What is the interface: volume or mass fraction?

jpina · September 23, 2015, 17:07

Hello,

I've carried out a simulation where a polymer gets into a cavity full of air.

The question is that in CFD-Post I can plot the contour of "Polymer - Mass fraction" or "Polymer - Volume fraction".

It turns out that, due to the different density of the two fluids, the result is very different.

So, the question is, which result plots the interface between the two fluids?

abhanja · September 24, 2015, 00:18

Hi,

If you are using the VOF model, then it tracks the volume fraction of each of the fluids throughout the domain (including the interface) by solving a single set of momentum equations. So might well use the volume fraction.

Regards

jpina · September 24, 2015, 04:20

Dear Abhanja,

If the 0.5 contour line of the volume fraction result is the interface between the two phases...

What is physically the 0.5 contour line of the mass fraction result??

landerfwc · September 29, 2015, 07:42

Hi,

I also would use volume-fraction, since this represents an interface, which you can see by eye. But in case the mesh is fine enough, I dont think these two contours shall be significantly different - regardless from the densities, the difference should be only within a few cells.

If you used mass-weighted and you want to find out, whether some cavity is filled with a polymer or not, this 0.5 contour would show a cell, which is filled with a polymer in a small part (depending on densities ratio). This, IMO, is not considered to be an appropriatelly filled cavity.

September 23, 2015, 17:07	What is the interface: volume or mass fraction?	#1
jpina Senior Member Jordi Pina Join Date: Mar 2015 Posts: 157 Rep Power: 11	Hello, I've carried out a simulation where a polymer gets into a cavity full of air. The question is that in CFD-Post I can plot the contour of "Polymer - Mass fraction" or "Polymer - Volume fraction". It turns out that, due to the different density of the two fluids, the result is very different. So, the question is, which result plots the interface between the two fluids?

September 29, 2015, 07:42	Interface	#4
landerfwc New Member Join Date: Jun 2014 Posts: 17 Rep Power: 12	Hi, I also would use volume-fraction, since this represents an interface, which you can see by eye. But in case the mesh is fine enough, I dont think these two contours shall be significantly different - regardless from the densities, the difference should be only within a few cells. If you used mass-weighted and you want to find out, whether some cavity is filled with a polymer or not, this 0.5 contour would show a cell, which is filled with a polymer in a small part (depending on densities ratio). This, IMO, is not considered to be an appropriatelly filled cavity.

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September 24, 2015, 00:18		#2
abhanja New Member AmBh Join Date: Aug 2015 Posts: 7 Rep Power: 11	Hi, If you are using the VOF model, then it tracks the volume fraction of each of the fluids throughout the domain (including the interface) by solving a single set of momentum equations. So might well use the volume fraction. Regards

September 24, 2015, 04:20		#3
jpina Senior Member Jordi Pina Join Date: Mar 2015 Posts: 157 Rep Power: 11	Dear Abhanja, If the 0.5 contour line of the volume fraction result is the interface between the two phases... What is physically the 0.5 contour line of the mass fraction result??