Help, outlet pressure on Hydrofoil tutorial

jordiboronat · October 27, 2016, 11:06

Hi,

I'm a completely new CFD user going through the tutorials in order to learn to simulate flow on hydrofoils.

As far as I understand I can set the default pressure to 1 atm, normal speed at inlet and relative pressure on the outlet to 0 or relative pressure where is venting.

Could anyone explain why is Ansys setting the Outlet pressure to 51957 ? Are they assuming the foil is submerged enough to have that pressure?

ghorrocks · October 27, 2016, 18:30

No, the example sets the reference pressure to 0 and the outlet is 51957 Pa. For the initial no cavitation model this makes no difference as the model is incompressible. But in the second model with cavitation it will affect the cavitation as cavitation uses absolute pressure.

I do not understand why this is done - this means the system is running at vacuum of about half an atmosphere and is more prone to cavitate. But I guess it could be simple as - if you run this at atmospheric pressure it does not cavitate nicely, so they lowered the pressure to get a clearly defined cavitation region.

October 27, 2016, 11:06	Help, outlet pressure on Hydrofoil tutorial	#1
jordiboronat New Member Jordi Join Date: Oct 2016 Posts: 1 Rep Power: 0	Hi, I'm a completely new CFD user going through the tutorials in order to learn to simulate flow on hydrofoils. As far as I understand I can set the default pressure to 1 atm, normal speed at inlet and relative pressure on the outlet to 0 or relative pressure where is venting. Could anyone explain why is Ansys setting the Outlet pressure to 51957 ? Are they assuming the foil is submerged enough to have that pressure?

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October 27, 2016, 18:30		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,816 Rep Power: 144	No, the example sets the reference pressure to 0 and the outlet is 51957 Pa. For the initial no cavitation model this makes no difference as the model is incompressible. But in the second model with cavitation it will affect the cavitation as cavitation uses absolute pressure. I do not understand why this is done - this means the system is running at vacuum of about half an atmosphere and is more prone to cavitate. But I guess it could be simple as - if you run this at atmospheric pressure it does not cavitate nicely, so they lowered the pressure to get a clearly defined cavitation region.