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

Is this a contradiction between primary assumptions and CFD ?

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

Like Tree1Likes
  • 1 Post By cdegroot

Reply
 
LinkBack Thread Tools Display Modes
Old   September 12, 2012, 04:03
Question Is this a contradiction between primary assumptions and CFD ?
  #1
MHP
Member
 
mhp
Join Date: Dec 2011
Posts: 33
Rep Power: 5
MHP is on a distinguished road
Hi everyone;

Consider a 2D flow past a circular cylinder ,the geometry is symmetric, all B.C. are symmetric for both upper and lower part, but what happens that the solution is not symmetric for Re>100? How can this problem be explained with Navier Stocks equations? Is there any terms in NS equations that explain this?
With CFD programs like fluent it can be solved with both steady and unsteady solutions but the solutions for blunt bodies like cylinder and stream wise bodies like NACA0012 are different.For NACA0012 both solutions are the same but for cylinder aren't.
MHP is offline   Reply With Quote

Old   September 12, 2012, 05:09
Default
  #2
Senior Member
 
cfdnewbie
Join Date: Mar 2010
Posts: 551
Rep Power: 11
cfdnewbie is on a distinguished road
Quote:
Originally Posted by MHP View Post
Hi everyone;

Consider a 2D flow past a circular cylinder ,the geometry is symmetric, all B.C. are symmetric for both upper and lower part, but what happens that the solution is not symmetric for Re>100? How can this problem be explained with Navier Stocks equations? Is there any terms in NS equations that explain this?
I am not sure what you are asking, is it about transition to turbulence? instability mechanisms? Of course, they are all incorporated in the NS equations.... maybe I'm not getting your question...??

Quote:
With CFD programs like fluent it can be solved with both steady and unsteady solutions but the solutions for blunt bodies like cylinder and stream wise bodies like NACA0012 are different.For NACA0012 both solutions are the same but for cylinder aren't.
For a NACA 0012 just increase the Reynolds number and your flow will certainly become unsteady.... or increase the AOA and see what happens...

again not sure what puzzles you here...
cfdnewbie is offline   Reply With Quote

Old   September 12, 2012, 05:13
Default
  #3
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,585
Rep Power: 20
FMDenaro will become famous soon enough
I don't know to have understood your question but symmetry in the geometry (and BC) does not imply symmetry in the flow solution... the NS equations are non-linear ...
FMDenaro is offline   Reply With Quote

Old   September 12, 2012, 05:21
Default
  #4
Senior Member
 
Joern Beilke
Join Date: Mar 2009
Location: Dresden
Posts: 185
Rep Power: 9
JBeilke is on a distinguished road
The use of symmetrical boundary conditions just violates the reality.
JBeilke is offline   Reply With Quote

Old   September 12, 2012, 08:42
Post
  #5
MHP
Member
 
mhp
Join Date: Dec 2011
Posts: 33
Rep Power: 5
MHP is on a distinguished road
Quote:
Originally Posted by cfdnewbie View Post
I am not sure what you are asking, is it about transition to turbulence? instability mechanisms? Of course, they are all incorporated in the NS equations.... maybe I'm not getting your question...??


For a NACA 0012 just increase the Reynolds number and your flow will certainly become unsteady.... or increase the AOA and see what happens...

again not sure what puzzles you here...
I think maybe this issue is about instability mechanisms, How we can express instability with NS equations?
Yes you are true, but in the same low Re number (suppose Re=100) you will never see unsteady flow for NACA0012 but it is seen in circular cylinder. more close to our discussion, for a simple cylinder up to Re~40 the flow is steady but with an increase in the Re number flow became unsteady, what is the reason for this ?
I read somethings about karman vortex street but what is the main reason for these vortex creation?
Attached Images
File Type: jpg Steady Vs. Unsteady.jpg (45.3 KB, 8 views)

Last edited by MHP; September 12, 2012 at 08:45. Reason: add a picture
MHP is offline   Reply With Quote

Old   September 12, 2012, 09:02
Post
  #6
MHP
Member
 
mhp
Join Date: Dec 2011
Posts: 33
Rep Power: 5
MHP is on a distinguished road
So we shall never use symmetric BC for problems and it is wrong to model semi circular cylinder in a flow with symmetry wall in Re>~50, because the result for full cylinder is not symmetric,
Also another question is that what caused the flow pattern became different when the Re number increased while flow is laminar ?

