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January 15, 2016, 00:42 
Question about VOF method

#1 
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Zhifang Hu
Join Date: Sep 2015
Location: Brisbane
Posts: 53
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I did a twophase flow simulation using VOF method, with alpha defining as 1 for water and 0 for air.
After the simulation, i tried to do some analysis on water level using sampling along a cross section. The results show there are sampling points with alpha value between 0 and 1, like 0.7 sth. I am wondering if there is any standard to determine if the point is considered to be air or water based on the alpha value or other factors. For instance, i may say the point is considered to be water if alpha is bigger than 0.5 at that point, or sth else. Anyone have any suggestion? Thanks in advance. 

January 15, 2016, 04:00 

#2 
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Hesam
Join Date: Feb 2015
Posts: 139
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Hi Zhifang Hu,
In VOF method any value between 0 and 1 show the interface. base on VOF,volume fraction (alpha) is : volume of liquid that occupy the cell/total volume of cell,therefore,you can argue that when alpha is 0.7,70% of cell occupied by liquid and 30% occupied by gas.therefore,in alpha=0.7 you have mixture of gas and liquid that has more contribution of liquid. 

January 17, 2016, 19:15 

#3  
Member
Zhifang Hu
Join Date: Sep 2015
Location: Brisbane
Posts: 53
Rep Power: 3 
Quote:
Thanks for the reply. I do understand the volume fraction alpha, but what i am thinking is that if there is some kinds standard threshold value for alpha to define the point if it can be considered as water or air. For instance, i can define alpha smaller than 0.5, the point to be air, else is water. or I calculate the point that which the mass of water is more than half to be water cell, or may be alpha is zero for air, else for water. etc I did some research on it, but nvr find one till now, thats why i am wondering if this kind of threshold exist in defining a point to be water or air. 

January 18, 2016, 03:24 

#4 
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Anton Kidess
Join Date: May 2009
Location: Delft, Netherlands
Posts: 1,163
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If you want to do it properly, you will need some sort of geometric reconstruction such as PLIC.
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