How interfaceHeight works?

Malyadeep · November 17, 2020, 03:16

Hi!

I want to track an interface with time and came across interfaceHeight function object. I am wondering how it calculates the interface position. Does the position it calculate has anything to do with the direction of gravity?

Thanks!

simrego · November 17, 2020, 08:48

Hi!

You can check the code here: https://openfoam.com/documentation/g...8C_source.html
You are interested in the writePositions() function starting from line 54.

As I see it is using the gravity.

But you can do some similar stuff on your own. For example write out the alpha value along a line for each time, and with a single python script for example you can run through the times, and in every time you just have to find the position where alpha = 0.5 for example.

As i see this code does something like integrating the alpha value through this line which will be your height.

Malyadeep · November 17, 2020, 09:08

Hi!

Thanks for your reply. I considered using the approach you suggested but haven't tried it yet. Will give it a try.

Quote:

Originally Posted by simrego

As i see this code does something like integrating the alpha value through this line which will be your height.

Could you explain the concept behind this? I am confused after seeing the results if there are more than one interfaces present.

Thanks!

simrego · November 17, 2020, 10:31

If there are more then one interface it won't work. But I just ran through the code and for me it seems like it is just integrating the alpha.
So imagine you have a line where you are looking for the interface. You divide it into segments. For each segment you have a p0 starting, and a p1 end point. So for every segment you calculate the segmengLenght*(a0+a1)/2, where a0 and a1 are the alpha values at the two ends of the segment. And if you add these values for all the segments, you numerically integrated the alpha through the line. And since alpha must be between 0 and 1, you can also imagine alpha as a weight, so you add up all the lengths where alpha is 1, and you won't add anything to this sum where alpha is 0. So this will gives you the length along a line where alpha is 1, which is your interface height.
At least from the code I think this is what's happening, and for me it makes sense.

But If you have for example water, air, water, and air "regions" from bottom to top, with this method the height will be the sum of the two water "regions" height, which will be incorrect(?). But for example if you have a really small bubble in your water column compared to the water column height, it will be still fairly accurate.

But if you write the alpha along a line, then you can find all of the interfaces with some simple programming. You only have to look for the heights where alpha changes from zero to one, or one to zero, and these will be your interface heights so you can find all of them.

Malyadeep · November 17, 2020, 11:18

Thank you so much for the explanation. It cleared a lot. I will consider writing alpha along a line to get the interface position.

November 17, 2020, 03:16	How interfaceHeight works?	#1
Malyadeep New Member Malyadeep Bhattacharya Join Date: Nov 2020 Posts: 5 Rep Power: 6	Hi! I want to track an interface with time and came across interfaceHeight function object. I am wondering how it calculates the interface position. Does the position it calculate has anything to do with the direction of gravity? Thanks!

November 17, 2020, 10:31		#4
simrego Senior Member anonymous Join Date: Jan 2016 Posts: 416 Rep Power: 14	If there are more then one interface it won't work. But I just ran through the code and for me it seems like it is just integrating the alpha. So imagine you have a line where you are looking for the interface. You divide it into segments. For each segment you have a p0 starting, and a p1 end point. So for every segment you calculate the segmengLenght*(a0+a1)/2, where a0 and a1 are the alpha values at the two ends of the segment. And if you add these values for all the segments, you numerically integrated the alpha through the line. And since alpha must be between 0 and 1, you can also imagine alpha as a weight, so you add up all the lengths where alpha is 1, and you won't add anything to this sum where alpha is 0. So this will gives you the length along a line where alpha is 1, which is your interface height. At least from the code I think this is what's happening, and for me it makes sense. But If you have for example water, air, water, and air "regions" from bottom to top, with this method the height will be the sum of the two water "regions" height, which will be incorrect(?). But for example if you have a really small bubble in your water column compared to the water column height, it will be still fairly accurate. But if you write the alpha along a line, then you can find all of the interfaces with some simple programming. You only have to look for the heights where alpha changes from zero to one, or one to zero, and these will be your interface heights so you can find all of them. M.W.G. likes this.

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November 17, 2020, 08:48		#2
simrego Senior Member anonymous Join Date: Jan 2016 Posts: 416 Rep Power: 14	Hi! You can check the code here: https://openfoam.com/documentation/g...8C_source.html You are interested in the writePositions() function starting from line 54. As I see it is using the gravity. But you can do some similar stuff on your own. For example write out the alpha value along a line for each time, and with a single python script for example you can run through the times, and in every time you just have to find the position where alpha = 0.5 for example. As i see this code does something like integrating the alpha value through this line which will be your height.

November 17, 2020, 11:18		#5
Malyadeep New Member Malyadeep Bhattacharya Join Date: Nov 2020 Posts: 5 Rep Power: 6	Thank you so much for the explanation. It cleared a lot. I will consider writing alpha along a line to get the interface position.