[Nek5000-users] Turbulent shear layer

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

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


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