[Nek5000-users] Perturbation Mode

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

Hi David,

The perturbation solver was developed roughly 10 years back
and hasn't been extensively exercised in recent years.  As it 
is updated only when used (as you guys are doing now) there may
be some maintenance that is overdue.

There is no intrinsic reason why the perturbation solver
should need a smaller timestep than the nonlinear one, so
it sounds as though something is amiss; and most likely
this is something that is fairly readily remedied.

Some questions:

   .Are you using IFCHAR = true for your nonlinear solver?

    (I'm not certain of the status of IFCHAR + IFPERT at the
     moment.)


Given that I'll be there next week we can look at this first-hand.
I understand that you guys have already encountered (and perhaps
resolved?) some issues with the perturbation solver, so hopefully
we can get it all straightened out.

If you'd like to send me the case files, I can take a quick look
now.  (You can email me off-list...)

I look forward to meeting you next week!

Best regards,

Paul

On Mon, 3 Jan 2011, nek5000-users at lists.mcs.anl.gov wrote:

> Hi Neks,
>
> We have been running the perturbation mode in order to study the linear
> evolution of disturbances in a couple of 2D boundary layer cases and are
> now setting up a 3D boundary layer case.
>
> Already for the 2D cases we experienced that the timestep needs to be
> decreased significantly compared to a corresponding nonlinear simulation
> in order to yield a numerically stable simulation. For the nonlinear
> simulations we put the disturbance with a small amplitude on top of the
> baseflow. Hence, running the perturbation mode for the 2D cases was more
> expensive than running a nonlinear simulation but still reasonable.
>
> However, the 3D case seems to become much more expensive. We first ran a
> nonlinear simulation as described above and compared the disturbance
> evolution to results of the parabolised stability equations which
> matched perfectly.
> Running then the perturbation mode for the same disturbance with the
> same base flow did not yield a stable simulation even when the timestep
> was reduced (divided by 5). The disturbance amplitude exploded very
> early. We then took a higher box and increased the resolution which made
> things a little better meaning that the simulation exploded much later.
> However, it is still not stable although the resolution is higher and
> the timestep is much smaller compared to the corresponding nonlinear
> simulation which worked fine. Also, the number of pressure iterations is
> around 20-30 times higher.
>
> Is there any reason why the linear stability equations in nekton should
> behave so much different than the full Navier-Stokes? Are there some
> parameters (filtering etc.) that have to be used differently in this case?
>
> Best regards,
>
> David
>
> -- 
> David Tempelmann
> Linné Flow Center, Mechanics KTH
> SE-100 44, Stockholm, Sweden
> Phone: +46 8 7907161
> E-mail: david at mech.kth.se
>
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