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

bondary conditions

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 23, 2009, 09:24
Unhappy bondary conditions
  #1
Member
 
nazareno mancinelli
Join Date: Mar 2009
Location: argentina
Posts: 35
Rep Power: 7
yochule is on a distinguished road
hi all...
i need to confirm what's the problem in my set up.
i have two domain: one thatīs is the whole inlet, outlet and stator, this domain is stationary. and the second is only around the rotor, this domain is rotating with the rpm of the rotor.
i set up one interface. this interface is only one surface. i set up it in: Interface Models: General Connection. Frame Change: Frozen Rotor.
i put the inlet with 350Pa and the outlet with 0Pa relative.
when i see the results it seems to be wrong, but i donīt know where is the probmem...
can anybody help me....
thanks!
Attached Images
File Type: jpg Dibujo1.jpg (37.4 KB, 42 views)
File Type: jpg Dibujo2.jpg (57.0 KB, 40 views)
File Type: jpg Dibujo3.jpg (103.3 KB, 46 views)
yochule is offline   Reply With Quote

Old   July 23, 2009, 18:33
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 9,383
Rep Power: 74
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
What seems to be wrong?

If you mean the streamlines going wild in the rotating section it is because you need to calculate the streamlines using the "Velocity in Stationary Frame" variable.

Glenn Horrocks
ghorrocks is offline   Reply With Quote

Old   July 24, 2009, 08:49
Default
  #3
Member
 
nazareno mancinelli
Join Date: Mar 2009
Location: argentina
Posts: 35
Rep Power: 7
yochule is on a distinguished road
hello glenn.
ok, you're right. but if i plot the velocity in the stationary frame i have streamline leaked (cut down in the interface) and this seems odd. i have this cut near the interface in all plots? (i plot an isosurface of turbulence kinetic energy and see the same).
i expect that the sreamlines have some curvature, but not that have a break. and the streamline in the first face would be the same number that in the back face....
are i'm wrong?
Attached Images
File Type: jpg Dibujo4.jpg (54.7 KB, 33 views)
File Type: jpg Dibujo5.jpg (52.7 KB, 29 views)
yochule is offline   Reply With Quote

Old   July 25, 2009, 06:36
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 9,383
Rep Power: 74
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Hi,

I can see the facetting of the inlet and outlet ducts and the mesh is very coarse. You are never going to get anything believable with a mesh as coarse as that. Start refining the mesh and things should behave themselves. Do a sensitivity analysis to determine how fine the mesh needs to be for the accuracy you require.

Anyone - we really need to write a FAQ on the basics of checking mesh, timestep and residuals convergence and sensitivity for these type of basic questions. 90% of the "my simulation is not accurate" questions do not do the basics here - any takers?

Glenn Horrocks
ghorrocks is offline   Reply With Quote

Old   July 25, 2009, 06:55
Default
  #5
Senior Member
 
ckleanth's Avatar
 
George
Join Date: Mar 2009
Location: Birmingham, UK
Posts: 257
Rep Power: 8
ckleanth is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
Anyone - we really need to write a FAQ on the basics of checking mesh, timestep and residuals convergence and sensitivity for these type of basic questions. 90% of the "my simulation is not accurate" questions do not do the basics here - any takers?

Glenn Horrocks
.............
__________________
Top 4 tips
1. Knowledge is everything and Ignorance is dangerous.
2. Understand your limitations and try to eliminate them.
3. Get yerself a bike and hoon the chuffer. You will soon learn why dogs like to hang their heads out the car window.
4. Please before asking any questions on how to run simulations in CFX, go though all the tutorials
ckleanth is offline   Reply With Quote

Old   July 27, 2009, 11:42
Default mesh analysis
  #6
Member
 
nazareno mancinelli
Join Date: Mar 2009
Location: argentina
Posts: 35
Rep Power: 7
yochule is on a distinguished road
hello guys..
i'm now running a test with a more small mesh...
how can i determine the quality of the mesh before the running? itīs there a tool like in FLUENT to see the best and the worse elements of my mesh?

i make a coarse mesh in the ducts far away of the blades because i think in this place the pressure and velocity gradients will be smooth...but, well iīm now be sure of it...i will post the results..
yochule is offline   Reply With Quote

Old   July 29, 2009, 13:45
Default the same thing
  #7
Member
 
nazareno mancinelli
Join Date: Mar 2009
Location: argentina
Posts: 35
Rep Power: 7
yochule is on a distinguished road
i have extended the domain and drawing the pathlines, but i still can see a break in the interface with "velocity", when i draw it with "velocity in Stn Frame" some pathlines were missing...it is normal?
Attached Images
File Type: jpg Dibujo11.jpg (67.6 KB, 25 views)
File Type: jpg Dibujo10.jpg (59.4 KB, 22 views)
yochule is offline   Reply With Quote

Old   July 29, 2009, 19:08
Default
  #8
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 9,383
Rep Power: 74
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
The streamlines are behaving as expected. The Velocity variable is in the local frame of reference so when you go through a change of reference frame streamlines drawn with the velocity variable will have a kink. Streamlines drawn using Velocity in Stn Frame are continuous but do hit the blades on occasion as the blades are rotating.

If you don't get this concept I recommend you only use streamlines inside a single domain, and always use the "velocity" variable. Then the streamlines will behave as expected and not hit walls.

Glenn Horrocks
ghorrocks is offline   Reply With Quote

Old   July 30, 2009, 09:26
Default interesting
  #9
Member
 
nazareno mancinelli
Join Date: Mar 2009
Location: argentina
Posts: 35
Rep Power: 7
yochule is on a distinguished road
i was thinking about the linepath.... i'm agree about the "velocity" drawn of streamlines. the difference of velocity of the coordinate system of both domain make the streamlines to break.
but, i still donīt understand the picture of "velocity in stationary frame". i think it would be exactly if i see the device in my hands, i expect to see all the lines reach the end. there is not a delta T, so i can't say that the line is cut because the particle reach this position in the end of analysis....
where can i read about this?
yochule is offline   Reply With Quote

Reply

Tags
bondary, domain, rotating, rotor

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
Implementation of boundary conditions for FVM Tom Main CFD Forum 7 August 26, 2014 05:58
Runtime changing of bondary conditions evrikon OpenFOAM Running, Solving & CFD 2 November 24, 2009 19:53
Fluent accuracy and boundary conditions Paolo Lampitella FLUENT 0 June 12, 2008 06:25
How to apply bondary conditions only having mesh. mitul CFX 1 February 10, 2007 04:28
A problem about setting boundary conditions lyang Main CFD Forum 0 September 19, 1999 18:29


All times are GMT -4. The time now is 21:55.