[Nek5000-users] perturbation mode

[Paul Fischer]

Hi Antonios,

I'll look into this.

One key thing is that the perturbation code is designed
to solve for perturbations about an evolving base flow,
so that one can compute Lyapunov exponents....

You should be able to override this by continually resetting
the base flow to the desired steady state base flow.

We could also likely modify the code to work in the mode
that you desire (i.e., by simply turning off the evolution
of the base flow).

Paul


On Fri, 15 May 2009, Antonios Monokrousos wrote:

> Hello!
> I have a question about the pertubation mode of the code.
>
> First I computed the base flow using the full non-linear solver and
> letting it run until I get a steady solution. This worked fine.
> Then I construct a perturbation which I add on the steady solution I got
> before (base flow) with a very small amplitude, around 0.1%. The
> solution, after I subtract the base flow looks good. (I'm comparing to a
> different code).
>
> Next I try to use perturbation mode since I want to use the exact
> linearised Navier-Stokes and the problems appear.
> I have the base flow and the perturbation in two different restart
> files, which I specify in .rea file like this:
> ------------------------------------------------------
>            2 PRESOLVE/RESTART OPTIONS  *****
> meanflow.u
> ic_pert_field.fld
> ------------------------------------------------------
> I activate the perturbation mode (option 31 in .rea).
>
> I damp both the solution of the base-flow and of the pertubation. I can
> see that the code is solving for the base-flow (which it shouldn't) but
> also has as initial condition the one that is supposed to be for the
> The simulation blows-up after a few time steps.
>
> I'm not sure I use the perturbation mode in the right manner.
> Thanks in advance for any feedback or help.
> Antonios
>
>
> -- 
> ---------------------------------------------------------------------------------------------
> Antonios Monokrousos
> Department of Mechanics                 email:    antonios at mech.kth.se
> Royal Institute of Technology (KTH)     phone:    +46 8 790 68 76
> Osquars Backe 18                        telefax:  +46 8 796 98 50
> SE-100 44 Stockholm, SWEDEN
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