a question on why output pressure has set fixedValue as minimum of it?
 

I have used swak4FOAM for pressure in outlet patch(named right)
and I expect all of(max,min and average) be constant what I assigned in boundary condition fixedValue:857198.8151.but the output is shown like this(some part is shown here):
how the way does fixedValue work then?
does it set the fixedValue only as a lower limit of that variable?

I saw some region of back flow in outflow region.could this issue be a result of backflow?
then how does OF calculates the pressure?

no one has had any experience?is this situation right or false?
I have set pressure at both inlet and outlet.
Maybe this is the cause?

i performed it again after sone changes.
Now values are most what i have assigned but in some times they rebel!from what i dictate!
Really why?
Shouldn't patch values be exactly what we put in fixedValue?

Hi Ehsan,

I vaguely remember having had similar problems in the past. One such situation was when the boundary conditions weren't cohesive enough and lead to conflicts between the boundaries.
In other words, it was a situation similar to this:

• One boundary on the left specified a fixed value for speed;
• The other one on the right specified a fixed value for pressure, which conflicted with the first boundary.
• The result was that the fixed value was not respected, leading to some weird results.

OpenFOAM possibly only allows for these kinds of situations because of how things are solved: it's all calculated through hundreds of iterations, by only partially solving matrices. And since each iteration doesn't do a complete solution of "A*X=B", it only does a partial calculation towards the solution, it therefore can allow for some crazy math...

And I've seen worse cases in older OpenFOAM versions, where solver would simply open a breach on a single point on the wall and make the flow explode through that single point! All because I had messed up the boundary conditions...

Best regards,
Bruno

thanks.
I had encountered what you said about exploding on a single point before and last night in a new boundary condition I made.
so could you clarify a little more what does your sentence mean?(because i was engaged in for a long time for bad BCs)
then in the case do you think I should make pressure in outlet be lower or upper?

Hi Ehsan,

I'll try to explain with a simple description, from what I can remember:

1. The case was a box with two opposite openings.
2. One opening would be the inlet.
3. The other opening would be the outlet.
4. The solver was simpleFoam.
5. I defined the inlet with U>0 and p=0; the outlet with U=0 and p=0.
6. Somehow the solver didn't complain about mass-flow problems.
7. What the solver would do is to open a small hole in one of the vertexes on a wall and forced the excess fluid to escape through that hole.
8. The mass flow was balanced simply because of that hole in the wall, namely the "breach".

I hope this is easier to understand.

:confused: AFAIK, theoretically the outlet should have a lower pressure value... there is a tutorial in OpenFOAM that demonstrates this: "incompressible/pimpleFoam/TJunction"

Best regards,
Bruno