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December 8, 2022 at 10:57 am
abhnv_01
SubscriberWith my two phase VOF simulation for a closed domain, I observed that I am loosing mass.
Mesh is refined and time step is of the order of 0.0001 sec.
What can be the reason for loss of mass in the system.
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December 8, 2022 at 12:14 pm
Rob
Ansys EmployeeConvergence and mesh resolution would be the first things to check. Are both phases fixed density?
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December 8, 2022 at 12:21 pm
abhnv_01
SubscriberLiquid phase - Function of temperature
Vapor phase - Ideal gas
Convergence criteria:
Continuity - 1e-05
Velocity - 1e-04
Energy - 1e-07
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December 8, 2022 at 2:26 pm
Rob
Ansys EmployeeAnd the mesh resolution?
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December 9, 2022 at 4:36 am
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December 9, 2022 at 4:38 am
abhnv_01
SubscriberMy Mesh quality
1. Avg Element quality – 0.9977
2. Avg Aspect ratio – 1.0419
3. Avg Skewness – 0.001083
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December 9, 2022 at 10:02 am
Rob
Ansys EmployeeIf you're wanting to model VOF phase change, ie you need to track the bubbles use a lot more cells. You're looking at putting 5-10 cells across each bubble, or the film if it's surface boiling.
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December 9, 2022 at 10:25 am
abhnv_01
SubscriberBut, using a lot more cells will also increase my computational time.
Except the cell resolution, can there be any other reason for loss of mass inside the system?
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December 9, 2022 at 11:56 am
Rob
Ansys EmployeeUsing more cells will cost compute and time, yes, but without running a finer mesh you won't know if the results are mesh dependent.
Accuracy is likely reason for mass loss. Whether that's convergence, cell resolution or solver related is a different question, and you'd need to run more models to change solver mesh & parameters to figure that out.
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December 9, 2022 at 12:06 pm
abhnv_01
SubscriberShould we say that with accurate and correct model one can get zero mass loss/gain?
Or zero mass loss/gain for a closed domain is an ideal case?
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December 9, 2022 at 1:32 pm
Rob
Ansys EmployeeIn an ideal case all mass is conserved. In most cases some mass is lost eventually, but if the domain is well resolved spacially and with time the loss should be negligable.
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December 9, 2022 at 1:44 pm
abhnv_01
SubscriberWith time step of 0.0001 sec(e-04) and after running 600,000 timesteps (=60sec), mass lost is approximately 0.3%, can this be considered negligible or the value is significant?
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December 9, 2022 at 1:48 pm
Rob
Ansys EmployeeI'd consider it negligable, but you're the one deciding that based on your work. You also need to see if it's 0.3% mass liquid or vapour, and how that impacts the domain volume.
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December 9, 2022 at 2:36 pm
abhnv_01
SubscriberThanks Rob for your help.
Highly appreciate it.
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