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July 3, 2010, 07:27 
Recirculation bubble and wall shear stress

#1 
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Daniele
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hi
I'm simulating an atmospheric boundary layer with a perturbation (an obstacle) on the bottom of domain. Downstream of the obstacle there is a Recirculation bubble, and i want to calculate the wall shear stress on the bottom. But the wss on the bottom has an overshooting when start the Recirculation bubble. How can i try to eliminate it? it's very important Thanks 

July 6, 2010, 06:40 

#2 
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Daniele
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No idea for this problem?


July 6, 2010, 08:18 

#3 
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Daniele
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I'm trying to use this Grad scheme in my fvSchems
gradSchemes { default Gauss upwind; } But I have this error when simulation start > FOAM FATAL IO ERROR: attempt to read beyond EOF file: /u/acconcia/OpenFOAM/acconcia1.6/run/blayer/system/fvSchemes::gradSchemes::default at line 26. From function ITstream::read(token& t) in file db/IOstreams/Tstreams/ITstream.C at line 83. 

July 6, 2010, 09:50 

#4 
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Francois Gallard
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Hi Daniele,
You may have a parenthesis problem in your file. Can you check it and share it if you didn't solve the problem ? 

July 6, 2010, 09:58 

#5 
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Daniele
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Thanks
If I use Gauss linear there isn't error 

July 6, 2010, 10:22 

#6 
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Francois Gallard
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Have you tried this ?
gradSchemes { default Gauss linearUpwind; } Does Gauss linear works ? Because it is second order so more precise. 

July 6, 2010, 10:28 

#7 
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Daniele
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Yes I used it, but I have the same error, I'm traying to eliminate the stress overshooting but I don't understand where is th problem


July 6, 2010, 11:08 

#8 
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Francois Gallard
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Ok, so the problem is in your file, can you share it?
I don't see the problem on the results, you have a recirculation so the flow is stalled after the obstacle so the stress become null at the stall point and then negative. Because your shape is not smooth (not derivable actually), the velocity might have a discontinuity or a jump anyway so its gradient will have an overshoot and consequently she wall shear stress too. 

July 6, 2010, 12:05 

#9 
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Daniele
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Yes, but if you consider the backwardfacing step the wall shear stress haven't overshooting


July 6, 2010, 12:16 

#10 
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Francois Gallard
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That would more be linked with your mesh I think. If it is sufficiently refined, then the singularity should disappear. It seems to be coarse when I see the x variations of the wss which leads to the discontinuity that I pointed in the previous post. Well, that is what i think.
Could you post a screen shot of your mesh please ? 

July 8, 2010, 10:28 

#11 
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Daniele
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I tried to refine the mesh but the overshooting don't disappear. I used 5000 cell over a 500 m domain.


July 8, 2010, 11:34 

#12 
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Francois Gallard
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Are you using 5000 cells in the X direction or 5000 cells on the whole domain ? Is your mesh 2D ? How many cells do you use globally ? Did you refine the mesh close to the obstacle ? Can you post a screenshot of your mesh ?


July 8, 2010, 12:02 

#13 
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Daniele
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The correct data are:
2D domain along x (main flow direction) 1000 cells along y 100 cells along z 1 cells 

July 8, 2010, 12:25 

#14 
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Francois Gallard
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Have you checked the y+ values ? Are they in the correct range for your turbulence model ?


July 8, 2010, 12:36 

#15 
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Daniele
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How can do it? What is the correct range for the keps model?
Thanks you 

July 8, 2010, 12:52 

#16 
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Francois Gallard
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There is an utility for that, yPlusRAS, and the correct range depends if you are using wall functions or not, which you should as you use a coarse mesh. Then the first cell point must be in the logarithmic area so with y+ between 50 and 200.


July 8, 2010, 13:16 

#17 
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Daniele
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I use wall function, with the mesh that I post. The y+ was more or less 800, now I change the mesh and it is 150


July 8, 2010, 13:26 

#18 
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Francois Gallard
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Ok, you can then play with that to check if your results can be improved but that should be ok like that.
I think that you could use much more cells in the region which you are interested in as you only use about 20 cells in it (if I had understood well your screen shot) and the gradients are here much more important than anywhere else. 

July 8, 2010, 14:32 

#19 
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Daniele
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Refined mesh. Then I'll post the simulation results


July 8, 2010, 18:29 

#20 
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Daniele
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I used a vertical step in my profile and this is the wall shear stress at bottom, They seem numerical error, because if I refine the mesh on chenge of shape don't change anything.


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