[Nek5000-users] Problem w/ outflow and DIV(V)

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

Hi Paul,
Thanks for your response.
In my coordinate, gravity points in -y direction. Nevertheless, the unit normal of the outflow is not exactly (0, -1, 0), since the outflow plane is a little tilted (i.e., n ~ (0, -0.99, -0.1). Should this plan be parallel to a coordinate plane?

Mohsen
________________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
Sent: Wednesday, May 28, 2014 8:39 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Problem w/ outflow and DIV(V)

Hi Mosen,

I'm assuming that gravity points in the -z direction for you?

If so, what is the value of the unit normal vector on your outflow plane?
Is it (0,0,-1), i.e., in the negative z direction?  Or something else?

Paul

________________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
Sent: Wednesday, May 28, 2014 6:48 PM
To: Nek 5000
Subject: [Nek5000-users] Problem w/ outflow and DIV(V)

Hello all,
I am working on a problem consisting of an axially-horizontal
cylindrical tank with an inlet (an opening) at its top and a
connected pipe to its bottom as an outlet. The boundary conditions
are set accordingly, i.e., inflow and outflow, respectively. Gravity
is in vertical direction. I need to use Pn-Pn formulation as I use
Moab for this problem.

Assuming a steady-state condition with a full tank, this problem can
be solved using single-phase NS solver. However, I noticed that there
is large mass conservation error, as indicated by warning of "DIV(V)-
QTL too large", when I run the case with a zero initial velocity
field. I expected that this error decreased as the time evolved, but,
it increased. Also a back-flow gradually starts to grow at the
outflow boundary as well.

I thought that this problem may be solved if I start from a realistic
initial condition, however, it is not trivial for this problem. I am
also skeptical about the zero pressure BC which is imposed at the
outflow in Pn-Pn formulation.  The other solution might be having an
inlet (but outward) velocity at the outlet.

So I wondered if anyone can give me a hint on this problem or the
solutions proposed.


Thanks,
Mohsen

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