Split Iso-Volume in CFX-Post

PeMo · April 18, 2013, 08:36

Hey guys,

I need your help at a Post processing issue. I am doing two phase cavitation simulations and visualize the cavitation volume as an Iso Volume (Vapour Volume Fraction = 0.5).
Additional I would like to calculate the Cavitation volume (Expression: volume()@IsoCav) to get a better comparison.
Here is the question: To calculate the volume values in different regions I have to split my computational domain, unfortunately you can only create an Iso Volume out of a domain not a region or volume.
I guess there are two options:
1. define Iso Volume in a specific region and calculate the cavitation volume
2. use the expression above and limit it to the required region

Any hints how to do this?
Thanks in advance

ghorrocks · April 18, 2013, 19:41

Cavitation is not an effect with a defined free surface, so I do not think taking a VF=0.5 isosurface is a good way of defining the cavitation region. Specifically, if a large region has VF=0.3 then it clearly has cavitation but your volume would be zero.

So rather than working out the volume of an isosurface I would simply integrate the vapour volume fraction over the area. This will include all levels of volume fraction, and is much simpler to implement.

mjgraf · April 18, 2013, 20:42

good suggestion glenn
wouldn't the calculation be the volumeInt of Vapor volume Fraction for the Domain/Volume?

suggestion for visualization of a cavitating volume?

Quote:

Originally Posted by ghorrocks

Cavitation is not an effect with a defined free surface, so I do not think taking a VF=0.5 isosurface is a good way of defining the cavitation region. Specifically, if a large region has VF=0.3 then it clearly has cavitation but your volume would be zero.

So rather than working out the volume of an isosurface I would simply integrate the vapour volume fraction over the area. This will include all levels of volume fraction, and is much simpler to implement.

ghorrocks · April 19, 2013, 07:11

Yes, VolumeInt of vapour VF is correct.

It can be visualised many ways - an isosurface of VF=(some value, not necessarily 0.5), cross section planes colour by volume fraction or many others.

PeMo · April 22, 2013, 03:41

Thanks for your comment Glenn,
I am still not sure if it is correct to take into account all the cavitation volumina if you just compare different operating points. But you are right the integrated volume fraction is much easier to handle, I will give it at try.

ghorrocks · April 22, 2013, 09:53

As I said in my first post, cavitation does not tend to end up with sharply defined free surfaces. There tends to be drops of liquid in the vapour cavity and vapour bubbles in the liquid region and a transition between the two. This is why I said drawing a line at VF=0.5 is not very helpful, because it does not separate the vapour from the liquid, as it does in a free surface modelling thing. So the VF integral is more physically relevant than the VF=0.5 contour in most cases.

April 18, 2013, 08:36	Split Iso-Volume in CFX-Post	#1
PeMo Member Peter Join Date: Sep 2011 Location: Germany Posts: 39 Rep Power: 15	Hey guys, I need your help at a Post processing issue. I am doing two phase cavitation simulations and visualize the cavitation volume as an Iso Volume (Vapour Volume Fraction = 0.5). Additional I would like to calculate the Cavitation volume (Expression: volume()@IsoCav) to get a better comparison. Here is the question: To calculate the volume values in different regions I have to split my computational domain, unfortunately you can only create an Iso Volume out of a domain not a region or volume. I guess there are two options: 1. define Iso Volume in a specific region and calculate the cavitation volume 2. use the expression above and limit it to the required region Any hints how to do this? Thanks in advance

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April 18, 2013, 19:41		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Cavitation is not an effect with a defined free surface, so I do not think taking a VF=0.5 isosurface is a good way of defining the cavitation region. Specifically, if a large region has VF=0.3 then it clearly has cavitation but your volume would be zero. So rather than working out the volume of an isosurface I would simply integrate the vapour volume fraction over the area. This will include all levels of volume fraction, and is much simpler to implement.

April 19, 2013, 07:11		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Yes, VolumeInt of vapour VF is correct. It can be visualised many ways - an isosurface of VF=(some value, not necessarily 0.5), cross section planes colour by volume fraction or many others.

April 22, 2013, 03:41		#5
PeMo Member Peter Join Date: Sep 2011 Location: Germany Posts: 39 Rep Power: 15	Thanks for your comment Glenn, I am still not sure if it is correct to take into account all the cavitation volumina if you just compare different operating points. But you are right the integrated volume fraction is much easier to handle, I will give it at try.

April 22, 2013, 09:53		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	As I said in my first post, cavitation does not tend to end up with sharply defined free surfaces. There tends to be drops of liquid in the vapour cavity and vapour bubbles in the liquid region and a transition between the two. This is why I said drawing a line at VF=0.5 is not very helpful, because it does not separate the vapour from the liquid, as it does in a free surface modelling thing. So the VF integral is more physically relevant than the VF=0.5 contour in most cases.