[Nek5000-users] Full restart with a different time step

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

Hi

For full_restart you should have four velocity fields and you could use 
nek5000 as a preprocessing tool. You can simply load all 4 fields in 
userchk, perform some kind of interpolation in time and finally write 
down new restart files. The question is how to deal with pressure. One 
solution would be simply interpolate pressure saved in the files. The 
other would be to run modified version of pressure solver with uzawa 
turned on to get pressure corresponding to interpolated velocity field. 
We've played with uzawa for pn-pn-2 and we use our version of 
full_restart, but we haven't done this kind of modified restart, so I 
don't know what is better. On the other hand you could restart nek with 
your current checkpoints and dt corresponding to the old simulation, and 
later (after 4th step) gradually decrease time step. I guess this would 
be the simplest and probably best solution. If time resolution in old 
simulation was insufficient, your restart files do not give correct 
evolution of the field and interpolation in time would not remove it. 
Anyhow you introduce some initial error to simulation. In this case I 
probably would simply play with dt keeping it constant during first 4 
steps and gradually decreasing later.
Regards
Adam

On 2015-11-16 20:19, nek5000-users at lists.mcs.anl.gov wrote:
>
> Dear Neks,
>
>
> I use full_restart option for my current simulations. One simulation 
> crashed after restarting for some time with the Courant number going 
> crazy. I suppose I will need to reduce the time step size.
>
>
> Basically, I want to continue my simulation using full_restart option 
> but with a smaller time step size. Somehow I remember I saw one post 
> saying one shouldn't change the time step size when using the 
> full_restart option. I'm just thinking of a way to work around the 
> problem.
>
>
> So does anyone have ideas on how I can interpolate the flow fields 
> between two time steps or any other ways to fully restart a simulation 
> with a different time step size?
>
>
> Thank you very much in advance for any suggestions or comments.
>
>
> Best regards,
>
> Tony
>
>
>
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