difrrence betwin external and internal flow

hmasenger · December 31, 2012, 11:20

hi all
i have a question that it seems easy to answer at first but i think its not that much easy !.

I am modeling cavitation in a butterfly valve and i need to validate my CFX results by checking sigma (cavitation index) with experimental data
but the problem is that experimental data which are available ,are for external flows over hydrofoils and other submerged bodies, and now i don't know is it possible for me to use does data for the disk of butterfly valve which is in a wall bounded water domain in a pipe ?

in what circumstances flow over a butterfly valve's disk can be modeled as an external flow?

by the way i just need to know the sigma value over the disk and its not important to know what would happen to flow(eddy viscosity) just after the disk
can i just compare cfx results for sigma and experimental ones for external flows?

any help will be appreciated .

ghorrocks · January 1, 2013, 18:51

Cavitation modelling is never simple. This is a very difficult problem and if you get accurate answers you are doing well.

If you have experimental data on hydrofoils and other sumerged bodies then I would model those and make sure you can model those accurately before moving to your flow. In terms of cavitation modelling it does not matter whether it is internal or external flow. But what does matter is details about the fluid such as impurities, dissolved air content and surface conditions - these make massive differences in cavitation modelling and is why it is so hard.

December 31, 2012, 11:20	difrrence betwin external and internal flow	#1
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	hi all i have a question that it seems easy to answer at first but i think its not that much easy !. I am modeling cavitation in a butterfly valve and i need to validate my CFX results by checking sigma (cavitation index) with experimental data but the problem is that experimental data which are available ,are for external flows over hydrofoils and other submerged bodies, and now i don't know is it possible for me to use does data for the disk of butterfly valve which is in a wall bounded water domain in a pipe ? in what circumstances flow over a butterfly valve's disk can be modeled as an external flow? by the way i just need to know the sigma value over the disk and its not important to know what would happen to flow(eddy viscosity) just after the disk can i just compare cfx results for sigma and experimental ones for external flows? any help will be appreciated .

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January 1, 2013, 18:51		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Cavitation modelling is never simple. This is a very difficult problem and if you get accurate answers you are doing well. If you have experimental data on hydrofoils and other sumerged bodies then I would model those and make sure you can model those accurately before moving to your flow. In terms of cavitation modelling it does not matter whether it is internal or external flow. But what does matter is details about the fluid such as impurities, dissolved air content and surface conditions - these make massive differences in cavitation modelling and is why it is so hard.