[Nek5000-users] Set gradient and/or pressure at outlet

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

Dear Nek users, I've been struggling to program an adecuate outlet BC. I
simulate a vertical isothermal surface in an open domain, forming a free
convection boundary layer. When there are vortex leaving the domain, the
code diverges. I've tried turb_outflow subroutine, stabilized outflow BC
(Dong 2014), convective BC (sommerfield radiation condition, thanks to
Phillip and Eric, from this comunity), etc. But I can't achieve to make the
code run propperly, and I can't go on with my research while I have this
problem.

There is a paper, from Desrayaud (2013), "Benchmark solutions for natural
convection flows in vertical channels submitted to different open boundary
conditions".

I want to try these BC, which can be stated as: (x: horizontal and
y:vertical.)

At the top face (outlet)

Ux=0
dUy/dy=0

If Uy > 0 (fluid going out)
dTEMP/dy=0
pressure=0

If Uy < 0 (fluid going in because there is backflow)
TEMP=0
pressure= -0.5Uy^2 or 0   (these are two options in the paper)



As far as I know, in Pn-Pn-2 formulation there are not pressure BC, but it
is possible to set it using Pn-Pn. Am I right? If so, how can I set BC for
pressure in Pn-Pn?

And for the velocity gradient and temperature gradient equal cero at the
top face, how can it be done? Would it be enough to set dirichlet BC and
replicate the last row of values to the last one?

Any help is welcomed, thanks in advance,

Juan Pablo.
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