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Difference of definition between HEM and VOF models

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Old   February 12, 2014, 02:16
Default Difference of definition between HEM and VOF models
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Baris (Heewa)
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Dear Foam USers,

I am trying to make simulation of cavitation in nozzle and confused about the differences between HEM and VOF models which are used in cavitatingFOAM and InterPhaseChangeFoam.

When i look at the definitions of the solvers:

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1. [B]interPhaseChangeFoam[/B]: (incompressible) VOF+ Transport equation of vapor fraction→ source terms for vaporization and condensation 

2. [B]cavitatingFoam[/B]:(compessible) HEM (homogeneous equilibirum model)+ Barotropic equation of state 
ρ=f(P)→EOS in continuity eq. to produce pressure eq. (linear model for compressibility)
Both model are isothermal and dont solve the energy equations while solving closure equations as transport eqn. and barotropic.

Also when i look at the original Schneer
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[Schnerr G, Sauer J. Physical and numerical modeling of unsteady cavitation dynamics.  Fourth International Conference on Multiphase Flow, New Orleans, USA2001.]
and Yuan
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[Yuan W, Schnerr GH. Optimization of Two-Phase Flow in Injection Nozzles: Interaction of Cavitation and External Jet Formation. ASME; 2002.]
model papers, he explains the VOF model as: The bubble flow is treated as homogeneous vapor-liquid mixture which means that The gas-liquid interface is assumed to be in dynamic and thermal equilibrium and, consequently, only one set eqns are solved for mixture phase.

and in another paper above explanation is called homogeneous mixtured model.:
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 Kunz RF, Boger DA, Stinebring DR, Chyczewski TS, Lindau JW, Gibeling HJ, et al. A preconditioned Navier–Stokes method for two-phase flows with application to cavitation prediction. Computers & Fluids. 2000;29:849-75.
==>So, This VOF explanation (inside of interPhaseChangeFoam) is totally same with definition of HEM model (inside of cavitatingFoam) . Then, why in the solver explanation of the Openfoam tutorials, it is written for interphaseChangeFoam as: Solver for 2 incompressible, isothermal immiscible fluids with phase-change (e.g. cavitation). Uses a VOF (volume of fluid) phase-fraction based interface capturing approach...(http://www.openfoam.org/docs/user/standard-solvers.php)

If i am wrong please someone can correct me...

Thanks in advance...
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Old   February 16, 2014, 14:48
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Bruno Santos
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wyldckat is just really nicewyldckat is just really nicewyldckat is just really nicewyldckat is just really nice
Hi shipman,

In my limited experience on this topic:
  1. Be very careful with which version of OpenFOAM you are dealing with. The latest release version is 2.2.2 and the bug fix version is 2.2.x. If you are using any older versions of OpenFOAM, you might be looking at code that is completely outdated.
  2. cavitatingFoam seems to rely on thermodynamic characteristics.
  3. interPhaseChangeFoam is explicitly isothermal. Quoting from the source code first big block of comments in the header section of "interPhaseChangeFoam.C" (from OpenFOAM 2.2.x):
    Quote:
    Code:
        Solver for 2 incompressible, isothermal immiscible fluids with phase-change
        (e.g. cavitation).  Uses a VOF (volume of fluid) phase-fraction based
        interface capturing approach.
    
        The momentum and other fluid properties are of the "mixture" and a
        single momentum equation is solved.
    
        The set of phase-change models provided are designed to simulate cavitation
        but other mechanisms of phase-change are supported within this solver
        framework.
  4. OpenFOAM mostly uses segregated numerical methods: http://www.openfoam.org/features/numerical-method.php - so it's natural that energy is not solved simultaneously with transport.
Beyond this, I need more information, namely regarding which specific equations do you see that are identical in both solvers.

Best regards,
Bruno
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