Is it true that unsteady flows are unstable flows?
MHP is offline   Reply With Quote

Old   September 12, 2012, 10:47
Default
  #7
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 1,585
Rep Power: 20
FMDenaro will become famous soon enough
Quote:
Originally Posted by MHP View Post
So we shall never use symmetric BC for problems and it is wrong to model semi circular cylinder in a flow with symmetry wall in Re>~50, because the result for full cylinder is not symmetric,
Also another question is that what caused the flow pattern became different when the Re number increased while flow is laminar ?

Is it true that unsteady flows are unstable flows?

in poor words, instability is when a solution, for example steady, when perturbed from its condition moves to a different solution instead of returning to be as the previous.
As example, the Poiseuille solution is valid for any Reynolds number but is not stable for all the Re numbers ...
FMDenaro is offline   Reply With Quote

Old   September 12, 2012, 14:13
Default
  #8
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 518
Blog Entries: 14
Rep Power: 17
sbaffini will become famous soon enough
If you want more puzzles, consider also that a general symmetric laminar and steady flow can be unstable and find a more stable state which is asymmetrical (of course, at some level, even if infinitesimal and not deterministically reproducible, there is a perturbation somewhere to have this situation)
sbaffini is offline   Reply With Quote

Old   September 13, 2012, 03:30
Default
  #9
Senior Member
 
Joern Beilke
Join Date: Mar 2009
Location: Dresden
Posts: 185
Rep Power: 9
JBeilke is on a distinguished road
Quote:
Originally Posted by MHP View Post
So we shall never use symmetric BC for problems ...
"Never say never Again" ;-)

Most of the time you are in a loose-loose situation. An one side you know that you can not solve the problem without simplifications. On the other side you know that simplification might lead to uncertain results. Now it's up to you how to go on.

Here are some nice animations of flow configurations with symmetrical geometries:

http://www.youtube.com/watch?v=5lSvDKv7sTg
http://www.youtube.com/watch?v=_TXRXYxUGU8
http://www.youtube.com/watch?v=QXaSmkAkqzc
JBeilke is offline   Reply With Quote

Old   September 13, 2012, 09:24
Default
  #10
MHP
Member
 
mhp
Join Date: Dec 2011
Posts: 33
Rep Power: 5
MHP is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
in poor words, instability is when a solution, for example steady, when perturbed from its condition moves to a different solution instead of returning to be as the previous.
As example, the Poiseuille solution is valid for any Reynolds number but is not stable for all the Re numbers ...
OK, so what happens when Poiseuille flow becomes unstable? Does it means that the mass flow rate fluctuates in time?
MHP is offline   Reply With Quote

Old   September 13, 2012, 09:28
Default
  #11
MHP
Member
 
mhp
Join Date: Dec 2011
Posts: 33
Rep Power: 5
MHP is on a distinguished road
Quote:
Originally Posted by JBeilke View Post
"Never say never Again" ;-)

Most of the time you are in a loose-loose situation. An one side you know that you can not solve the problem without simplifications. On the other side you know that simplification might lead to uncertain results. Now it's up to you how to go on.

Here are some nice animations of flow configurations with symmetrical geometries:

http://www.youtube.com/watch?v=5lSvDKv7sTg
http://www.youtube.com/watch?v=_TXRXYxUGU8
http://www.youtube.com/watch?v=QXaSmkAkqzc
Thank you for your comment and movies...
MHP is offline   Reply With Quote

Old   September 13, 2012, 20:19
Default
  #12
Senior Member
 
Chris DeGroot
Join Date: Nov 2011
Location: Canada
Posts: 387
Rep Power: 6
cdegroot is on a distinguished road
Quote:
Originally Posted by MHP View Post
OK, so what happens when Poiseuille flow becomes unstable? Does it means that the mass flow rate fluctuates in time?
Turbulence happens
sbaffini likes this.
cdegroot 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
CAD-free CFD Herbert Main CFD Forum 4 July 25, 2005 09:31
CAD-free CFD Herbert FLUENT 1 July 25, 2005 09:13
CAD-free CFD Herbert Phoenics 0 July 24, 2005 12:01
Errors in CFD Lily Kabanj FLUENT 8 May 1, 2000 07:52
CFD Design...The CFD Future John C. Chien Main CFD Forum 19 October 6, 1999 11:57


All times are GMT -4. The time now is 10:20.