CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

Recirculation bubble and wall shear stress

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 3, 2010, 07:27
Default Recirculation bubble and wall shear stress
  #1
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
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
Attached Files
File Type: pdf wss.pdf (4.2 KB, 39 views)
Daniele111 is offline   Reply With Quote

Old   July 6, 2010, 06:40
Default
  #2
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
No idea for this problem?
Daniele111 is offline   Reply With Quote

Old   July 6, 2010, 08:18
Default
  #3
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
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/acconcia-1.6/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.
Daniele111 is offline   Reply With Quote

Old   July 6, 2010, 09:50
Default
  #4
Member
 
Francois Gallard
Join Date: Mar 2010
Location: Edinburgh
Posts: 39
Rep Power: 7
fgal is on a distinguished road
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 ?
fgal is offline   Reply With Quote

Old   July 6, 2010, 09:58
Default
  #5
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
Thanks
If I use Gauss linear there isn't error
Daniele111 is offline   Reply With Quote

Old   July 6, 2010, 10:22
Default
  #6
Member
 
Francois Gallard
Join Date: Mar 2010
Location: Edinburgh
Posts: 39
Rep Power: 7
fgal is on a distinguished road
Have you tried this ?

gradSchemes
{
default Gauss linearUpwind;
}

Does Gauss linear works ? Because it is second order so more precise.
fgal is offline   Reply With Quote

Old   July 6, 2010, 10:28
Default
  #7
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
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
Daniele111 is offline   Reply With Quote

Old   July 6, 2010, 11:08
Default
  #8
Member
 
Francois Gallard
Join Date: Mar 2010
Location: Edinburgh
Posts: 39
Rep Power: 7
fgal is on a distinguished road
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.
fgal is offline   Reply With Quote

Old   July 6, 2010, 12:05
Default
  #9
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
Yes, but if you consider the backward-facing step the wall shear stress haven't overshooting
Daniele111 is offline   Reply With Quote

Old   July 6, 2010, 12:16
Default
  #10
Member
 
Francois Gallard
Join Date: Mar 2010
Location: Edinburgh
Posts: 39
Rep Power: 7
fgal is on a distinguished road
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 ?
fgal is offline   Reply With Quote

Old   July 8, 2010, 10:28
Default
  #11
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
I tried to refine the mesh but the overshooting don't disappear. I used 5000 cell over a 500 m domain.
Daniele111 is offline   Reply With Quote

Old   July 8, 2010, 11:34
Default
  #12
Member
 
Francois Gallard
Join Date: Mar 2010
Location: Edinburgh
Posts: 39
Rep Power: 7
fgal is on a distinguished road
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 ?
fgal is offline   Reply With Quote

Old   July 8, 2010, 12:02
Default
  #13
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
The correct data are:
2D domain along x (main flow direction) 1000 cells
along y 100 cells
along z 1 cells
Attached Images
File Type: jpg Mesh.jpg (101.2 KB, 23 views)
Daniele111 is offline   Reply With Quote

Old   July 8, 2010, 12:25
Default
  #14
Member
 
Francois Gallard
Join Date: Mar 2010
Location: Edinburgh
Posts: 39
Rep Power: 7
fgal is on a distinguished road
Have you checked the y+ values ? Are they in the correct range for your turbulence model ?
fgal is offline   Reply With Quote

Old   July 8, 2010, 12:36
Default
  #15
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
How can do it? What is the correct range for the k-eps model?

Thanks you
Daniele111 is offline   Reply With Quote

Old   July 8, 2010, 12:52
Default
  #16
Member
 
Francois Gallard
Join Date: Mar 2010
Location: Edinburgh
Posts: 39
Rep Power: 7
fgal is on a distinguished road
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.
fgal is offline   Reply With Quote

Old   July 8, 2010, 13:16
Default
  #17
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
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
Daniele111 is offline   Reply With Quote

Old   July 8, 2010, 13:26
Default
  #18
Member
 
Francois Gallard
Join Date: Mar 2010
Location: Edinburgh
Posts: 39
Rep Power: 7
fgal is on a distinguished road
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.
fgal is offline   Reply With Quote

Old   July 8, 2010, 14:32
Default
  #19
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
Refined mesh. Then I'll post the simulation results
Attached Images
File Type: jpg Mesh.jpg (100.5 KB, 5 views)
Attached Files
File Type: pdf wss.pdf (4.2 KB, 6 views)
Daniele111 is offline   Reply With Quote

Old   July 8, 2010, 18:29
Default
  #20
Senior Member
 
Daniele
Join Date: Feb 2010
Posts: 134
Rep Power: 7
Daniele111 is on a distinguished road
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.
Attached Images
File Type: jpg wss.jpg (32.6 KB, 15 views)
Daniele111 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
Two-phase air water flow problems by activating Wall Lubrication Force challenger85 CFX 5 November 5, 2009 06:44
Simulation of a single bubble with a VOF-method Suzzn CFX 18 October 2, 2009 04:18
air bubble is disappear increasing time using vof xujjun CFX 9 June 9, 2009 07:59
gas hold up in bubble Shelly Main CFD Forum 0 November 13, 2006 01:28
hwo to model rising bubble interact with wall Dusky.He Main CFD Forum 2 December 19, 2005 09:53


All times are GMT -4. The time now is 03:35.