Learning Forum

[Chenxi Wang]

Hi, I am simulating the inner flow field of part of water pipeline. The VOF model was appplied as the flow is open channel flow. The inlet boundary was set as mass flow inlet.

I am confused about the outlet boundary condition now. Should I use pressure outlet or outflow boundary condition? If I use pressure outlet boundary, what gauge pressure should I give when the outlet boundary do not connect with atmosphere? If I use outflow boundary condition, the air phase is compressible , which Ansys User's guide doesn't recommed using outflow boundary for compressible phase.

And I also noticed there is an option for gauge pressure named from neighboring cell in pressure outlet boundary. Is this option suitable to my problem? Thanks in advance.

[Rob]

Please can you post some images?  "Open Channel" is a way of setting inlet and outlet conditions in VOF to allow part of the boundary to be liquid and the remainder gas. 

Outflow is an older boundary type, and is not commonly used: other than for testing I've not used it in many years. Pressure outlet is probably the option you want. 

[Chenxi Wang]

 

Thanks for your reply. Below figure is the profile of my geometry. Because it includes full-pipe flow and open channel flow simultaneously, I didn’t use open channle flow function in VOF setting. And I don’t know the water depth at outlet boudary before the simulation either, and can’t give an estimation of proper gauge pressure for pressure outlet boundary. That’s the reason why I was thinking about outflow boundary condition. Any suggestions would be appreciated. 

 

[Rob]

OK, for the outlet if you want any hold up you probably want the open channel.  Not sure what you're gaining by including all of the vertical section, but open channel or pressure outlet will work find there too. 

[Chenxi Wang]

The vertical section works as a chamber for air ventilation. In term of pressure outlet boundary condition, I am confused about the gauge pressure. Because I just cut off the pipeline, the 0 gauge pressure representing atmosphere seems inappropriate. 

[Rob]

Why? The operating pressure sets atmospheric pressure (default of 101325 Pa), so we then set gauge against that. How much of a pressure change are you expecting to need compressible gas? 

[Chenxi Wang]

I suppose to have around 120 Pa pressure to compress air. But this is just maximum value.

[Rob]

What's the density difference in air between 0 Pa gauge and 120 Pa gauge?