[Nek5000-users] Turbulent shear layer

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

Alex,

It seems that the reason you're unable to subscribe is that
email to your return address is rejected?

Can you try from a different email address?

>From your description below - my initial guess about what is
happening is that the 2D case probably should relaminarize,
whereas 3D would not.   In the case where it does relaminarize,
if it also ultimately reaches a steady state, then a flaw in
my implementation of the projection scheme (Fischer CMAME '98)
will result in linearly-dependent vectors and blow-up.  I expect
that this is the issue you encounter at large step counts. 
It's something that is on our to-be-fixed list, but it's not
encountered in unsteady cases that constitute the bulk of the
code usage.

Hope this helps.

Paul



On Tue, 3 May 2011, nek5000-users at lists.mcs.anl.gov wrote:

> Hi,
>
> I'm trying to set up a turbulent shear layer similiar to the test cases used
> in http://www.springerlink.com/content/h7263g6268875412/fulltext.pdf (DNS of
> Compressible Inert and Infinitely Fast Reacting Mixing Layers, Mahle,
> Sesterhenn, Friedrich; New Res. in Num. and Exp. Fluid Mech. VI, NNFM 96, pp.
> 372-380, 2007) and http://maeresearch.ucsd.edu/SARKAR/PantanoSW_jfm_03.pdf
> (Mixing of a conserved scalar in a turbulent reacting shear layer, Pantano,
> Sarkar, Williams; J. Fluid Mech. (2003), vol. 481, pp. 291-328). However, my
> simulations converge to the laminar flow and/or explode. I know that the
> papers used compressible flows, but I hoped that it may work for
> incompressible flows as well. I'm not interested chemical reactions since I
> just want to observe the mixing of passive scalars in a turbulent flow using
> DNS.
>
> It is a 2D test case at the moment so it can be run on my small workstation
> (only 4 Cores). It is a channel ([0,2]x[0,1]) with periodic boundary
> conditions in x-direction and moving walls with velocity u=1 on the bottom
> side of the mesh and u=-1 on the upper side. The flow is initialized with a
> hyperbolic tangent profile with random perturbations added. The mesh has
> 64x32 elements with p=9. I made some runs with to different settings. One
> case is only the unsteady NS equation and in the second case the temperature
> and one passive scalar are added.
>
> In the first case, the flow gets laminar and after approx. 269000 iterations
> the solution explodes. In the second case, the solver has trouble with the
> flow although I use the same initial and boundary conditions for the velocity
> field. It takes 384 time steps until the divergence of the velocity field is
> smaller than 1e-1 while in the first case it only takes six time steps.
> Furthermore, the Helmholtz solver for the passive scalar fails right from the
> beginning. It does not matter if I initialize the scalar with zeros or
> something else. The temperature field doesn't make problems like this. The
> velocity field also looks a bit strange. You can identify the edges of the
> elements and the solution begins to blow up pretty soon (approx. 20000 time
> steps).
>
> The meshes was generated using genbox and prenek/prex.  All files are
> attached to the mail.
>
> Has anyone experienced a similar problem or can give me a hint what I'm doing
> wrong? I also tried other cases, but the flows always gets laminar or
> explodes so it is seems to be my fault. If anyone has an idea for a
> better/easier test case I would appreciate any suggestions. I just want a DNS
> of a turbulent flow in an easy geometry that can be handled with about 32
> CPUs when running as 3D case.
>
> Just two more things:
> 1. Why can't I join the mailing list? I think I subscribed 2-3 times, but I
> don't get any mails.  2. Where can I report (probably) useful information or
> small bugs that are not directly related to the solver?
>
> Thanks!
> Alex
>
>
>