CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FLUENT

Roughness height

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   July 21, 2014, 10:31
Question Roughness height
  #1
Senior Member
 
Goutam Saha
Join Date: Dec 2011
Location: UK
Posts: 131
Rep Power: 7
Goutam is on a distinguished road
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?
Goutam is offline   Reply With Quote

Old   July 21, 2014, 11:11
Default
  #2
Senior Member
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,389
Rep Power: 22
flotus1 will become famous soon enoughflotus1 will become famous soon enough
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.
flotus1 is offline   Reply With Quote

Old   July 22, 2014, 08:38
Default
  #3
Senior Member
 
Goutam Saha
Join Date: Dec 2011
Location: UK
Posts: 131
Rep Power: 7
Goutam is on a distinguished road
Quote:
Originally Posted by flotus1 View Post
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
Goutam is offline   Reply With Quote

Old   July 22, 2014, 10:39
Default
  #4
Senior Member
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,389
Rep Power: 22
flotus1 will become famous soon enoughflotus1 will become famous soon enough
The only thing left to know is whether the Moody-chart you want to compare your results with uses the equivalent sand-grain roughness k_S or some other roughness parameter like R_Z.

If the relative roughness R_\text{rel} in the Moody-chart is defined using the equivalent sand-grain roughness, no further conversion is needed.

R_\text{rel} = \frac{D}{k_S} \rightarrow D = R_\text{rel} \cdot k_S

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 D 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.
flotus1 is offline   Reply With Quote

Old   July 22, 2014, 11:07
Default
  #5
Senior Member
 
Goutam Saha
Join Date: Dec 2011
Location: UK
Posts: 131
Rep Power: 7
Goutam is on a distinguished road
Quote:
Originally Posted by flotus1 View Post
The only thing left to know is whether the Moody-chart you want to compare your results with uses the equivalent sand-grain roughness k_S or some other roughness parameter like R_Z.

If the relative roughness R_\text{rel} in the Moody-chart is defined using the equivalent sand-grain roughness, no further conversion is needed.

R_\text{rel} = \frac{D}{k_S} \rightarrow D = R_\text{rel} \cdot k_S

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 D 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.
Goutam is offline   Reply With Quote

Old   July 22, 2014, 11:34
Default
  #6
Senior Member
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,389
Rep Power: 22
flotus1 will become famous soon enoughflotus1 will become famous soon enough
This is my view. But honestly I cant think of any other way to interpret this equation with only 3 variables.
flotus1 is offline   Reply With Quote

Old   July 22, 2014, 16:23
Default
  #7
Senior Member
 
Goutam Saha
Join Date: Dec 2011
Location: UK
Posts: 131
Rep Power: 7
Goutam is on a distinguished road
Quote:
Originally Posted by flotus1 View Post
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
Goutam is offline   Reply With Quote

Old   July 31, 2014, 09:49
Default
  #8
Senior Member
 
Goutam Saha
Join Date: Dec 2011
Location: UK
Posts: 131
Rep Power: 7
Goutam is on a distinguished road
Quote:
Originally Posted by flotus1 View Post
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
Goutam is offline   Reply With Quote

Old   February 17, 2017, 07:55
Default
  #9
Member
 
seyedashraf's Avatar
 
Omid Seyedashraf
Join Date: May 2010
Posts: 49
Rep Power: 8
seyedashraf is on a distinguished road
Send a message via AIM to seyedashraf Send a message via Yahoo to seyedashraf


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

seyedashraf is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Setting of Roughness Constant and Height Sing FLUENT 6 January 21, 2016 09:53
UDF: for the setting of roughness height maximus FLUENT 1 September 10, 2014 16:20
Roughness Height Sam0508 STAR-CCM+ 2 March 29, 2013 04:29
atmospheric roughness height Alain BASTIDE CD-adapco 5 December 30, 2002 06:26
atmospheric roughness height Alain BASTIDE Main CFD Forum 0 December 27, 2002 17:32


All times are GMT -4. The time now is 20:44.