Fluids

Fluids

Possible to set a constant contact diameter BC for a droplet/bubble (VOF)?

    • tallred11
      Subscriber

      I'm wondering if there is a way to fix a constant contact diameter during bubble growth? For instance, evaporation is known to generally occur in either a constant contact angle mode or a constant contact diameter mode. Obviously, I can set a constant contact angle for a droplet or bubble very simply. But is there a way to set a constant contact diameter where the contact angle is allowed to change as the droplet shrinks, but the contact line remains fixed? Ultimately, I'd like to use a UDF to toggle between these BCs during different conditions.


       


      Thanks!

    • DrAmine
      Ansys Employee
      There is no diameter description in VOF model. The parameter you might influence is the volume fraction in the cell which influences the curvature and hence the morphology.
    • tallred11
      Subscriber

      Thanks for the reply. Can you be a bit more specific on how you would control the contact diameter by altering the volume fraction in a cell? You would still require a contact angle to have wall adhesion correct? So, if you altered the volume fraction in the cells near the surface to try to change the droplet morphology, would it just recorrect itself back to the contact angle provided?



      Perhaps I'm not understanding what needs to change with the contact angle boundary conditions to do what you're describing.


       


      Thanks!

    • Abhaysinghcharan
      Subscriber
      Hello, I'm a student doing a project which requires me to maintain a bubble diametre of 400 microns.
      Would you please help me by guiding me through this ?

      Thank you.
    • Rob
      Ansys Employee

      Can you add more details of what you're trying to model? One way is to use the Eulerian model where you set the secondary phase diameter but whilst that answers your question it may not be suitable for what you're trying to do. 


      Multiphase flows require a good understanding of the physics before you go near the CFD code, otherwise you may model the wrong thing! 

    • DrAmine
      Ansys Employee
      There is nothing magic about the contact angle in Fluent as it is just used to describe the surface normal of the near wall cell and thus will affect the curvature. So if one needs to change the curvature either you manipulate contact angle or the volume fraction.
    • tallred11
      Subscriber

      I understand that I can affect the curvature with either the contact angle or manipulation or the cell volume fractions, but I suppose what I'm asking is - won't the contact angle BC and the manual manipulations of the cell volume fractions be in competition? Say I have a contact angle set to 60°. If I keep the diameter fixed by changing the cell volume fractions near the based such that the contact angle would now be 80°, won't it just try to adjust back to 60° at the next time step? So, would I need to alter the contact angle boundary condition step by step as well? Or is there a way to have wall adhesion without a specific contact angle?


      Also, I have some follow-up questions necessary for implementation, 


      1) How exactly does Fluent determine the contact angle? How many cells does it consider in determining the surface normal?


      2) Can you point me towards any resources for developing a UDF that manipulates volume fraction in a particular cell (e.g. sections of the UDF manual or examples?)

    • DrAmine
      Ansys Employee

      1/ near wall cell is the active live cell where the surface normals are adjusted. 


      2/ degassing udf as an example 

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