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-   -   reversed flow and divergence (https://www.cfd-online.com/Forums/fluent/63409-reversed-flow-divergence.html)

 mechfloeng April 7, 2009 15:24

reversed flow and divergence

Hi, I have following message:

reversed flow in 80 faces on pressure-outlet 4.
Error: divergence detected in AMG solver: x-momentum
Error Object: ()

Can I get help, how can I solve it?

Thanks
HW

 callofduty April 8, 2009 05:05

Hi Howard,
first of all i recommend you to increase your maximum limits as much as possible(solve>controls>limits) (i think your face number-which reversed flow problem occurs- is controllable.)
Moreover you can start fmg initilization by solve>initialize>fmg initilization. FMG initilization provides better convergence.
If problem continues i recommend you to check your grid if your skewness is high.
But i hope you dont need it :)

Ciaooo
Tenar

 mechfloeng April 8, 2009 11:12

Hi, Tenar,

maximum limits of 1e+20 is allowable.
I cannot find menu FMG initilization in solve>initialize>fmg initilization. Can you tell me
how to get this menu? My skewness is high, how can I fix it?

Thanks
HW

PS here is my grid check
-------------------
Grid Check
Domain Extents:
x-coordinate: min (m) = 9.288283e-015, max (m) = 3.139997e+000
y-coordinate: min (m) = 8.000000e+000, max (m) = 1.000000e+001
z-coordinate: min (m) = -1.570000e+000, max (m) = 1.570000e+000
Volume statistics:
minimum volume (m3): 6.394585e-008
maximum volume (m3): 1.183709e-002
total volume (m3): 9.413165e+000
Face area statistics:
minimum face area (m2): 1.300699e-004
maximum face area (m2): 1.211852e-001
...............
---------------------

 callofduty April 8, 2009 14:22

You wont use fmg from menu.On fluent when you pres enter you will see some directories as

.... .... solve>

you will write "solve" and press enter , so you'll be on 'solve' directory..
when you press again again sub-directories will be seen. You will choose 'initialize' and under sub-directory of 'initialize', there is function 'fmg initialization'. When you write the name of function, fluent will ask if you want to run 'fmg initialization'. So you can run it. For further detail you can look on FLUENT manual.

About your grid, i dont have any idea because your note does not contain any info. about grid skewness. If I were you, first i would check if my grid properly fit on geometry(if your model is complex), then i would be careful about my surface skewness is less then 0.8.

Let me know about your work, these are the basic solutions. There may be a problem about your boundary conditions or solution technique also!

Tenar

 mechfloeng April 9, 2009 01:31

Hi, Tenar,

My case is for mixture (two phase), unsteady,
So computer show me:
---------------------------
/solve/initialize> fmg-initialization
FMG initialization is not available with the unsteady solver.
-------------------------------
so I can send email to you.

Thanks
HW

 irineupetri April 25, 2009 23:46

The message "reversed flow" can to cause a problem on the convergence, for take off this message, you should put in the comand line [solve/set/flow-warnings?/no] + enter

or

[solve] + enter
[set] + enter
[flow-warnings?] + enter
[no] + enter

 Mohsin August 10, 2010 00:24

Hello

I have following querries regarding FMG initialization. I will be grateful for your kind look at these:

1. When I run Fmg initialization without starting any iterations then it gives me the following error:

/solve/initialize> fmg-initialization
invalid command [fmg-initialization]

Please specify. Why it is so. As its prime function is to give the initial solution so isn't it should be specified before iterations?

2. When FMG initialization is complete I get the following message:

FMG: Initialize flow for Segregated solution.. . end

It means that FMG initialization has been done and after this I dont need to change the initial values from solve...> initialize. And I only have to iterate now?

3. I am using multiphase flow DPM. But first i think i should run FMG initialization without DPM and then introduce DPM later(making it single phase in order to for FMG initialization to run). Is it possible.