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

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Old   April 23, 2013, 08:50
Default a question on why output pressure has set fixedValue as minimum of it?
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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):
Code:
#             Time              min              max          average
  0.0003328733296      857198.8151      1035170.079      914519.9161
  0.0003328891634      857198.8151      1034805.569      914349.6685
  0.0003329049972      857198.8151      1034382.104      914176.4348
   0.000332920831      857198.8151      1033897.606      914000.5358
  0.0003329366648      857198.8151      1033350.681      913822.4189
  0.0003329524986      857198.8151      1032740.664      913642.6328
  0.0003329683324      857198.8151       1032067.75      913461.7711
  0.0003329841662      857198.8151      1031333.055      913280.4695
         0.000333      857198.8151      1030538.555      913099.3907
  0.0003330172414      857198.8151      1029604.911      912902.7454
  0.0003330344828      857198.8151      1028602.729      912707.1124
  0.0003330517241      857198.8151      1027537.217      912513.4208
  0.0003330689655      857198.8151      1026414.828      912322.6111
  0.0003330862069      857198.8151      1025242.146      912135.5541
  0.0003331034483      857198.8151      1024025.705      911952.7739
  0.0003331206897      857198.8151      1022771.847      911774.5666
   0.000333137931      857198.8151      1021486.612      911601.0205
  0.0003331551724      857198.8151      1020174.904      911431.7817
  0.0003331724138      857198.8151      1018841.625      911266.3243
  0.0003331896552      857198.8151      1017491.651      911104.1384
how the way does fixedValue work then?
does it set the fixedValue only as a lower limit of that variable?
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Old   April 24, 2013, 12:50
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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?
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Old   April 25, 2013, 02:07
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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?
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Old   April 26, 2013, 22:30
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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?
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Old   April 29, 2013, 18:30
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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
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Old   April 30, 2013, 06:47
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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)
Quote:
solver would simply open a breach on a single point on the wall
then in the case do you think I should make pressure in outlet be lower or upper?
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Old   April 30, 2013, 18:17
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Hi Ehsan,

Quote:
solver would simply open a breach on a single point on the wall
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.

Quote:
Originally Posted by immortality View Post
then in the case do you think I should make pressure in outlet be lower or upper?
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
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