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steven1 June 28, 2013 03:51

reversed flow
 
hello everyone,

my cas is a 'T' tube connected by two cylindres with an inlet and 2 outlet.
the fluide is turbulent. i made the boundary condition as 'outflow', the warning came to me was "reversed flow". i looked for the former thread, it is said that i could change it to 'pressure outlet'. after operation, the warning is always "reversed flow".
does someone know what happened?

thank you
steven

-mAx- June 28, 2013 06:33

you have a vortex crossing the outlet.
It is just a warning, let iterate, the warning may disappear

azt June 28, 2013 06:36

Extend your computational domain at the outlets, i.e. ensure that the outlet is far away from the tee

steven1 June 28, 2013 07:46

Ok, thank you, i will wait a bit.

steven1 June 28, 2013 07:48

Quote:

Originally Posted by azt (Post 436527)
Extend your computational domain at the outlets, i.e. ensure that the outlet is far away from the tee

Thank you. in this case the distance of outlets are at least 25 times that of diameter

steven1 June 28, 2013 07:54

Quote:

Originally Posted by -mAx- (Post 436526)
you have a vortex crossing the outlet.
It is just a warning, let iterate, the warning may disappear

hello, Maxime,this is what i get. do you have some suggestions?
https://www.dropbox.com/s/30deqc29eq...ed%20flow.docx

-mAx- June 28, 2013 08:44

Quote:

Originally Posted by steven1 (Post 436548)
hello, Maxime,this is what i get. do you have some suggestions?
https://www.dropbox.com/s/30deqc29eq...ed%20flow.docx

you have a convergence problem, and I don't think it comes to your reversed flow
-check Mesh
-how are your turbulent bc?

steven1 June 28, 2013 08:58

Quote:

Originally Posted by -mAx- (Post 436552)
you have a convergence problem, and I don't think it comes to your reversed flow
-check Mesh
-how are your turbulent bc?

the mesh is not too bad. i chose the standard wall function. here are the informations https://www.dropbox.com/s/30deqc29eq...ed%20flow.docx

-mAx- June 28, 2013 09:19

-what is the quality of worst element? (show worst element option in gambit)
-what are the tubulent bc at your inlet (you showed the turbulent model)
-did you scaled your model?
-can you copy and paste the output from checkMesh

steven1 June 28, 2013 09:46

Quote:

Originally Posted by -mAx- (Post 436559)
-what is the quality of worst element? (show worst element option in gambit)
-what are the tubulent bc at your inlet (you showed the turbulent model)
-did you scaled your model?
-can you copy and paste the output from checkMesh

hello, this is the information. thank you
https://www.dropbox.com/s/xml8ctvagskchbf/0628-2.docx


Mesh Quality:
Orthogonal Quality ranges from 0 to 1, where values close to 0 correspond to low quality.
Minimum Orthogonal Quality = 2.75142e-01
Maximum Aspect Ratio = 1.61014e+01

-mAx- June 28, 2013 09:50

ok, your mesh is ok and seems to be scaled
in the velocity BC, you should have turbulent parameters, waht do you have?
How is you material? density, viscosity?

steven1 June 28, 2013 10:01

Quote:

Originally Posted by -mAx- (Post 436567)
ok, your mesh is ok and seems to be scaled
in the velocity BC, you should have turbulent parameters, waht do you have?
How is you material? density, viscosity?

i have the parameters of K and epsilon. k=0.053985 and epsilon=1.4722. the velocity is 5m/s, it is calculated with the Re=100000. the material is water-liquid. density is about 1000kg/m3, and viscosity is about 0.001kg/m-s

-mAx- June 28, 2013 10:15

Ok I have to hurry
Try to switch with realizable-kepsilon
Try with and without enhanced wall treatment

steven1 June 28, 2013 10:21

sorry, i didn't understand what you mean 'realizable-kepsilon'. the k,epsilon values that i used are also calculated with Re=100000.

-mAx- June 28, 2013 10:43

Quote:

Originally Posted by steven1 (Post 436576)
sorry, i didn't understand what you mean 'realizable-kepsilon'. the k,epsilon values that i used are also calculated with Re=100000.

Other turbulence model

steven1 July 2, 2013 03:33

Quote:

Originally Posted by -mAx- (Post 436580)
Other turbulence model

hello Max, i have tried the realizable k-epsilon, but the situation didn't change, there are still problem of conbergence and warnings of reversed flow. By the way, the Y+ is between 5 and 30.

rayan24 July 2, 2013 05:12

Hi,

I have same problem and I think it is the boundary conditions imposed on your two outputs which poses the problem .. when you have two output can not impose "pressure" at outlet..

What are your boundary conditions at the outlets??



Quote:

Originally Posted by steven1 (Post 437232)
hello Max, i have tried the realizable k-epsilon, but the situation didn't change, there are still problem of conbergence and warnings of reversed flow. By the way, the Y+ is between 5 and 30.


steven1 July 2, 2013 05:24

Quote:

Originally Posted by rayan24 (Post 437259)
Hi,

I have same problem and I think it is the boundary conditions imposed on your two outputs which poses the problem .. when you have two output can not impose "pressure" at outlet..

What are your boundary conditions at the outlets??

I use outflow as the BC of outlet. in fact, i had wanted to try the pressure outlet, but i don't know the factors that i have to fill

rayan24 July 2, 2013 08:20

The condition of pressure outlet is valid when we have "one outlet" for two and more of outlet this condition is not valid, you must use the "equal flow rate" at outlets.. try to reduce the underelaxation at the minimum (0.1 or 0.2)


Quote:

Originally Posted by steven1 (Post 437262)
I use outflow as the BC of outlet. in fact, i had wanted to try the pressure outlet, but i don't know the factors that i have to fill


steven1 July 2, 2013 08:23

Quote:

Originally Posted by rayan24 (Post 437304)
The condition of pressure outlet is valid when we have "one outlet" for two and more of outlet this condition is not valid, you must use the "equal flow rate" at outlets.. try to reduce the underelaxation at the minimum (0.1 or 0.2)

so what should I put as the gauge pressure?


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