# How does ANSYS FLUENT model the surface tension in the Eulerian (multi-fluid) model?

 Register Blogs Members List Search Today's Posts Mark Forums Read September 1, 2018, 18:33 How does ANSYS FLUENT model the surface tension in the Eulerian (multi-fluid) model? #1 New Member   Join Date: Feb 2018 Posts: 5 Rep Power: 7 I am using ANSYS FLUENT to model a two-phase flow of liquid and gas where the surface tension force is included. In the theory of the VOF model, the surface tension force is added to the momentum equation as a source term. However, in the Eulerian model, we have two momentum equations, one for the liquid and one for the gas. How is the surface tension force included? is it added to the liquid momentum equation?is it added to the gas momentum equation? is it split between the two phases? If it is split, is it split by two volume averaged factors or two mass averaged factors? Unfortunately, the theory manual does not mention related information under the Eulerian model theory. Last edited by mohamedh; September 3, 2018 at 09:33.   September 3, 2018, 09:29 #2 Senior Member   Join Date: Aug 2014 Location: Germany Posts: 292 Rep Power: 12 This is a very interesting question! I always wondered if and how the surface tension force in an Eulerian-Eulerian framework is modelled. My current view on this is, that it is actually non relevant in E-E framework, since it is the force that describes interface curvature. This is only relevant in a VOF type simulation. In E-E the interfaces are not simulated. I would be very happy if anyone could confirm this.   September 4, 2018, 11:09 #3 New Member   Join Date: Feb 2018 Posts: 5 Rep Power: 7 I found the answer. It is split by two volume averaged factors. In other words, it is multiplied by the volume fraction of the given phase and added to its momentum equation as a source term.   September 5, 2018, 03:14 #4
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 Originally Posted by mohamedh I found the answer. It is split by two volume averaged factors. In other words, it is multiplied by the volume fraction of the given phase and added to its momentum equation as a source term.
Do you have a source for this?   September 5, 2018, 09:33 #5
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 Originally Posted by BlnPhoenix Do you have a source for this?
I asked the same question in the ANSYS Student Community, someone from ANSYS stuff answered it.

The answer does make sense for me. If you looked at a momentum equation for the Eulerian model, you will find that each term ( pressure force, shear force ...etc) is multiplied by the volume fraction so that if you add the two momentum equations of the phases the volume fractions add up and become unity. .
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They deal with the surface tension force in a similar manner.   September 5, 2018, 11:15 #6
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 Originally Posted by mohamedh I asked the same question in the ANSYS Student Community, someone from ANSYS stuff answered it. The answer does make sense for me. If you looked at a momentum equation for the Eulerian model, you will find that each term ( pressure force, shear force ...etc) is multiplied by the volume fraction so that if you add the two momentum equations of the phases the volume fractions add up and become unity. . . They deal with the surface tension force in a similar manner.
Yes, but still i don't understand why modelling the Surface Tension is important in an E-E type simulation. When you have a dispersed like flow and the interface is not resolved why should i include surface tension in my simulation? To my knowledge the surface tension describes the bubble shape and therefore the drag, the drag force is modelled in E-E. So the effect of surface tension force is embedded in the drag force calculation. What extra force is acting in the momentum exchange, when i include the surface tension? It would make sense to use it if i actually resolve an interface but for a fully dispersed (under-resolved) situation it's not a force acting like Lift, drag or Virtual Masse force imho..

Am i missing something? Thx!   September 5, 2018, 11:41 #7
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 Originally Posted by BlnPhoenix Yes, but still i don't understand why modelling the Surface Tension is important in an E-E type simulation. When you have a dispersed like flow and the interface is not resolved why should i include surface tension in my simulation? To my knowledge the surface tension describes the bubble shape and therefore the drag, the drag force is modelled in E-E. So the effect of surface tension force is embedded in the drag force calculation. What extra force is acting in the momentum exchange, when i include the surface tension? It would make sense to use it if i actually resolve an interface but for a fully dispersed (under-resolved) situation it's not a force acting like Lift, drag or Virtual Masse force imho.. Am i missing something? Thx!
The effect of surface tension is not included in the drag!

The drag force is a result of the friction at an interface separating two phases. If the two phases are solid and air, we do not have surface tension! Surface tension only exists on liquid free surfaces.

Moreover, the flow does not have to contain bubbles. You may have cases like stratified flow where you have a wavy interface between the two phases with no bubbles exist! The surface tension is there at the liquid-gas interfaces either bubbles exist or not.

One difference between the drag and the surface tension is that the drag force is in the direction of the relative velocity, while the surface tension force is always normal to the interface. For an air bubble in water, for example, the surface tension force on the bubble surface add up to the pressure force acting on the bubble surface. So if the surface tension is so small compared to the inertia force, it can be neglected.   September 6, 2018, 03:59 #8
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 Originally Posted by mohamedh The effect of surface tension is not included in the drag!
It depends on the drag Force model. If i use Schiller-Naumann it's only for rigid objects without shape deformation. If i use Ischii-Zuber, the bubble shape is calculated with influence of the surface tension.

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 Originally Posted by mohamedh The drag force is a result of the friction at an interface separating two phases. If the two phases are solid and air, we do not have surface tension! Surface tension only exists on liquid free surfaces.
I agree! It's basically what i said above.

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 Originally Posted by mohamedh Moreover, the flow does not have to contain bubbles. You may have cases like stratified flow where you have a wavy interface between the two phases with no bubbles exist! The surface tension is there at the liquid-gas interfaces either bubbles exist or not.
I agree. It's what i meant, when i said, it's used when there is an interface modelled in the E-E simulation.

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 Originally Posted by mohamedh One difference between the drag and the surface tension is that the drag force is in the direction of the relative velocity, while the surface tension force is always normal to the interface.
The drag force acts against the relative velocity. It slows down rising or sinking velocity of an object other than that, i fully agree.

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 Originally Posted by mohamedh For an air bubble in water, for example, the surface tension force on the bubble surface add up to the pressure force acting on the bubble surface. So if the surface tension is so small compared to the inertia force, it can be neglected.
Ok, but this does not contribute anything extra to interface shape, does it.
So in total: if i assume rigid spheres and do not resolve any wavy interface, surface tension Force does not contribute anything to momentum exchange between the E-E phases and i do not need to include it.  Tags eulerian model, fluent, multiphase flow, surface tension, vof model Thread Tools Search this Thread Show Printable Version Email this Page Search this Thread: Advanced Search Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are On Refbacks are On Forum Rules Similar Threads Thread Thread Starter Forum Replies Last Post tom96 FLUENT 3 October 12, 2018 08:43 roi247 FLUENT 4 October 15, 2015 13:41 adilsyyed CFX 15 June 24, 2015 19:42 Attesz CFX 7 January 5, 2013 03:32 Descott88 CFX 11 April 20, 2012 07:41

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