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

Reversed flow problem

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

Like Tree1Likes
  • 1 Post By QIAN06

Reply
 
LinkBack Thread Tools Display Modes
Old   December 10, 2015, 09:35
Default Reversed flow problem
  #1
New Member
 
Vasudevan
Join Date: Dec 2015
Posts: 6
Rep Power: 2
VasCFD is on a distinguished road
Hi

I am doing a turbulent flow simulation(K-ephsilon model) of air in a square duct with internal rectangular fins. I previously did the same case in workbench using design modeller and did not get any problem. But now i am doing this in gambit and after a few hundreds of iterations, i am receiving reversed flow in a lot of faces. What should i do??
VasCFD is offline   Reply With Quote

Old   December 11, 2015, 08:33
Post
  #2
New Member
 
wangzhijun
Join Date: Aug 2013
Posts: 5
Rep Power: 5
QIAN06 is on a distinguished road
HI,
could you give specific problem? e.g., give the printscreen of the problem.
VasCFD likes this.
QIAN06 is offline   Reply With Quote

Old   December 11, 2015, 12:07
Default
  #3
New Member
 
Vasudevan
Join Date: Dec 2015
Posts: 6
Rep Power: 2
VasCFD is on a distinguished road
Quote:
Originally Posted by QIAN06 View Post
HI,
could you give specific problem? e.g., give the printscreen of the problem.
I have attached geometry as modelled in gambit. I have tried various turbulence models like k-ephsilon, k-omega(So far the best as the reversed flow occurred as late as 890 iterations) and tried to use different schemes for solving momentum equation. But all it did was to delay the problem than solve it.
Actually the problem begins with ever increasing "scaled residuals" and after a point the reversed flow occurs. Please help me out.
Thanks in advance.
Attached Images
File Type: png Untitled.png (47.6 KB, 7 views)
VasCFD is offline   Reply With Quote

Old   December 15, 2015, 07:28
Default
  #4
New Member
 
Join Date: Mar 2014
Posts: 16
Rep Power: 4
divergence is on a distinguished road
It seems to me that your outlet is quite near the last rectangular object. This would imply that the flow would have a hard time getting back on the track and axial instead of swirling in the wake of the last fin.

You could try to extend the square duct in order to give the flow a better chance to stabilize before exiting the domain. If you would still have back flow in the outlet, you could make the outlet somewhat smaller, say 60% of the area of the duct. Mind you that reducing the area is a bad idea if you would do it straight after the last fin.

Hope this helps!
divergence is offline   Reply With Quote

Old   December 15, 2015, 08:03
Default
  #5
New Member
 
Vasudevan
Join Date: Dec 2015
Posts: 6
Rep Power: 2
VasCFD is on a distinguished road
Quote:
Originally Posted by divergence View Post
It seems to me that your outlet is quite near the last rectangular object. This would imply that the flow would have a hard time getting back on the track and axial instead of swirling in the wake of the last fin.

You could try to extend the square duct in order to give the flow a better chance to stabilize before exiting the domain. If you would still have back flow in the outlet, you could make the outlet somewhat smaller, say 60% of the area of the duct. Mind you that reducing the area is a bad idea if you would do it straight after the last fin.

Hope this helps!
Thanks for the idea. I came across this suggestion but will not be able to implement it, as my test case does not allow deviation with the current geometry.
Also i heard through professors that reversed flow does not occur in back flow, but occurs only when the scheme used is not able to solve the governing equations properly and occurs due to generation of absurd values for the parameters.
Thanks for the suggestion though. Please help me with my problem.
VasCFD is offline   Reply With Quote

Old   December 15, 2015, 08:42
Default
  #6
New Member
 
Join Date: Mar 2014
Posts: 16
Rep Power: 4
divergence is on a distinguished road
So it's more of an academic test you have in your hands there, okay.

I read your first message second time. You mentioned succeeding in your calculations when you created the volume with DM and Meshing (if I understood you right), but got some negative feedback when you created the flow volume and mesh with Gambit.

I assume that the duct itself alongside with the fins are identical in both of the cases as well as the boundary conditions and the discretizations. So actually the only thing that has changed would be the mesh. Therefore, I would proceed with studying the mesh, trying to find some low quality cells that might mess things up for you. Check if there's sudden and drastic changes in the aspect ratio or if the mesh near the fins are inadequate. Additionally, you should check if the under-relaxations are at least moderate if not low.

Kinda sounds weird if you already managed to solve the case and now would have some mysterious problem. I'm sure you'll be victorious sooner than later.
divergence is offline   Reply With Quote

Old   December 15, 2015, 12:04
Thumbs up
  #7
New Member
 
Vasudevan
Join Date: Dec 2015
Posts: 6
Rep Power: 2
VasCFD is on a distinguished road
Quote:
Originally Posted by divergence View Post
So it's more of an academic test you have in your hands there, okay.

I read your first message second time. You mentioned succeeding in your calculations when you created the volume with DM and Meshing (if I understood you right), but got some negative feedback when you created the flow volume and mesh with Gambit.

I assume that the duct itself alongside with the fins are identical in both of the cases as well as the boundary conditions and the discretizations. So actually the only thing that has changed would be the mesh. Therefore, I would proceed with studying the mesh, trying to find some low quality cells that might mess things up for you. Check if there's sudden and drastic changes in the aspect ratio or if the mesh near the fins are inadequate. Additionally, you should check if the under-relaxations are at least moderate if not low.

Kinda sounds weird if you already managed to solve the case and now would have some mysterious problem. I'm sure you'll be victorious sooner than later.
Thanks for the suggestion. I have figured out that the mesh would be only difference as i gave assembly mesh in workbench (which gives the best suited mesh for the geometry). I'll definitely review my mesh for error or inconsistencies. But can you suggest me some mesh methods or patterns that would help me solve the problem.
Thanks again.
VasCFD is offline   Reply With Quote

Reply

Tags
ansys 14, gambit, reversed flow

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
ask for help about the reversed flow yuhehuan FLUENT 3 January 9, 2013 06:30
Reversed flow cyclone Mohsin FLUENT 0 August 7, 2012 20:46
transient, impregnating flow problem fgommer FLUENT 0 February 29, 2012 17:10
Reversed flow problem & null point error narendra FLUENT 0 May 20, 2010 12:43
Reversed flow error with thin film heater MK FLUENT 1 February 12, 2009 11:26


All times are GMT -4. The time now is 15:31.