Fluids

Fluids

Adding momentum source in VOF multiphase flow.

    • friday.abolorunke
      Subscriber


      Hi All,

      Description of my problem:

      Immiscible gas-liquid multicomponent two-phase free surface flow. The gas and liquid phases are immiscible, while their components or species are miscible in both phases. I initially started with Eulerian model, but I want to switch to VOF method to get quick result for sanity check before running the full Euler model.

      Questions:

      For my eulerian model, I have a UDF for species interphase mass transfer (define-linearized-mass-transfer) and interface momentum transfer during to the transfer of mass across the interface( define-exchange-property). Now, i see that I can't use the define exchange udf anymore in my VOF setup, bcos the option is not available on the phase interaction panel. And I must some how include my interface momentum balance in the set up.

      1. Will Define_Source do the same job? If I reformulate my define-exchange property to define-source and add to the fluid zone as source term will that do same thing as my define-exchange-property udf.
      2. Do I have to include "if(grad_of_VolumeFraction ! = 0)" to my define-source udf to restrict the momentum change to the interface? or fluent will automatically do that for me?
    • DrAmine
      Ansys Employee
      You need to restrict mass transfer to occur only in interfacial cells if you are using UDF. You can do everything Souce Macros but it's better to rely on dedicated mass transfer macros.
    • friday.abolorunke
      Subscriber
      Thank you for your response. Yes, I restricted my mass transfer to the interfacial cells using grad_vof !=0. I used the define linearized mass transfer macro.
      I am asking if I should restrict the momentum source to the interfacial cells using same flag bcos the momentum transfer occurs at the interface or fluent will automatically know to do that.
      More so, if i use define-source macro for my momentum source at the interface in the VOF model does it do same thing as define-exchange-property macro available in Eulerian model?
    • DrAmine
      Ansys Employee
      Mdot will be zero elsewhere if done properly so you do not to restrict it again.
    • DrAmine
      Ansys Employee
      You do not require momentum source for phase change with Vof model.
    • friday.abolorunke
      Subscriber
      Thank you! This really helped a lot.
    • DrAmine
      Ansys Employee
      Welcome: why you do not need it -> in Vof both phases share same velocity so it is on bulk level zero if you want to add it .
    • friday.abolorunke
      Subscriber
      Oh. Yes, your response made me think carefully about it and I realized it will be zero. Since the velocity of the liquid, gas and interface is the same that contribution to the momentum bal at the interface will be zero. See figure below for momentum bal at the interface (surface tension effect was neglected). The viscous normal stress term won't cancel out as the liquid and gas viscosities are different. However, the magnitude of this term (eqn in red) is small and won't really affect the physics that much.
      To rigorously capture the physics of my problem I will be using the Eulerian model. In my problem, the liquid and gas velocities are different and pressure discontinuity is expected on the interface. For now, i just wanna use VOF to get a quick result and see if the model behaves as expected.

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