[Nek5000-users] Restarting simulation in same run

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

Thanks a lot. That will be a great help. Currently I am doing it calling
nek via system function.

Best regards
Praveen

On Monday, October 27, 2014, <nek5000-users at lists.mcs.anl.gov> wrote:

>  Hi Parveen
>
> There is implementation of arnoldi time-stepper for spectra calculation
> with parpack for nek5000. I think I've already sent you some info about it.
> The code you can find at
> ftp://ftp.mech.kth.se/pub/adam/Nek5000/toolbox/
> There are sfd and arnoldi set-ups based on ext_cyl example form
> repository. The restart of the stepper you can find in the code. We do it
> by setting istep to 0 (forces solver to go through BDF1->BDF2->BDF3->BDF3)
> and resetting some vectors to zero.
> Best regards
> Adam
>
> On 22/10/14 15:07, nek5000-users at lists.mcs.anl.gov
> <javascript:_e(%7B%7D,'cvml','nek5000-users at lists.mcs.anl.gov');> wrote:
>
>  Hello Paul
>
>  I dont want to restart in the middle.
>
>  Every time I want to start the time stepping from scratch. So I want to
> know how to reset everything for a fresh simulation. Every time ARPACK
> gives me a vector, I have to start with BDF1->BDF2->BDF3->BDF3... until
> final time.
>
>  I think accuracy of time stepper also matters since we are approximating
> exact evolution operator exp(A*t) with the time stepper.
>
>  I have read some papers on time stepping approach but the details are
> not clear.
>
>  Thanks
> praveen
>
> On Wed, Oct 22, 2014 at 5:06 PM, <nek5000-users at lists.mcs.anl.gov
> <javascript:_e(%7B%7D,'cvml','nek5000-users at lists.mcs.anl.gov');>> wrote:
>
>> Hi Praveen,
>>
>> I would guess you need to develop your own timestepper because
>> Nek uses BDFk & EXTk  to time march, where typically k=3.  At
>> start-up, it uses k=1 and k=2 to bootstrap.   If you start timestepping
>> in the middle of the simulation then you need to overwrite the
>> existing coefficients so that you can restart the timestepper,
>> plus you need to store your initial condition (which presumably
>> amounts to overwriting vx,vy,vz, and pr).
>>
>> There are others in the community who have done these kinds of
>> things (I have not), who might have some further comments.
>>
>> Best,
>>
>> Paul
>>
>> PS ----- Are you trying to find eigenvalues associated with
>> the spatial operators ?   i.e., does the accuracy of the
>> timestepper matter?  If not, you can likely get away with
>> k=1, in which case you don't need much history in the backward
>> difference formula or extrapolation.  Pressure, however, is
>> a bit of a tricky business because we use splitting and there
>> is a residual O(dt^k) impact...
>>
>
>
>
>
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