reconstruction scheme (least squares, green gauss cell based, or green gauss node based). But of course other settings can indirectly affect the cell-node value relationship (like limiters) or non-linearities.

The relationship between cell-node values is strictly dependent on the schemes used and the properties of the solution (the properties of the flow). A general rule would be based purely on flow properties (e.g. gradient magnitudes). The influence of geometry is limited to the boundary conditions, i.e. constraints on the flow. Geometry plays no direct role in CFD beyond that.

Ionut G · October 8, 2019, 11:22

Hello,

I am running simulations in Code_Saturne, which stores the variables in the centre of the cell.
The results file is in CGNS format and I importing this file in Fluent for post processing the results.
Because I study the velocity variation near the wall, it is important for me to know what is the interpolation scheme that Fluent uses to represent the variables values on the nodes.

Does anybody know what is the interpolation scheme used by Fluent when it represents the variables (velocity) values on nodes?

Thank you,
Ionut

May 15, 2016, 12:59	Node or Cell Values	#1
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	When plotting cell or node values they deviate like 10% from each other and I was wondering which values u could trust more. In particular I was modelling diffusion losses due to reactions of the species at the wall. Right now I'm plotting (X-Y plot) the concentration along the crosssection of the tube. I know the difference of both. I just am not quite sure which values (maybe by experience) you could trust more in this situation.

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May 16, 2016, 10:42		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,737 Rep Power: 66	They are different values. Node values are on nodes and cell values are on cell centers. Just take it as it is. The cell values are generally the values actually being stored at cells and node values are generally interpolated (interpreted) from the cell values. But cell values are not node values. If I want temperature on a surface, I wouldn't go after cell values because a surface node is a node and not a cell center. There are too many other things going on to be worrying about cell versus node values. Things like your discretization scheme for example.

May 17, 2016, 08:32		#3
Diger Senior Member Join Date: Nov 2015 Posts: 135 Rep Power: 10	At the moment I'm using the SIMPLE scheme. Are you refering to that? So depending on the situation one is better over the other!? Probably there is not something like a general rule, but could you say that for certain geometrics this scheme is better over the other or what does it depend on?

May 18, 2016, 13:59		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,737 Rep Power: 66	SIMPLE is an predictor-corrector algorithm for solving the P-V coupling problem. It has nothing to do with interpolation and node & cell values. Face values of various quantities are determined by your discretization schemes. In particular, they are highly dependent on your gradient reconstruction scheme (least squares, green gauss cell based, or green gauss node based). But of course other settings can indirectly affect the cell-node value relationship (like limiters) or non-linearities. The relationship between cell-node values is strictly dependent on the schemes used and the properties of the solution (the properties of the flow). A general rule would be based purely on flow properties (e.g. gradient magnitudes). The influence of geometry is limited to the boundary conditions, i.e. constraints on the flow. Geometry plays no direct role in CFD beyond that.

October 8, 2019, 11:22		#5
Ionut G New Member G Ionut Join Date: Sep 2016 Posts: 29 Rep Power: 9	Hello, I am running simulations in Code_Saturne, which stores the variables in the centre of the cell. The results file is in CGNS format and I importing this file in Fluent for post processing the results. Because I study the velocity variation near the wall, it is important for me to know what is the interpolation scheme that Fluent uses to represent the variables values on the nodes. Does anybody know what is the interpolation scheme used by Fluent when it represents the variables (velocity) values on nodes? Thank you, Ionut