Fluids

Fluids

VOF – Drag force on moving Droplet

    • Rusbel.Ayala
      Subscriber

      Is it possible to determine drag force between particles on a VOF model?


      I have an 2d (air) domain with a droplet, patched via volume fraction.

      I have set up the drag force that the wall will experience over time. However is it possible to track the droplet over the plate, and give the drag?

      Using a custom UDF, or another method?

    • Rob
      Ansys Employee
      You want the drag on the free surface? You can get the pressure on the iso-surface but not sure if the facet normal (assuming there is one) is stored to let you get any vector information.
    • Rusbel.Ayala
      Subscriber
      Thank you for your response. I am trying to determine the lift and drag force the droplet experiences as it rolls, detached, and reattaches along a surface.
      I am trying to make lift vs time, and drag vs time plots. As the droplet moves through the domain.
      Mentioned VOF model is not compatible with DEFINE_DRAG_FORCE. particle_drag_force.
      Is there a method of determining these values?
      Thank you, truly
    • Rob
      Ansys Employee
      You're tracking an isosurface of phase. I'm not sure the force vector is available, so you may need to calculate from something using a UDF. It's very definitely not simple. It's also not going to roll as the liquid is likely to wet the wall surface.
    • DrAmine
      Ansys Employee
      Drag over the wall is straightforward via Report definition
    • DrAmine
      Ansys Employee
      Along the droplet you need to get shear rate and pressure and do the whole math to get shear stress and pressure force to derive drag and lift.
    • Rusbel.Ayala
      Subscriber
      By my understanding I could save additional data quantities in fluent such as: strain rate, static pressure, pressure coefficient, dynamic, absolute, total, and relative total pressure to further analyze in Post. and provide some code to determine the drag and lift force.
      However, the location of the droplet is crucial to determine these values in post, correct?
      I would have to determine a function or location after every droplet, such tracking a particle through the domain, or selecting a point in post to determine the drag and lift force?
      Thank you in advance!
    • DrAmine
      Ansys Employee
      Yes location is important.
    • DrAmine
      Ansys Employee
      You need to identify each droplet that is challenging. We gave lump detection feature which might be helpful and still require coding but I need to understand your final goal: what do want to do and why?
    • Rusbel.Ayala
      Subscriber
      I will look into lump detection.
      I am ultimately trying to determine the lift and drag forces of a droplet over time, in order to cross referencing my simulation methods and data to reference experimental data.
      The aim is to validate current simulations to experimental results. I am aware that at high air speeds, the droplet shape/surface tension breaks which would cause lots of problems with determining a lift force for example. But a spherical droplet traveling through space is the idea.
      I hope to further analyze other droplets, with varying properties and derive data from this.
      Thank you once again Dr. Amine!
    • DrAmine
      Ansys Employee
      Do you have a single droplet?
    • Rusbel.Ayala
      Subscriber
      I have 2 droplets that are static - a certain distance away from one another. They then merge into one another, forming a single larger droplet.
      Cheers
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