[Nek5000-users] Internal boundary condition

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

Dear all,
I have a question regarding the internal boundary condition in the code.
My problem consists of a fan-shaped nozzle, which parts of its walls  
are located within the flow domain (a cylindrical tank with inflow/ 
outflow boundary conditions). The question is how to set the boundary  
conditions for such internal zero-thickness walls?

I thought the walls could be defined as internal faces by creating a  
side-set (in the Cubit) and setting the velocity and the temperature  
fields of these faces to 'W  ' and 'E  ', respectively, similar to  
the solid-fluid interface  in the conjugate heat transfer example.  
Nevertheless, there is a fluid at both sides of the interface in this  
case.
Below is part of the boundary conditions in .rea file.
.....
.....
1 0     	! 1 fluid set, 0 other/solid sets
10	    	! fluid set is 10
1	 	! block 10 is material 1
10      	! no. bc sets (side-sets); bc set id,
100  1    W  , 	(external walls)
100  2    I  ,	(external walls)
120  1    W  ,	(internal walls corresponding to the nozzle)
120  2    E  ,	(internal walls corresponding to the nozzle)
200  1    v  ,	(inflow)
200  2    t  ,	(inflow)
300  1    O  ,	(outflow)
300  2    O  ,	(outflow)
400  1    SYM,
400  2    SYM,
.....
.....

When I run this case, warning messages appear during assigning the  
sidesets to the element faces, saying that: "face shared by 2 Hexes  
(say) 2331 5227". It seems that this warning is only because I  
defined some faces as "internal", however, no "other solid/fluids"  
has been defined in the input file.
Nevertheless, the issue with such definition is that, although, the  
velocity field is zero at the internal walls, there seems to be a  
pressure gradient across the wall, which is not physically consistent  
with internal walls, and drives a flow through the walls.

It would be appreciated if anyone could help me on this problem.

Thanks,
Mohsen

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