[Nek5000-users] Perturbation mode

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

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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