Learning Forum

[Cheng Peng]

Hi everyone, I'm new to Fluent and have some questions. When I try to mix two blood flows in a Y-shape tube, they do not mix. Instead, their streamlines are parallel and do not cross to form vortices. I only used laminar model and set the boundary conditions two "velocity inlet" and one "pressure outlet".

My goal is to use the blood from inlet2 to "push" the blood from inlet1 back(see the figure). But even I set the inlet1 as 0 velocity, there is still no any flow through it. Is this because of the "velocity inlet" boundary conditions? Can anyone help me about this problem?

[Rob]

Streamlines won't mix as they're showing where the "lump" of fluid is going. If you use species and have Blood-a and Blood-b (for example) with identical material properties you will see some mixing. Make sure you set the diffusivity correctly. 

A velocity boundary sets that value uniformly on the surface. That's probably sensible for inside the body, not sure about what you're trying to model to comment. 

 

[Cheng Peng]

Hi Rob, thanks for your reply. I understand the streamlines don't mix now. I have a udf to describle the flowrate in the inlet 1, and I want to use the same waveform but different amplitued to push the inlet 1 back. This model is used in cerebral artery, the inlet 2 is the graft to cut the inlet 1's normal blood flow. What boundary condition should I set?

[Rob]

If you know the flow in both channels then you can do that, leave the outlet as pressure. However, unless you have reverse flow (negative velocity) on inlet-1 I don't see how you'll get back flow.  If you use pressure bc's then you may see the effect but you'd need to calculate mass flow in the solver, and that may not be overly stable if there are multiple "correct" results for a given set of boundary conditions.