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

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

Hi Mohsen,

For incompressible Navier-Stokes, you can add a body force such that the pressure will be
uniform at outflow in the no-flow configuration.  It's important to do this --- see Eq. (13) on p. 9
of

http://www.mcs.anl.gov/~fischer/nek5000/examples.pdf

(which is accessible from the Nek5000 documentation page).

If you don't compensate for the variance in pressure force due to the hydrostatic
load then you will indeed find backflow coming in via the outflow boundary.

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 8:35 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] Problem w/ outflow and DIV(V)

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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