Reply To: How to mix blood in a Y-shape tube

[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.