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Too many iterations causing long simulation time :(

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Old   February 26, 2018, 18:55
Default Too many iterations causing long simulation time :(
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Shafik Walakaka
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Hi everyone,

I am trying to simulate a two-phase stratified flow in a pipe. However, my simulations are taking extremely long. It states that it takes 1001 iterations to obtain pressure , does anyone know what could be the cause of this issue?

PIMPLE: iteration 1
smoothSolver: Solving for alpha.water, Initial residual = 0.000529543, Final residual = 2.66809e-11, No Iterations 2
Phase-1 volume fraction = 0.517887 Min(alpha.water) = 0 Max(alpha.water) = 1.06837
MULES: Correcting alpha.water
MULES: Correcting alpha.water
Phase-1 volume fraction = 0.517887 Min(alpha.water) = -8.72144e-07 Max(alpha.water) = 1.06794
DICPCG: Solving for p_rgh, Initial residual = 1, Final residual = 0.880426, No Iterations 1001
time step continuity errors : sum local = 0.00204219, global = -2.39327e-05, cumulative = -4.39327e-05
DICPCG: Solving for p_rgh, Initial residual = 0.270516, Final residual = 0.753417, No Iterations 1001
time step continuity errors : sum local = 0.00579181, global = -2.39253e-05, cumulative = -6.7858e-05
DICPCG: Solving for p_rgh, Initial residual = 0.510678, Final residual = 0.1488, No Iterations 1001
time step continuity errors : sum local = 0.00170815, global = -1.19552e-05, cumulative = -7.98132e-05
ExecutionTime = 12.26 s ClockTime = 12 s

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Shafik
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Old   February 27, 2018, 03:03
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Uwe Pilz
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You get such effects if something wrong with your boundary or initial conditions. Or you set up your case in a non physical way.
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Old   February 27, 2018, 15:52
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Quote:
Originally Posted by piu58 View Post
You get such effects if something wrong with your boundary or initial conditions. Or you set up your case in a non physical way.
Hi, thank you for your reply!

Could you kindly elaborate what you mean by wrong boundary/initial conditions or non physical?
Because I am getting accurate results when modelling single phase flow (Unless it is possible to obtain good results with my error). Which is why I am confused, what do you mean by wrong boundary/initial conditions or non physical way?

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Shafik
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Old   February 28, 2018, 02:14
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Good morning,

if yopu take additional effects in to account, the behaviour of the solver may change. Somtimes, it ist more or less a different solver.

I see two ways that anybody may help you. Either you reduce the effect you see to the simples thinkable case and report is as an error. Or you describe what is your real problem and which connection to the physical worls it has. May be there are other ways to solve it.
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Old   February 28, 2018, 06:41
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Quote:
Originally Posted by piu58 View Post
Good morning,

if yopu take additional effects in to account, the behaviour of the solver may change. Somtimes, it ist more or less a different solver.

I see two ways that anybody may help you. Either you reduce the effect you see to the simples thinkable case and report is as an error. Or you describe what is your real problem and which connection to the physical worls it has. May be there are other ways to solve it.
Good morning!

Thank you for your kind reply. I am currently using the interFoam solver which uses the Pimple algorithm. I notice that when I change the deltaT, the number of iterations required for pressure correction to converge reduces!

Is there an explanation for this?

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Shafik
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