[Nek5000-users] Effect of CFL number.

Wed May 28 03:21:48 CDT 2014

Thanks Mr.Fischer and Mr.Schlatter for your brief explanation.

Best Regards,
Kamal

On 28/05/2014 00:43, nek5000-users at lists.mcs.anl.gov wrote:
> Hi Kamal,
>
> Not only different numerical methods have different transitions, but different
> experiments as well.   The point here is that the sensitivity you observe
> (across numerical methods, or across spatial/temporal resolution for a
> given method, or across the initial condition, or across different experiments)
> is due to physical phenomena, modulated by the method of investigation.
>
> If we take, say, an experiment as a gold standard, the question is: which one?
>
> Even Osborne Reynolds experimental apparatus does not give the same
> transition point today as it did when he did his experiments in the 1800s.
> Why?   Vibrations due to truck traffic in the street...    That input is not
> accounted for in any numerical simulations of pipe flow.
>
> The problem of course is that this is a subcritical bifurcation, unlike
> Rayleigh-Benard, for which the transition is triggered by a classic
> linear instability and is therefore repeatable.
>
> 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 4:27 PM
> To: nek5000-users at lists.mcs.anl.gov
> Subject: Re: [Nek5000-users] Effect of CFL number.
>
> 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
>>>> mailing list Nek5000-users at lists.mcs.anl.gov
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>>>>
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