## Fluids

#### Diameter secondary phase

• prebenjs
Subscriber

I want to simulate a two-phase system with the eulerian multifluid-VOF. The system contains two continuous phases (gas-liquid), where the liquid is set as secondary phase and should not contain any granular particles. My question is, Why do i Fluent automatically set a diameter for the secondary phase ? When i would not like to set a diameter here. Does this mean that one cannot simulate two continuous fluid in the mutlifluid VOF?

• DrAmine
Ansys Employee
Because the Fluent terminology assumes you have a primary and secondary phase and that the secondary phase is the dispersed phase. As you have two continuous phases you can leave that diameter unchanged and just use symmetric drag laws or AIAD or Anisotropic Drag on top of symmetric or AIAD interfacial area. In that sense the diameter is just a mean to express interfacial area and influence drag!
• prebenjs
Subscriber
Thanks for the great answer. Would this diameter affect the "breakage" of the water ? Since I'm interested in the when the water "breaks". Let's say we have a T-junction, and the water is supplied at vertical piper while air in the horizontal. Would it be better to use interfacial area concentraion to allow for sauter mean diameter ?
Summarized : Two continuous phases where water breaks in a T-junction. What would be the most appropriate setting here (not found anything about in the Guides) ? Just find it odd that the multifluid-VOF contains so many more parameters than the VOF (and i have to use Multifluid-VOF as i need the separate pressure of the phases).
• Rob
Ansys Employee
Multifluid VOF is designed for when the phases may become mixed in part of the domain. In your case, if the phases are mixed at the inlet you need multifluid VOF, if the inlet is stratified/annular you may want to consider VOF. Not sure why you need a separate pressure field?
• prebenjs
Subscriber
The need for separate pressure field is due to post-processing of results. Do you have any tips/answer on my question above?
• Rob
Ansys Employee
How big is the pipe? How fast is the flow? What flow regime to you expect on the corner? With VOF you'll need to resolve any spray in the model which could get expensive due to to the cell count. With Eulerian I'd tend to estimate a sensible droplet size based on an understanding of the physics. What did your supervisor suggest?
• prebenjs
Subscriber
Unfortunately I have gotten no answer from him. What does this diameter parameter affect? Will it affect the breakage and size of the droplets formed?
• Rob
Ansys Employee
Read the theory guide on how the Multifluid VOF model works.
• DrAmine
Ansys Employee
It will affect drag and interfacial area. If you do not have proper resolution and you interested in all of these breakage and size evolution then M-VOF alone is not sufficient. M-VOF is used to model VOF like applications where the single velocity VOF solution is not valid anymore.