## Fluids

#### Loss of mass in a closed domain.

• abhnv_01
Subscriber

With 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.

• Rob
Ansys Employee

Convergence and mesh resolution would be the first things to check. Are both phases fixed density?

• abhnv_01
Subscriber

Liquid phase - Function of temperature

Vapor phase - Ideal gas

Convergence criteria:

Continuity - 1e-05

Velocity - 1e-04

Energy - 1e-07

• Rob
Ansys Employee

And the mesh resolution?

• abhnv_01
Subscriber

• abhnv_01
Subscriber

My Mesh quality

1. Avg Element quality – 0.9977

2. Avg Aspect ratio – 1.0419

3. Avg Skewness – 0.001083

• Rob
Ansys Employee

If 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.

• abhnv_01
Subscriber

But, 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?

• Rob
Ansys Employee

Using 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.

• abhnv_01
Subscriber

Should 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?

• Rob
Ansys Employee

In 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.

• abhnv_01
Subscriber

With 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?

• Rob
Ansys Employee

I'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.

• abhnv_01
Subscriber

Thanks Rob for your help.

Highly appreciate it.

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