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

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

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