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May 4, 2012, 06:10 
wall roughness in enhanced wall treatment

#1 
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Oluwalogbon Akinnola
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Hi,
Does anyone know the value of wall roughness used by fluent when the enhanced wall treatment option is selected? I'm aware that you can't set a value when you select this option. what I'd like to know is what value Fluent uses when calculating the flow or how it calculates that value. I ran a series of flows through a thin pipe using an axissymmetric ke model with enhanced wall treatment. I also ran a simulation with standard wall functions and set the value to zero. The results were different which suggests that fluent gives the walls a roughness value. I also carried out some theoretical calculations for the flow rate with different wall roughnesses for comparison to my enhanced results. It appears that the value of wall roughness used by fluent changes with the size of the geometry. If anyone could offer any guidance I would be very appreciative. 

May 4, 2012, 13:41 

#2  
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Lucky Tran
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Quote:
Last edited by LuckyTran; May 4, 2012 at 22:43. 

May 8, 2012, 10:13 

#3 
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Oluwalogbon Akinnola
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I am unsure as to what you mean. I ran a ke with enhanced wall treatment of the flow through a pipe and also ran a simulation of the same geometry also with the ke model but this tim eusing the standard wall function. I then set both the wall roughness and the roughness factor to zero to simulate a smooth wall. As the results were significantly different I concluded that the enhanced wall treatment doesn't treat the walls of the geometry as smooth. I'd like to know how it does treat them as from my undertsanding the roughness of a wall will affect the flow over it. so my question is how does fluent determine the roughness of walls when enhanced wall treatment is enabled.


May 8, 2012, 13:06 

#4  
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Lucky Tran
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Quote:
Anyway, the enhanced wall function approach is not compatible with wall roughness. If you want to use wall roughness, you should be using the standard or scalable wall function approach. Using the enhanced wall treatment, if you even somehow magically were able to do it, will lead to errors. the enhanced wall treatment does treat the geometry as hydraulically smooth, same as the standard and scalable wall functions. 

May 9, 2012, 09:31 

#5 
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Oluwalogbon Akinnola
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I'm not sure you understand my question. I am aware that you can't apply wall roughness when using enhanced wall treatment, only when using standard or scalable. However I am under the impression that for a turbulent flow through a pipe, the wall roughness has an effect on the flow. I'd like to know how fluent estimates the wall roughness to determine this effect. Can you please define hydraulically smooth? I ask because like I said I ran two separate simulations. One with enhanced wall treatment, one with standard wall treatment but with a wall roughness height of zero. I got very different results between the two. My understanding that a pipe is hydraulically smooth if the roughness height is roughly 0.5% of the diameter. For my pipe of 2mm this should be around 10 microns, or 0.1e5m as a maximum. What I'd like to know is how exactly does fluent determine the roughness to use in the flow.


May 9, 2012, 12:29 

#6  
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Lucky Tran
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May 10, 2012, 09:42 

#7 
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Oluwalogbon Akinnola
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Ok I think I understand, but do you have any idea why there is such a significant change in the mass flow rate? With EWT I got a flow rate of 5e3 kg/s but only changing the wall treatment to standard and the roughness height to zero resulted in a flow rate of of 3e2 kg/s. Is k+ or k* anything to do with the Von Karman constant?


June 29, 2012, 05:43 

#8 
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Oluwalogbon Akinnola
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I have been testing out different settings on fluent to try and find the difference between the treatment of wall roughness using the standard wall settings and enhanced wall treatment. However I've found that no matter what I set the wall roughness to on the wall boundary condition under standard wall, there is no change in flowrate. I've gone from zero to ridiculous heights such as 1 metre. Does anyone know of a reason why the simulation is ignoring the wall roughness?


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enhanced wall treatment, wall roughness 
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