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Old   June 17, 2015, 15:00
Default Gas-liquid pipe flow
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ivan
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Hi guys, im new in CFX and im trying to simulate a gas liquid multiphase flow but im getting weird results, i just want to be able to identify some horizontal pipe flow patterns (wave, stratified, etc) but i reallt dont know very well how to set the bc and multiphase models, i have read some tutorial and i have done this.

i set a simple horizontal pipe 3 m lenght and 0.20 m diameter

at inlet i set 0.5 of gas and 0.5 of water
1 m/s both

a simple outlet with av pressure 0 atm

im starting with laminar flow, both continuos phases, inhomogeneuos. fluid pair with mixing model, and drag coefficient.

i just want to know what best settings i have to usea to model stratified flow

as i run the model i get some waves but at some time i have "picks" in convergence and i see like suddenly "explosions" in the flow. i know i have alotof things wrong.

In simple words what a have to do to set a stable model.
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Old   June 17, 2015, 19:13
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Glenn Horrocks
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For free surface flows - they are quite sensitive to time step size. I would recommend you use adaptive time stepping so it can automatically find a good time step.

Use adaptive time stepping, homing in on 3-5 coeff loops per iteration. Make sure the max and min time step size are wide enough that you do not hit them, and give it a sensible starting time step size.
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Old   June 18, 2015, 09:33
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thanks glenn ill try that today.
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Old   June 18, 2015, 10:13
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Curious question for the knowledgeable:

Can you run this problem in steady state, with a small physical time step (e.g. t = 0.001 s) for 10000 iterations?

This will give you a pseudo simulation time of 0.001 * 10000 = 10 s.

Then use the steady state results as the initial guess for a transient run, but setting the initial start time as 10 s?

Thanks
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