# number of equations/variables in VOF model

 Register Blogs Members List Search Today's Posts Mark Forums Read December 18, 2015, 11:18 number of equations/variables in VOF model #1 Senior Member   Join Date: Oct 2014 Posts: 124 Rep Power: 9 Hi members, the VOF model (for a turbine in my case) uses the momentum and continuity equation. In the VOF, the momentum equation is solved for the mixture, while the continuity equation is solved for the secondary phase. The further equation that the sum of the VOF is equal to 1. But, since there are two phases, is there a missing equation?? Because if I model a single phase problem, the continuity and monentum equations are enough, but for a two phase flow, I need more equations. The further equation that the sum of the VOF is equal to 1 is not sufficient... Could anyone explain this to me better? Where I am wrong? Thank you   December 21, 2015, 21:00 #2 Senior Member   Join Date: Mar 2014 Posts: 375 Rep Power: 10 The volume fraction is not solved for the primary phase, if you know the sum of the two volume fractions is 1 then it makes sense to just compute the primary phase faction from the logic that the sum is 1, why would you want to solve another equation when you can just calculate the volume fraction of the primary phase by 1-vf of secondary phase. That is what the solver actually does.   December 22, 2015, 10:13 #3 Senior Member   Join Date: Oct 2014 Posts: 124 Rep Power: 9 Thank you for your response. My doubt is this. Consider a single phase problem, without turbulence and incompressible flow (water). Solving the continuity and momentum equation the solver calculates pressure and velocity in each point. Two equations for two variables are satisfactory. But when the phases are two (with VOF method), I need the mixture pressure, the mixture velocity and the volume fraction of the secondary phase in each point, thus 3 variables, and I need an additional equation. The equation sum(phases)=1 is used to calculate the primary phase volume fraction, but only after that the secondary phase volume fraction has been calculated by the previous three ones (my doubt pertains to the third equation...). Thank you!!   December 22, 2015, 19:23 #4 Senior Member   Join Date: Mar 2014 Posts: 375 Rep Power: 10 As far as i understand the continuity equation is used to solve for the interface between the faces and only the momentum equations are shared. Read section 17.3 in the fluent theory guide, the full explanation is there,   December 23, 2015, 10:40 #5 Senior Member   Join Date: Oct 2014 Posts: 124 Rep Power: 9 Thank you. therefore, the solver solves the phase continuity equation for the secondary phase, then it calculates the primary phase volume fraction by (alpha_2=1-alpha_1) and then solves both the momentum and the continuity equation for the mixture. Thank you soumitra2102 likes this.   January 14, 2020, 20:48 #6
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 Originally Posted by Ema40 Thank you. therefore, the solver solves the phase continuity equation for the secondary phase, then it calculates the primary phase volume fraction by (alpha_2=1-alpha_1) and then solves both the momentum and the continuity equation for the mixture. Thank you
Thank You.
I hope it is right (please someone confirm), because, this thing have been bugging me since a long time.  Tags continuity equation, equations, momentum equation, phase equation, vof 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 minhaj_iut FLUENT 5 February 26, 2017 23:51 [mesh manipulation] Mesh Refinement Luiz Eduardo Bittencourt Sampaio (Sampaio) OpenFOAM Meshing & Mesh Conversion 42 January 8, 2017 12:55 Shogan FLUENT 1 May 28, 2014 15:03 blacksoil2012 Fluent Multiphase 2 February 8, 2014 23:59 qascapri FLUENT 0 January 24, 2011 10:48

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