How to: define fluid-fluid boundaries as 'wall'

Hi all

My case set-up is the following: I have 3 fluid domains. 1) the smallest, inner tube (representing a simplified catheter), 2) the outer tube surrounding the catheter, and 3) the rest of the geometry, where the two tubes have merged. What I want to model is that fluid flows in from both the inlets of "1" and "2" and then start to mix when they have reached "3".

What I used to do is use mesh interfaces to define the contact regions between the three domains and set the appropriate conditions. Now that I've switched to Fluent Meshing for my meshing and use the "Share Topology" option, that doesn't seem to do the trick anymore.

What I've tried is I've defined both the contact region between "1" and "2" (i.e. the 'wall' of the inner tube), and the contact regions between "1"/"2" and "3" (i.e. the outlets of "1"/"2" and the inlet of "3") as 'interior', because they're fluid-fluid boundaries. However, there is a distinct difference between these types of boundaries: the region between "1" and "2" should be treated as a wall (no mixing), while the region between "1"/"2" should just allow the fluid to pass as if the contact region isn't there. My question is now: how do I define that?

Using just default 'interior' setting, I get this. However, I'm not sure if this is what I need. As the catheter wall is 'real', the fluid should slow down to 0 velocity at that wall, but that doesn't seem to be happening (which makes sense, because I didn't model it that way. Also: I defined the catheter inflow much larger than the rest of the inflow, and it does seem like the catheter flow is slowing down, which also shouldn't happen because the 2 regions should be separated.

I would be very grateful for any advice!

Best Answer

  • RobRob UKForum Coordinator
    Accepted Answer

    As blood and saline mix you want to use species. If blood and saline didn't mix (eg oil and water) then VOF would be the correct choice.

    As an aside, blood is technically a slurry so I'd suggest some background reading into blood viscosity.

Answers

  • DrAmineDrAmine GermanyForum Coordinator

    Then define the boundary as wall instead of interior. If the mesh is conform Fluent will create wall/wall-shadow. If not conform then you can create interfaces with side 1 and side 2. Even in your current setup you can try to transform the interior 2d boundary to wall. Have you tried that? (also avoid talking about 1 , 2 and 3 if they are mentioned in screenshots..)

  • Thanks for this advice.

    However, when I define the boundary as 'wall', and I run a simulation with 2 different phases in the smaller tabe and the outer tube, I see they're already mixing before the point where the two tubes come together - which shouldn't happen. I don't understand why this is happening.


  • RobRob UKForum Coordinator

    Are you seeing wall and wall:shadow? Why are you using phases if the fluids are mixing? We use species for that, and if you don't turn on the reactions they just mix.

  • DrAmineDrAmine GermanyForum Coordinator

    Is it really a wall? Can you you show boundary condition secreenshot? Also the screenshot of the inlet condition (left side in the picture) and the mesh of it.

  • @Rob: yes, I can see wall and wall:shadow (name of the wall here is 'inlet-cath:solid:1'). As for the phases, I'm trying to model a catheter injection of saline in an artery filled with blood. I'm using the VOF model - you think it's not appropriate?

    @DrAmine: below you find a screenshot of the inner tube (meshed) and an outline of the outer tube, with the rest of the geometry meshed. For the inlet, I define a volume fraction of 1 for 'saline' for the inner tube, and a volume fraction of 1 for 'blood' for the outer tube.


  • RobRob UKForum Coordinator
    Accepted Answer

    As blood and saline mix you want to use species. If blood and saline didn't mix (eg oil and water) then VOF would be the correct choice.

    As an aside, blood is technically a slurry so I'd suggest some background reading into blood viscosity.

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