How does ANSYS FLUENT model the surface tension in the Eulerian (multi-fluid) model?
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. |
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. |
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.
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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. |
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Am i missing something? Thx! |
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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. |
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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. |
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Then, How can I calculate / generate result of finding value of surface tension ? |
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