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Old   January 5, 2005, 00:48
Default Converge Problem
  #1
Jen
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I am using FLUENT to model spray in a big tank, the spray gun has several outlet, the one in the center is for water and the others are for air. I set these openings as velocity_inlet, and the outlet of tank is set to be pressure_outlet. I tried some ways, but it does not converge especially the residule of volume fraction of water( the water speed is 1.9m/s, and the air is 20m/s).

What's more, when I calculate, the warning message "reversed flow in 94 faces on pressure_outlet" appear. Does this means there is something wrong with my codes?

This is really a big headache to me, I really wish some one could help me or give me some suggestion, Thanks so much!!!
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Old   January 5, 2005, 00:51
Default Re: Converge Problem
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Jen
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BTW: I am using Multiphase model. I wonder if I should use Discrete Phase Models. The turbulent model is k-epsilon
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Old   January 5, 2005, 07:49
Default Re: Converge Problem
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An Modh Coinniolach
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Are the values you defined for backflow on the pressure outlet reasonable?

Moving your pressure outlet further downstream might be beneficial.

You might also try the target mass flow rate option on the pressure outlet.
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Old   January 5, 2005, 12:19
Default Re: Converge Problem
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Jen
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Now, I can get the solution converge without slip velocity, but when I add slip velocity, all the residuals fly to the sky.

Is dicreted phase model a better choice?
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Old   January 6, 2005, 08:22
Default Re: Converge Problem
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soundar
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I think this problem caused due to the precision of ur geometry.

U just open your case file in 2ddp when u have made the model in 2d or in 3ddp when u have made the model in 3d.

then try to solve it.

see whether it is working .

--soundar
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Old   January 6, 2005, 11:28
Default Re: Converge Problem
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Jen
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After about 5000 iterations, it converged without slip velocity. But the contours are very strange, and the velocity seems so small. I don't know how to get better results. I never use 3ddp, does it has any difference with 3d?

I am going to put my outlet downstream to see if it will become better.

In the inlet boundary condition, I specify turbulent intensity (I) and hydraulic diameter (Dh), but why other parameters do not change, eg. k and epsilon. Is it normal?

Thanks a lot!!!
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Old   January 6, 2005, 11:34
Default Re: Converge Problem
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Jen
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Acctually, I don't know what I could specify at the pressure_outlet. I just know the pressure there, nothing else. Do you have some suggestion on how to specify pressure_outlet?

Where can I choose target mass flow rate option?
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Old   January 7, 2005, 11:39
Default Re: Converge Problem
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Jen
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Now the solution is converged. But the reversed flow warning is still exist. I have moved the outlet downstream. Is the warning an error? Any other method I can solve this problem?
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Old   January 25, 2005, 01:24
Default Re: Converge Problem
  #9
soundar
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Any body could suggest for my convergence problem.

i'm running at steady state species transport model.

while iterating it shows reversed flow in faces 14 at pressure-outlet 5.

however it is running for next iterations

is it relates with convergence or it may cause err in the result.

my outlet is outflow and inlets are mass inflow.

waiting for response soundar

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Old   January 25, 2005, 08:29
Default Re: Converge Problem
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Jason
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I'm assuming when you say "my outlet is outflow" that you're talking about the pressure-outlet 5. If it's a pressure-outlet, then you defined some values for reversed flow in the boundary conditions window. "reversed flow in XX faces at XXXX" just tells you that you're getting some flow into your system through what you assumed was your pressure outlet and it's using the reversed flow values you defined. If the reversed flow is real, and your reversed flow settings are true, then it shouldn't mess with convergence or cause an error. If this isn't real, then either it will work itself out during convergence, or your model setup is wrong. One example of where I've seen this: we had a flat plate representing a portion of a fuselage, and a large bluff body sticking out of it. The pressure outlet wasn't far enough downstream, so the recirculation behind the bluff body was pulling air back into the control volume through the pressure outlet. We ended up relocating the B.C. to avoid this, but it wasn't actually causing problems with the results.

Hope this helps, and goodluck Jason
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Old   January 28, 2005, 09:11
Default Re: Converge Problem
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soundar
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Thank you jason for your detail explanation.

I'm doing species transport model with various chemical reactions for gasification. After 400 iterations it has shown inert as 99.99% where as for mole fraction of CO,Co2 it shows very less value of the order 4.0 *10^-16. any suggestion please for rectifying this so that reaction will take place inside.

thanks from soundar
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Old   January 28, 2005, 15:20
Default Re: Converge Problem
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Jason
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Now you're getting outside my knowledge base. I've never dealt with species transport (I'm strictly an aerodynamics guy).

Sorry, and I hope someone else on here can help.

Goodluck, Jason
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