[Nek5000-users] Perturbation mode

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

Hi Jean-Christophe,
One thing we found a couple of months ago is that the pertubation mode
is unstable with time scheme of 3rd order.
Try to switch to 2nd order, change in the rea file line 31 (TORDER)
Regards,
Antonios


nek5000-users at lists.mcs.anl.gov wrote:
> Hi Nek's,
>
> I am still struggling with the perturbation mode.
> Cavity_20x20x10_6x6x6_R1000_25D0.f00001 contains my base flow. It is a
> 2D lid-driven cavity extruded in the third direction (z).
> Perturbation_20x20x10_6x6x6_25D0.f00001 contains my divergence-free
> perturbation. It is subject to no-slip conditions at the walls and
> periodic BC at zmin and zmax.
>
> Following is how I set it up.
>
> **********************************************
>
> In my blah.rea
>
> -1.           p31 NPERT: #perturbation modes
>
> [...]
>
> 2 PRESOLVE/RESTART OPTIONS *****
> Cavity_20x20x10_6x6x6_R1000_25D0.f00001
> Perturbation_20x20x10_6x6x6_25D0.f00001
>
> **********************************************
>
> In SIZE:
>
> parameter (lpelv=lelv,lpelt=lelt,lpert=1) ! perturbation
> parameter (lpx1=lx1,lpy1=ly1,lpz1=lz1) ! array sizes
> parameter (lpx2=lx2,lpy2=ly2,lpz2=lz2)
>
> **********************************************
>
> I have tried with various initial amplitude for the perturbation, and
> various time steps. However, I always have a very small CFL and after
> six timesteps I get:
>
>     Failed in HMHOLTZ: VELX NAN
>
>
> Same for vely and velz. I may have missed a key point but being not
> particularly familiar with Nek numerics yet I'm not sure where I
> missed it.
>
> Best regards,
> -- 
> Jean-Christophe
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>
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