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Old   November 15, 2012, 14:57
Default LES using Fluent
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Hi

i have tried to simulate the case that has been mentioned in the attached paper, a backward step flow, but when i draw the mean x-velocity profile at specific location (x/H=14), i do not get good results. i did this procedure for simulation:

1) run the case with k-e model

2) wrote this command: solve/initialize/init-instantaneous-vel (i write this command to generate the instantaneous velocity out of the steady state RANS results as Fluent manual mentioned)

3) wrote this command: solve/initialize/init-flow-statistics (i write this command to zero out the initial statistics as Fluent manual mentioned)

4) wrote (rpsetvar 'les-2d? #t) to enable on 2D LES model

5) turned on Transient

6) select LES model

7) second order model for time step

8) for perturbation i used turbulent intensity and hydrolic diameter

9) i saved data for every 1 time step and also turned on the "Data Sampling for time statistics"

10) the time step used in the paper is 0.005 but i used even smaller one :0.003

11) run the cod for 2000 time steps

when i look at mean x-vel contour, it has the fully developed format, but i do not know why the results are different from the paper. i used both outflow and pressure outlet boundary condition for outlet and also a channel with larger length (paper is 34H mine 40 H where H=26.7 mm).

i used both 34H and 40H for the simulations and i got different results for the vel. profile at x/H=14. i used 259 and 25 nodes in x and y direction respectively as paper mentioned. and Samgosinesky model.

do you have any idea? how many nodes should be used in each direction and how can i find that my results are correct?

thanks for your time
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Old   November 15, 2012, 16:04
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1) LES must be used for a 3D flow condition

2) You have to sample the fields for the statistics after the numerical transient is finished. Therefore, you need to run in time until an energy equilibrium is reached.

3) Results will strongly depends on the inflow condition. I suggest to use a supplementary part of domain before the step

4) use dynamic modelling
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Old   November 15, 2012, 16:06
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I shudder to think what a "2D LES model" might be....
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Old   November 15, 2012, 16:35
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But there are many papers used 2D LES
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Old   November 15, 2012, 16:39
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Quote:
Originally Posted by FMDenaro View Post
1) LES must be used for a 3D flow condition

2) You have to sample the fields for the statistics after the numerical transient is finished. Therefore, you need to run in time until an energy equilibrium is reached.

3) Results will strongly depends on the inflow condition. I suggest to use a supplementary part of domain before the step

4) use dynamic modelling
Thanks, u mean I have to model the channel before the
Step too?
Also how much should be the amount of Yplus?
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Old   November 15, 2012, 16:42
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Quote:
Originally Posted by mazdak View Post
Thanks, u mean I have to model the channel before the
Step too?
Also how much should be the amount of Yplus?
Maybe you have read about 2D LES for geostrophic flows??? But it is very different from your case

You need to put some points in the BL, that means 3 - 4 cells within y+<1
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Old   November 15, 2012, 16:42
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Also if I want to use 3D model, how much
Should be the length in the z-direction?
How many times of H? For example 4H is enough?
I have one more question.
How the cell size affect the results?
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Old   November 15, 2012, 16:44
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Quote:
Originally Posted by FMDenaro View Post
Maybe you have read about 2D LES for geostrophic flows??? But it is very different from your case

You need to put some points in the BL, that means 3 - 4 cells within y+<1
How many nodes do u propose for each direction?
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Old   November 15, 2012, 16:45
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Quote:
Originally Posted by mazdak View Post
Also if I want to use 3D model, how much
Should be the length in the z-direction?
How many times of H? For example 4H is enough?
I have one more question.
How the cell size affect the results?
that depends on which case you use... periodic condition or walls
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Old   November 15, 2012, 16:49
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Quote:
Originally Posted by FMDenaro View Post
that depends on which case you use... periodic condition or walls
Honestly I do not which case
Cuz I'm not familiar with LES
And if you know any good paper that can
Help me I would be appreciate if introduce that
For me
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Old   November 15, 2012, 16:54
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literature is "full" of papers about the BFS

https://www.google.it/search?q=large...=hp&channel=np
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Old   November 15, 2012, 17:00
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Quote:
Originally Posted by FMDenaro View Post
literature is "full" of papers about the BFS

https://www.google.it/search?q=large...=hp&channel=np
No i mean paper about that periodic condition
That you told
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Old   November 15, 2012, 17:11
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many of those papers are based on periodic conditions along the span-wise direction ... see also the book of Sagaut
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Old   November 15, 2012, 17:13
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Quote:
Originally Posted by FMDenaro View Post
many of those papers are based on periodic conditions along the span-wise direction ... see also the book of Sagaut
Thanks for your help
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Old   November 15, 2012, 17:19
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see also chapter 8 in the Agard database http://www.efluids.com/efluids/databases/agard.html
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