[Nek5000-users] Heat/passive scalar solver - Helmholtz

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

Hi Paul,

thank you for your answer. What I misunderstood was the part 'dt will not exceed param(12)’. I was setting param(12) to a small value.

Thanks,
Marco 
> On Aug 22, 2016, at 3:28 PM, nek5000-users at lists.mcs.anl.gov wrote:
> 
> 
> Hi Marco,
> 
> If param(12) is positive, then it means that
> 
>        dt will not exceed param(12).
> 
>        and that
> 
>         dt will be chose such that CFL ~ CFL_target (specified in the .rea file).
> 
> Thus, if you have:
> 
> 
>      .001              p12 DT
>        :                :
>      0.5000000         p26 COURANT/NTAU
> 
> then dt will be the min of (.001, dt_cfl ), where dt_cfl is such that CFL=0.5 (p26).
> 
> Note that there is some tolerance in resetting dt ... i.e., I think you can get to CFL=0.55
> before it would reset dt to target CFL of 0.5
> 
> Similarly, it would not increase dt until CFL < 0.45.
> 
> ONE ADDITIONAL POINT:
> 
> If you are using characteristics, then COURANT (p26) takes on a different meaning.  
> 
> In that case, it means how many substeps the characteristics scheme takes for each
> Navier-Stokes timestep. (1, 2 or 3 are most common values, but it can vary from 
> case to case...)
> 
> See:   www.mcs.anl.gov/~fischer/new.pdf  , last section on timestepping, for a 
> description of characteristics as implemented in Nek.
> 
> 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: Monday, August 22, 2016 1:19 PM
> To: nek5000-users at lists.mcs.anl.gov
> Subject: Re: [Nek5000-users] Heat/passive scalar solver - Helmholtz
> 
> Dear Neks,
> 
> I am a little confused by the setup in the rea file to run a simulation with a constant cfl. My understanding (from the Nek5000 archives) is that by setting p012 to a positive value in the rea file, it should be sufficient to ensure a constant cfl condition during the run. When I do so, my time step is constant and thus my cfl varies.
> 
> I tried to set p012 to a positive value in different rea file from the examples directory but none of them yield a constant cfl during the run.
> 
> If someone has an answer to this problem I would really appreciate the help.
> 
> Thanks,
> Marco
> _______________________________________________
> 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
> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>