[dickes]

Hi there,

I am trying to extract the Volume of entrained air by an impinging jet during the filling process of liquids.

I inserted an Isovolume (Volume Fraction of Liquid < 0.65) which results in an Isovolume with two bodies (One for the Air above the Free Surface and One for the Bubble below the Free Surface).


I am able to quantify the volume of this Isovolume, but I have difficulties to seperate the volumes of the two bodies.

How can i extract the volume of only one of the bodies?

[ghorrocks]

Good point. This appears to be something missed in CFD-Post - that is, you can only define the domain and element type it makes a isovolume from. You cannot define it gets it from some other volume (such as another isovolume).

To work around this, define a new variable, equal to the volume fraction, but using a "if" statement to set the variable to zero on one side of the the free surface. Then you can do a isovolume of the new variable and get what you want.

[dickes]

Originally Posted by ghorrocks

Good point. This appears to be something missed in CFD-Post - that is, you can only define the domain and element type it makes a isovolume from. You cannot define it gets it from some other volume (such as another isovolume).

To work around this, define a new variable, equal to the volume fraction, but using a "if" statement to set the variable to zero on one side of the the free surface. Then you can do a isovolume of the new variable and get what you want.

Hello Glenn,

thanks for the reply, but I am not sure how to realize your suggestion.
My isovolume as of right now appears as following:


I implemented the expression if(water.Conservative Volume Fraction.Beta<=0.65,1,0) but I have no success in seperating the two volumes.

Thanks in advance for any further advice!

[Gert-Jan]

you should create a variable that is equal to your expression. Then you can create an isovolume using that variable.

[dickes]

When i use the following expression
if(water.Conservative Volume Fraction.Beta<=0.65,1,0)
as a variable, and create an isovolume for the variable at value = 0 i get the following Volume.


I tried all options for the isovolume (at value, below value etc.) but could not get the right isovolume

As i understand the expression, the variable will take the value 1 for all parts in the domain where the volume fraction of the liquid is less than 0.65, and the value 0 otherwise. This will not seperate the two volumes to result in only the volume inside of the liquid.

How should i write the expression?

[Gert-Jan]

You could include the Y coordinate.
You should determine the Y value that lies between the bubble and the free surface. Say this is Y1. Then you should multiply the volume fraction with step(-Y1). This will be zero above Y1 and 1 below Y1. In this way you can separate both volumes.

Certainly this won't work if the bubble is much more complicated in shape, but here it should work. Alternatively, you can include X and Y.

[dickes]

Yeah, I was thinking about this, but as I want to track the bubble volume during the whole transient simulation this won`t help for more complicated bubble types.

I also started with image processing of the transient video file with matlab but think a solution within ANSYS CFX Post will be more accurate.

EDIT: I used the wrong variable with water.Conservative Volume Fraction.Beta, I should have used water.Conservative Volume Fraction

With the inclusion of the Y-Coordinate in my expression I am able to get the Volume of the bubble. But I will not be able to track the bubble size during the whole transient simulation with this method.

[ghorrocks]

Yes, sound like you need a more sophisticated technique for tracking this over the whole simulation. Look at blob detection algorithms from image processing techniques, these may have some useful ideas. There are some nice libraries in python to do this which I have used often, such as opencv, scikit (https://scikit-image.org/) and mahotas. Interfacing these with CFX to run automatically will be an interesting challenge