In air-water multiphase flow CFD simulation's it is common to use a VOF = 0.5 to track or identify the position of interface or free surface. I would like to know why this criteria is used and if it can always be assumed the same. Otherwise, I would like to know if volume fraction can be taken different to 0.5 and, if so, ask them about a methodology to find this volume fraction.
Below I describe a case that I am simulating in Ansys Fluent:
-Sewer pipe with Length L=6 m (aligned with the x axis), Diameter D=200 mm (8 in) and longitudinal slope So = 0.005 m/m.
-Simulation in steady state of sewer pipe considering biphasic flow air-water flow.
-Model Volume of Fluid (VOF) with implicit formulation and Open Channel's submodel.
-Boundary conditions: Inlet as mass flow rate = 26.51 kg / s; outlet = Pressure outlet. The mass flow entered is such that the ratio y / d = 0.61 (that is, occupation of 61%).
-Turbulence model: k-epsilon RNG with Enhanced Wall Treatment function (y +
In the volume of fluid (VOF) scheme, the value of F equals to one for water and zero for air domain. Commonly, the value of F in the flow free surface is assumed as 0.5.
The iso-value = 0.5 is a highly suggested value for capturing iso-surface. By changing this value the elevation of the free surface will change because of different taken cell.
I recommend to compute a sharpen free surface instead of modifying iso-value.
Regards.
Dear Leonardo Henao
Sorry for the late reply. In the case of flow in a channel, in fact, what you are using is the Eulerian method which means all grids stay fixed without any motion in their location, and this is the volume of fluid that determines the portion of each phase of volume in meshes over the interface. So let me put it in this way, we suppose that you are going to start your simulation with two phases of fluids including water and air. Since the flow at the incipient moment of simulation is completely still over the water surface (interface), consideration of 0.5 means that you allocated the same portion of water and air for the cells covered the interface which makes sense. based upon the foundation of the volume of fluid, the step function actually determines the portion of each phase in the cell through ε(cell)=∫∫∫(x,y,z)dxdydz/∫∫∫dxdydz.
if ε=0 for cell denotes cell is empty of fluid, some allocate it for whole-cell contains water
ε=1 denotes cell is full of fluid, some allocate it for whole-cell contains air
and 0
Hamed Azimi thank you for answer. In fact, this is what I initially assumed, because the free surface is localiced in a cell, in which volume fraction changes between 0 and 1, that is, it could perfectly be assumed to be at 0.80. F = 0.5 seems good to me and it's most obvious choice (as a kind of average), only the doubt arose because in my opinion it is still an arbitrary choice.
Reza Khazaee thank you for answer. You left me thinking about in these of that "by changing the value of 0.5, the elevation of the free surface will change due to different cells taken". What I had understood is that free surface is located in a single cell and that to know the location within this cell, then it could be assumed that F = 0.5. Thank you for the information because I don't know that free surface could be located in more than one cell. Regarding what you say about calculating a sharpness-free surface, do you recommend a method? I am using the Sharp option and the Compressive model
Hi Hamed Majidian. Thank you for you value contribution. It is very logical what you say and I agree. After reading the previous answers, I would think that at end of the simulation, the value to take of 0.5 is very logical, but not necessarily the only one that can be taken. If the mesh is correct in terms of quality and refinement, a free surface taken with F = 0.3 or F = 0.7 should be very close to that corresponding to F = 0.5, that is, as if they converged to said value of F = 0.5. I think that my issue consists of the mesh and therefore simulated flow profile with F = 0.5 it is not matching with analytical profile estimated.
Regards!
Leonardo Henao; Use the HRIC Scheme and refine the mesh in the z-direction (the wave elevation direction).
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