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-   -   Is this a contradiction between primary assumptions and CFD ? (http://www.cfd-online.com/Forums/main/106904-contradiction-between-primary-assumptions-cfd.html)

 MHP September 12, 2012 04:03

Is this a contradiction between primary assumptions and CFD ?

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.:rolleyes:

 cfdnewbie September 12, 2012 05:09

Quote:
 Originally Posted by MHP (Post 381342) 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.:rolleyes:
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...

 FMDenaro September 12, 2012 05:13

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 ...

 JBeilke September 12, 2012 05:21

The use of symmetrical boundary conditions just violates the reality.

 MHP September 12, 2012 08:42

1 Attachment(s)
Quote:
 Originally Posted by cfdnewbie (Post 381353) 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?

 MHP September 12, 2012 09:02

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?

 FMDenaro September 12, 2012 10:47

Quote:
 Originally Posted by MHP (Post 381401) 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 ...

 sbaffini September 12, 2012 14:13

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)

 JBeilke September 13, 2012 03:30

Quote:
 Originally Posted by MHP (Post 381401) 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:

 MHP September 13, 2012 09:24

Quote:
 Originally Posted by FMDenaro (Post 381432) 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 September 13, 2012 09:28

Quote:
 Originally Posted by JBeilke (Post 381534) "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...

 cdegroot September 13, 2012 20:19

Quote:
 Originally Posted by MHP (Post 381620) 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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