[Nek5000-users] Restarting simulation in same run

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

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> 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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