[Nek5000-users] Effect of CFL number.

[nek5000-users at lists.mcs.anl.gov]

Hi Kamal,
Depending on what you want to do, I would stay away from the 
transitional region altogether, as you also might see spatial 
localisation which is maybe not what you want. The standard case for 
low-Re turbulence, which is sustained even in small boxes, is the 
Re_tau=180 case, which corresponds to Re_b=5300 or ReD=3464 
(approximately). At that Re you also have a good deal of data to compare to.
Best regards,
Philipp

On 2014-05-27 23:11, nek5000-users at lists.mcs.anl.gov wrote:
> Thanks Paul and Philipp. I have small cross wise velocity of v =
> 0.001 as a perturbation at the inlet to trigger the turbulence but as
> you said the sensitivity of the system determines the transition.
>
> Mr.phillip If you don't mind, could you please tell me at above which
> Re I can expect a sustained turbulence independent of the Mesh and
> order , Because I tend to see different Numerical methods (FVM,FEM)
> have different critical point of transition for a similar case .
>
> Thanks in advance for your replies
>
> Kamal
>
> On 27 May 2014, at 23:01, nek5000-users at lists.mcs.anl.gov wrote:
>
>> Dear all, just to add also the domain length to Paul's answer;
>> Re=2000 for pipe flow is in fact slightly below the "critical"
>> Reynolds number for sustained turbulence. In a shorter pipe,
>> turbulence will decay even faster. This indicates, as Paul
>> mentions, the sensitivity to initial conditions and geometry, even
>> though I would expect all DNS to eventually decay to laminar at
>> that Re.
>>
>> Best regards, Philipp
>>
>> On 2014-05-27 18:35, nek5000-users at lists.mcs.anl.gov wrote:
>>>
>>> Hi Kamal,
>>>
>>> In my view, you are trying to run in a regime that is actually
>>> not reproducible. You are right at the transition Re, which
>>> implies strong sensitivity to initial conditions and under such
>>> circumstances you should not expect two simulations at two
>>> different resolutions to converge to the same result, just as you
>>> would not expect two turbulent simulations to follow the same
>>> trajectory point-by-point, only on average.
>>>
>>> Paul
>>>
>>> ________________________________________ From:
>>> nek5000-users-bounces at lists.mcs.anl.gov
>>> [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of
>>> nek5000-users at lists.mcs.anl.gov
>>> [nek5000-users at lists.mcs.anl.gov] Sent: Tuesday, May 27, 2014
>>> 11:23 AM To: nek5000-users at lists.mcs.anl.gov Subject:
>>> [Nek5000-users] Effect of CFL number.
>>>
>>> Dear Neks,
>>>
>>> I would like to know what is the effect of CFL number on a
>>> turbulence simulation in SEM.
>>>
>>> I use polynomial order of lx1 = 5 and lxd = 8 and get a C = 0.176
>>> for a Re = 2000 with dt = 0.001  in pipe flow where I find a
>>> transition to turbulence. when I decrease my time step 'dt =
>>> 0.0001 ' further my pipe becomes fully turbulent for the same
>>> mesh.
>>>
>>> When I increase the order lx1 = 7 and lxd = 10, I find a
>>> completely steady solution, instead of a transition to
>>> turbulence.
>>>
>>> It would be nice what if some could please tell me what
>>> parameter determines the accuracy of the simulation in terms of a
>>> turbulent flow because I am not able to reproduce the result with
>>> a different mesh or with higher order.
>>>
>>> Thank you,
>>>
>>> Kamal _______________________________________________
>>> Nek5000-users mailing list Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>> _______________________________________________ Nek5000-users
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