[Nek5000-users] Effect of CFL number.

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

 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
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