# Roughness height

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 July 21, 2014, 10:31 Roughness height #1 Senior Member   Goutam Saha Join Date: Dec 2011 Location: UK Posts: 131 Rep Power: 8 I am working on tube flow with L = 1000 mm, D = 19 mm and tube thickness = 0.5 mm. I am using SST k-w model. Now I want to use Wall Roughness case. There are two input needed. These are 1. Roughness height 2. Roughness constant. I set Roughness constant = 0.5. It means uniform roughness along the tube. But what is called Roughness height? How to calculate this? Any suggestions pls?

 July 21, 2014, 11:11 #2 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,648 Rep Power: 26 Did you already consult the manual? The description of the wall-roughness model is quite complete. The roughness height is the equivalent sand-grain roughness. There are formulas relating this parameter to roughness units used in engineering nowadays.

July 22, 2014, 08:38
#3
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Goutam Saha
Join Date: Dec 2011
Location: UK
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Quote:
 Originally Posted by flotus1 Did you already consult the manual? The description of the wall-roughness model is quite complete. The roughness height is the equivalent sand-grain roughness. There are formulas relating this parameter to roughness units used in engineering nowadays.
Yeah, I did. I want to compare my results with the Moody chart where Relative roughness is defined as = roughness/Diameter. Since I am using Copper pipe, so roughness of copper pipe is = 0.0015 mm.

Now I want to know, what's the relation between relative roughness and roughness height? How to setup this in Fluent?

I am really confused. Thanks

 July 22, 2014, 10:39 #4 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,648 Rep Power: 26 The only thing left to know is whether the Moody-chart you want to compare your results with uses the equivalent sand-grain roughness or some other roughness parameter like . If the relative roughness in the Moody-chart is defined using the equivalent sand-grain roughness, no further conversion is needed. You already know the roughness of the pipe (again, check what kind of roughness this is supposed to be), all you need to do is choose the right diameter to achieve the desired relative roughness. But may Iask what is the point of this analysis? The way I read the manual, the roughness model is designed to yield solutions in accordance with the moody chart. So basically what you are doing is validating Fluents roughness model again.

July 22, 2014, 11:07
#5
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Goutam Saha
Join Date: Dec 2011
Location: UK
Posts: 131
Rep Power: 8
Quote:
 Originally Posted by flotus1 The only thing left to know is whether the Moody-chart you want to compare your results with uses the equivalent sand-grain roughness or some other roughness parameter like . If the relative roughness in the Moody-chart is defined using the equivalent sand-grain roughness, no further conversion is needed. You already know the roughness of the pipe (again, check what kind of roughness this is supposed to be), all you need to do is choose the right diameter to achieve the desired relative roughness. But may Iask what is the point of this analysis? The way I read the manual, the roughness model is designed to yield solutions in accordance with the moody chart. So basically what you are doing is validating Fluents roughness model again.
According to your suggestion, since I know the roughness of the pipe and then I need to chose diameter in such a way that it satisfy the value of relative roughness given in Moody diagram. So, roughness height = relative roughness * pipe roughness. Could you please tell me, Is this your view?

I am not validating Fluent roughness model. What I want to know first, what is the correct input of roughness height in fluent? Then I will validate the moody chart to make sure that I am heading towards right way and then I will apply this model for another application.

 July 22, 2014, 11:34 #6 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,648 Rep Power: 26 This is my view. But honestly I cant think of any other way to interpret this equation with only 3 variables.

July 22, 2014, 16:23
#7
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Goutam Saha
Join Date: Dec 2011
Location: UK
Posts: 131
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Quote:
 Originally Posted by flotus1 This is my view. But honestly I cant think of any other way to interpret this equation with only 3 variables.
Thanks. I will use this first and will try to validate the Moody chart. It will tell me the right or wrong. I fully agreed with you. I am also thinking the same way as you are thinking.

I believe, I am not the first one who is doing work on rough surface for tube. I am really interested to know, how other user of fluent handling this problem. Many thanks for your time. I will let you know the outcomes. Thanks

July 31, 2014, 09:49
#8
Senior Member

Goutam Saha
Join Date: Dec 2011
Location: UK
Posts: 131
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Quote:
 Originally Posted by flotus1 This is my view. But honestly I cant think of any other way to interpret this equation with only 3 variables.
Hi Flotus, This will not work. Still trying to understand, how to define roughness height in Fluent if someone knows the roughness of copper tube and also diameter of a tube? Seems very confusing.

Thanks

 February 17, 2017, 07:55 #9 Member     Omid Seyedashraf Join Date: May 2010 Posts: 49 Rep Power: 9 fluent manual: 7.3.14 Wall Boundary Conditions the Roughness Height, Ks in fluent boundary conditions: Roughness Height (m) so this has to be the same as the Ks in the sand-grain roughness Table. for copper, it has to be 0.0000015 m

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