VOF model

piyusharas · July 2, 2015, 08:14

I am working on a multiphase model .
It consist of a tank with a velocity inlet and 6 pressure outlets.
I have used V.O.F model
I want to find mass flow rates in 6 outlets is steady state.
I asked this doubt before and someone said that vof model do not converge in steady state so now I am using finding in transient condition.
everything is almost done but my solution is still not converging and also there are reversed flow at my all outlets. To this doubt everywhere its posted that extend the outlet . Can someone tell me what does it mean.

hwet · July 5, 2015, 21:58

It is normal to have reversed flow.
If you draw a vector plot on a plane inside your geometry you will see at the outlet some vectors pointing backwards into the flow domain.
If you have an extended outlet then this reverse flow will not cause any difficulty in convergence in cases it is not supposed to occur, the flow field has enough space to "steady out".
But it is turbulence modeling, when the solver solves the equations there is a fluctuating pressure field inside the domain, (since flow is turbulent), so at times it has a lower pressure inside the domain and high pressure at the outlet which can cause reverse flow. (just an example)
If you extend the geometry and still have reversed flow dont worry about it. Imagine even in the physical world if flow is concentrated on one part of the outlet it is possible that some fluid (very tiny bit) flows back frm the other end of the outlet.
And this is my personal research on the topic cant guarantee if i am right or wrong. Hope i make sense

piyusharas · July 6, 2015, 01:17

I am confused whther to use turbulance model or not.
Can you please help me out with it.
And when I use turbulance model what are values if parameters which I will have to give in the model.

hwet · July 6, 2015, 22:21

It depends on your conditions, i cant just tell if all i know is there is an inlet and a few outlets to use a turbulence model or not.

But, well most real world flows are turbulent in nature...

July 2, 2015, 08:14	VOF model	#1
piyusharas Member Piyush Aras Join Date: Jun 2015 Posts: 34 Rep Power: 10	I am working on a multiphase model . It consist of a tank with a velocity inlet and 6 pressure outlets. I have used V.O.F model I want to find mass flow rates in 6 outlets is steady state. I asked this doubt before and someone said that vof model do not converge in steady state so now I am using finding in transient condition. everything is almost done but my solution is still not converging and also there are reversed flow at my all outlets. To this doubt everywhere its posted that extend the outlet . Can someone tell me what does it mean.

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July 5, 2015, 21:58		#2
hwet Senior Member Join Date: Mar 2014 Posts: 375 Rep Power: 13	It is normal to have reversed flow. If you draw a vector plot on a plane inside your geometry you will see at the outlet some vectors pointing backwards into the flow domain. If you have an extended outlet then this reverse flow will not cause any difficulty in convergence in cases it is not supposed to occur, the flow field has enough space to "steady out". But it is turbulence modeling, when the solver solves the equations there is a fluctuating pressure field inside the domain, (since flow is turbulent), so at times it has a lower pressure inside the domain and high pressure at the outlet which can cause reverse flow. (just an example) If you extend the geometry and still have reversed flow dont worry about it. Imagine even in the physical world if flow is concentrated on one part of the outlet it is possible that some fluid (very tiny bit) flows back frm the other end of the outlet. And this is my personal research on the topic cant guarantee if i am right or wrong. Hope i make sense

July 6, 2015, 01:17		#3
piyusharas Member Piyush Aras Join Date: Jun 2015 Posts: 34 Rep Power: 10	I am confused whther to use turbulance model or not. Can you please help me out with it. And when I use turbulance model what are values if parameters which I will have to give in the model.

July 6, 2015, 22:21		#4
hwet Senior Member Join Date: Mar 2014 Posts: 375 Rep Power: 13	It depends on your conditions, i cant just tell if all i know is there is an inlet and a few outlets to use a turbulence model or not. But, well most real world flows are turbulent in nature...