# Is this a contradiction between primary assumptions and CFD ?

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 September 12, 2012, 04:03 Is this a contradiction between primary assumptions and CFD ? #1 Member   mhp Join Date: Dec 2011 Posts: 33 Rep Power: 6 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.

September 12, 2012, 05:09
#2
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cfdnewbie
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Quote:
 Originally Posted by MHP 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...

 September 12, 2012, 05:13 #3 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 2,613 Rep Power: 33 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 ...

 September 12, 2012, 05:21 #4 Senior Member   Joern Beilke Join Date: Mar 2009 Location: Dresden Posts: 196 Rep Power: 10 The use of symmetrical boundary conditions just violates the reality.

September 12, 2012, 08:42
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mhp
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Quote:
 Originally Posted by cfdnewbie 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?
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Last edited by MHP; September 12, 2012 at 08:45. Reason: add a picture

 September 12, 2012, 09:02 #6 Member   mhp Join Date: Dec 2011 Posts: 33 Rep Power: 6 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?

September 12, 2012, 10:47
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Filippo Maria Denaro
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Quote:
 Originally Posted by MHP 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 ...

 September 12, 2012, 14:13 #8 Senior Member     Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 601 Blog Entries: 17 Rep Power: 20 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)

September 13, 2012, 03:30
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Joern Beilke
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Quote:
 Originally Posted by MHP 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:

September 13, 2012, 09:24
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mhp
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Quote:
 Originally Posted by FMDenaro 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?

September 13, 2012, 09:28
#11
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mhp
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Quote:
 Originally Posted by JBeilke "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...

September 13, 2012, 20:19
#12
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Chris DeGroot
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Quote:
 Originally Posted by MHP OK, so what happens when Poiseuille flow becomes unstable? Does it means that the mass flow rate fluctuates in time?
Turbulence happens

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