[Nek5000-users] Effect of CFL number.

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

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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