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

Problem at the interface in CFX

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 15, 2014, 08:28
Default Problem at the interface in CFX
  #1
kwg
New Member
 
Koen
Join Date: Jan 2014
Posts: 11
Rep Power: 12
kwg is on a distinguished road
Hi,

I am simulating a (static) guide vane plus some ducting. Some weird things seem to occur at the interface. I am using a stage interface (mixing plane) with a GGI mesh connection.

If I look at some random parameter (in the pictures density is plotted) the averaging on most of the interface seems to be fine. However, where the domain is placed there is some uniformity for basically every parameter.
Does anyone have an idea what causes this? Is this something which disappears with running more iterations?

I tried to change the 'pitch change' option from automatic to manually specifying the pitch angles, this didn't make any difference. So the problem doesn't seem to be here.

Thanks in advance for your help,
Koen
Attached Images
File Type: png 001.png (49.6 KB, 27 views)
File Type: png 002.png (63.2 KB, 24 views)
kwg is offline   Reply With Quote

Old   October 15, 2014, 18:14
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,844
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
Can you show on your images what the problem is? Also please show your full domain and the flow path from inlet to outlet.
ghorrocks is offline   Reply With Quote

Old   October 16, 2014, 04:59
Default
  #3
kwg
New Member
 
Koen
Join Date: Jan 2014
Posts: 11
Rep Power: 12
kwg is on a distinguished road
The problem is the non uniform region in picture 003. Where I would expect a minimal variation of any parameter at a certain radius. Except from that region the solution seems to be uniform at a certain radius. So I am wondering what causes this non uniform region and since it is located at a point where also the 60 degree domain of the guide vane is placed, I am suspecting it has to do something with the connection between the two domain. However, I cannot figure out what is wrong..
Attached Images
File Type: png 003.png (81.0 KB, 16 views)
File Type: png 004.png (44.9 KB, 15 views)
kwg is offline   Reply With Quote

Old   October 16, 2014, 06:32
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,844
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
I can see facets in your cross section. This suggests your mesh is very coarse. Can you show a plot of your mesh? Have you done a mesh refinement study?
ghorrocks is offline   Reply With Quote

Old   October 16, 2014, 06:58
Default
  #5
kwg
New Member
 
Koen
Join Date: Jan 2014
Posts: 11
Rep Power: 12
kwg is on a distinguished road
I have done similar simulations where I did a mesh refinement study. No problems occurred at the interface at that time and the results were good. Now as a start I used the same mesh settings as for those previous studies.
Attached Images
File Type: png 006.png (80.2 KB, 14 views)
File Type: png 007.png (92.5 KB, 17 views)
kwg is offline   Reply With Quote

Old   October 16, 2014, 19:19
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,844
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
The tet mesh in image 007.png looks pretty coarse. You will probably have to refine that.
ghorrocks is offline   Reply With Quote

Old   October 17, 2014, 05:39
Default
  #7
kwg
New Member
 
Koen
Join Date: Jan 2014
Posts: 11
Rep Power: 12
kwg is on a distinguished road
I tried to refine the mesh at the interface, but that didn't make any difference.

However, I managed to find a solution to the problem! Before I didn't have an 'Outlet Domain' in TurboGrid. By adding this outlet domain to the mesh the problem was solved. Now the conditions at the interface look uniform in the circumferential direction (see picture 008). However, I don't really understand why this solved the problem. Since the outlet was close to the trailing edge of the blade, I thought I didn't need an outlet domain. Do you know what is the function of this domain?

I agree that the mesh still looks relatively coarse. I will perform a sensitivity analysis on this, but the problem at the interface seems to be solved.
Attached Images
File Type: jpg 008.jpg (32.5 KB, 8 views)
kwg is offline   Reply With Quote

Old   October 17, 2014, 06:59
Default
  #8
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,844
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
You have not shown us the full geometry you are modelling (at least I don't think you have) so I cannot comment. You will need to show the full geometry.
ghorrocks is offline   Reply With Quote

Old   October 17, 2014, 08:07
Default
  #9
kwg
New Member
 
Koen
Join Date: Jan 2014
Posts: 11
Rep Power: 12
kwg is on a distinguished road
See picture 009 and 010 for the full geometry.

Initially I created a mesh in TurboGrid without an 'Outlet Domain', picture 011. Now what I did is I moved the outlet a little bit inwards, so that I could create an 'Outlet Domain' in the mesh, picture 012. That solved the problem, but I don't understand why..
Attached Images
File Type: png 009.png (51.3 KB, 6 views)
File Type: png 010.png (30.2 KB, 6 views)
File Type: png 011.png (70.3 KB, 12 views)
File Type: png 012.png (59.5 KB, 10 views)
kwg is offline   Reply With Quote

Old   October 18, 2014, 04:00
Default
  #10
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,844
Rep Power: 144
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
When you have the outlet so close to the blade I would expect the wake of the blade to go to the outlet. This will cause convergence difficulties. When you put in a GGI before the outlet it diffuses the wake (especially with some frame change options which do stage averaging) and reduces the problem.

But it is bad practise to put the outlet so close. You need it further downstream for reliable convergence and accuracy.
ghorrocks is offline   Reply With Quote

Reply

Tags
cfx 13, interface domains


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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
convergenceof natural convection prob. in cfx cpkewat CFX 15 January 31, 2014 07:29
CFX parallel hp MPI problem fluidmechanics CFX 5 June 19, 2013 20:05
Problem Importing Geometry ProE to CFX fatb0y CFX 3 January 14, 2012 20:42
CFX Casting problem: Can it be done all in one? jfasl CFX 1 September 14, 2010 04:14
CFX GGI Interface Error (non-overlapping) surge519 CFX 1 August 3, 2009 19:54


All times are GMT -4. The time now is 14:08.