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-   -   I have a problem about the turbulence flow! (http://www.cfd-online.com/Forums/fluent/31722-i-have-problem-about-turbulence-flow.html)

yuchang June 26, 2003 05:51

I have a problem about the turbulence flow!
 
Hello! ALL

This is my first time to be here and to ask some

question!

As we know, the velocity profile of turbulence in the

circular tube is about obeying 1/7 power law.

I use this knowledge to practice the turbulence model,

but I cannot get the value like the profile I said

above. What happened, If anybody check the turbulence

model in Fluent and get the same answer as the 1/7

power law! Thx for your help!

Best regards.


Alex Munoz June 26, 2003 12:12

Re: I have a problem about the turbulence flow!
 
Hi

The power low 1/n apply to a especific Reynolds number therefore, you must to check your Reynolds number before choosing the value of n

I sugguest that you read the document of Zagarola and also this web side to compare your simmulation with Zagarola's experiments

http://www.princeton.edu/~gasdyn/Superpipe_data

Best regards

Alex Munoz

yuchang June 26, 2003 20:38

Re: I have a problem about the turbulence flow!
 
Thank you for your advice!

From the Book written by Prof. Bird " Transport Phenomena", he noted that the 1/7 power law is applied in the range of Reynold no. between 10000 and 100000. and I set the condition in the range.

And do you have another comments?

thx for your help

Alex Munoz June 26, 2003 21:14

Re: I have a problem about the turbulence flow!
 
Hi

Sometimes authors write in a generic scenario !!

If you read the Boundary Layer Theory by Schlichting Hermamm, you will find that at at different Reynolds Numbers you have a different 1/n power number

Re n 4e03 6.0 2.3E04 6.6 1.1E05 7.0

Now you can see why some writers use n=7.0, however this value only applies for Re=1.1E05.

Now If you cannot get the profile that you are expecting could be because:

1) Boundary conditions are poor defined 2) Your are using Tetrahedral mesh 3) Your are using first order discretization of the momentum equations 4) the turbulence model should not be used for this type or case 5) the number of cells near to the wall are not enough to describe the boundary layer.

Therefore, I sugguest that you read and read, especially some cases already posted on this web.

Best Regards

Alex Munoz


yuchang June 26, 2003 21:21

Re: I have a problem about the turbulence flow!
 
thx your for your advice again! I just want to know if Fluent can predict more correct values in turbulent case. So I choose the simplest case to try it! About what the reasons you suggest me, I think I all consider it! Maybe I should read more. Thank you after all!


Alex Munoz June 27, 2003 00:33

Re: I have a problem about the turbulence flow!
 
Hi

I can ensure you, because I have done this case, that fluent is able to project a parabolic velocity profile at the inlet of a pipe up to several meter down stream with a neglectable error!!

Regards

Alex Munoz

yuchang June 27, 2003 00:50

Re: I have a problem about the turbulence flow!
 
Excuse me, what turbulent model do you use in that case? And how long is the entrance length if I want to get a fully-developed profile? Because I try it using the ratio of L/D over 100 times and can get the approximate value. Maybe something wrong in my operation. Thank you again! see u


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