[CutyTeemo]

Hello

I made transient pipe model as below

■ Type
- Transient(Total time 1s, Timestep 0.01s)

■ Fluid model
- 2 Fluid pair model(air and water)

■ Boundary condtion
- 1 inlet(opening, cart vel 1m/s, Xwater=0.8/Xair=0.2)
- 1 outlet(opening, opening pres. and dirn 0 Pa)
- 1 conditional connection(pressure condition) at middle point.

■ Initial condtion : 0 Pa, 0 m/s and Xair=1

But when I start solver, it is stopped by overflow at FIRST step.
Please help me );

[ghorrocks]

Are the fluids incompressible? You have a velocity inlet, so if you block off the connection the solver will crash as you have said fluid must go in but given it nowhere to go.

You have to be careful with the conditional opening condition that the open and closed conditions are physically realistic.

Also, note that opening and closing is also very numerically aggressive and if your simulation is unstable it could trigger divergence and crashing.

[CutyTeemo]

Originally Posted by ghorrocks

Are the fluids incompressible? You have a velocity inlet, so if you block off the connection the solver will crash as you have said fluid must go in but given it nowhere to go.

You have to be careful with the conditional opening condition that the open and closed conditions are physically realistic.

Also, note that opening and closing is also very numerically aggressive and if your simulation is unstable it could trigger divergence and crashing.

Thanks for your answer. I use Water and Air at 25℃ in CFX library. Is there any options about incompressibility?
And I turn off conditional connection option, but it doesnt work.
It works in static state, so maybe tranisent setting problem?

[ghorrocks]

The fluids you chose are incompressible. So my comment is correct, when your condition condition closes the solver will crash.

If you say it crashes when you turn the conditional condition off then something else is also happening. Please post your output file and an image of what you are modelling defining where the boundary conditions are.