# VOF - slug flow modeling

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 February 9, 2007, 09:53 VOF - slug flow modeling #1 Nick Guest   Posts: n/a Dear All, I wonder if someone knows how to model a two phase flow (water + air) slug flow in a vertical tube with VOF with Fluent. The two phase flow is coming together at the inlet of the tube but I don't know how to define that boundary condition (like 50-50 water-air) where each phase has a velocity of 0.5 m/s. Thank you for the help Nicolas

 February 12, 2007, 07:08 Re: VOF - slug flow modeling #2 Rizwan Guest   Posts: n/a if u have only 1 inlet, then this should not be a problem. define water also as the the other material in the material panel. once this is done, define ---> phases becomes active. there you can select air and water as primary & secondary phase or vice versa. at the inlet boundary condition, u specify inlet velocity (for example) for the mixture, but for individual phases u can only select 50 % VOF of secondary phase. the primary phase wil be the rest regards

 February 12, 2007, 10:57 Re: VOF - slug flow modeling #3 Nick Guest   Posts: n/a Dear Rizwan, Thank you for your help, but I already tried what you mentioned. These are my simulation conditions: -Solver: Pressure based, implicit formulation, 2D space, unsteady, -Multiphase model: VOF, implicit and implicit body force. -Operating conditions: I left the default values, but in the experiment the inlet pressure is 15kPa. And I specified the operating density of air. -Boundary conditions: Inlet (velocity inlet): Mixture: 1 m/s, Water: nothing, Air: volume fraction 0.5 Outlet: outflow Solution: Velocity-pressure coupling: PISO, Pressure: PRESTO, Momentum & Volume fraction: 1st order upwind, Under-relaxation factor: default values Also I don't understand how I should fill the "patch" function? Regards

 February 12, 2007, 17:08 Re: VOF - slug flow modeling #4 Rizwan Guest   Posts: n/a Operating condtions: enable gravity, and enter value of gravity in the component forms. Pressure: PRESTO, Change this to Body force weightage average. Patch: this can be done by first adapting regions. example like a certain region in ur domain is always occupied by air. fluent tutorial on vof has description how registers can be created and patched. remember that patching can be done only after solution is initialized. hope this helps regards

 February 13, 2007, 09:29 Re: VOF - slug flow modeling #5 Nick Guest   Posts: n/a Thank you very much for your help, I added the gravity and I changed the pressure solver from PRESTO to body force weighted. Regarding the Patch, because in the geometry I don't have a specific region where water (or air) is always occupied I think I don't need to use it (the two flows are coming simultaneously)? I am assuming that density and viscosity are constant but I will modify them later on. When I display the contours of density is not possible to see the slugs (liquid and gas), I mean that when I select for example mixture it shows an average density (like 500 kg/m3), if I choose air or water it shows the air and water density respectively with a small changes in the decimals so I am not sure is the solution is OK or not. Because the next step in my restart is to model the was of the tube like a porous media and take into account the viscosity (share rate-stress) Regards,

 February 13, 2007, 11:26 Re: VOF - slug flow modeling #6 Rizwan Guest   Posts: n/a I think it is better u look at ur statement of assuming density and viscosity constant when ther r two fluids air and water whose density ratio is 1000. i think the solution is not correct if this assumption is applied in ur case. regards

 February 14, 2007, 05:33 Re: VOF - slug flow modeling #7 Nick Guest   Posts: n/a My model is isothermal (15oC) therefore the density will be constant at least for air, I am planning to replace the water by sludge, which has some particles, and depending the concentration of particles the density and the viscosity (non-Newtonian fluid) will change a long the tube (depending on the shear rate-stress), so I would like that the model at least for air and water works right in the first part, I mean that the slug flow will be observe in the contours results. Regards,

 February 14, 2007, 05:48 Re: VOF - slug flow modeling #8 Rizwan Guest   Posts: n/a ok all the best.

 March 24, 2014, 12:35 #9 New Member   West Bengal Join Date: Mar 2014 Posts: 1 Rep Power: 0 dear all, I have got to simulate the velocity of rising air bubble in vertical bottle filled with water which is getting emptied under the effect of gravity. What boundary conditions should i give at the outlet of the bottle. I didn't understand since air will be rising up through outlet and at the same time water will be draining down. Thank you. Regards.

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