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I want to simulate two fluids shearing past each other, to show that they form a vortex naturally. (That is, I don't want to put an artificial vortex to make the fluids rotate. I want it to happen as a natural result of the physics simulation.) I don't need to render it, but I would like to have it show the velocity of the particles, as done in the tutorial below.

I followed the technique in Maxim Sachs' tutorial on YouTube on how to do Fluid Flow Simulations. I made an emitter of particles and set its physics to "fluid." I duplicated it, flipped it 180 degrees and aimed it at the first emitter. (So that both fluids belong to the same particle system.) But when I animate it, the fluids flow past each other as if neither was there. (That is, no collisions between the two particle systems.)

Can such a simulation be done in Blender and what's the trick to allowing the two fluids to interact?

==== Update: Since asking the question, I've tried another tack. I am using a fluid domain and created two inflow volumes. I get some mixing that way, although I haven't gotten parameters right to get a vortex yet. But even if I get the vortex, I don't see a way to color the two inflows. Blender sees the fluids one (probably related to doing it all in the domain.) I've tried coloring the inflows with red and green materials, but no difference.

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  • $\begingroup$ Do the domains overlap? $\endgroup$ – VRM Apr 29 '15 at 16:04
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    $\begingroup$ I was under the impression (from Andrew Price) that you could only have one domain in a simulation. So I didn't even try two domains. $\endgroup$ – mikie Apr 29 '15 at 16:58
  • $\begingroup$ I think you might be right. Could you include a screenshot of the simulation as is? $\endgroup$ – VRM Apr 29 '15 at 18:19
  • $\begingroup$ Here's the screen shot so far. postimg.org/image/wgf2kb2d5 I've got upflow in red, downflow in green. I think a vortex is forming at the blue. In the Advanced Fluid tutorial, we use particles and can paint their velocity. I need a way to link the liquid to particles (I think.) Then I would move the mesh to a layer and only show the particle velocity. $\endgroup$ – mikie Apr 29 '15 at 18:31
  • $\begingroup$ I'm thinking a texture for the liquid using voxel data might be the way to go (I remember seeing Andrew Price do that in his flame thrower tutorial.) But no clue how to do that. Will review his tutorial now. $\endgroup$ – mikie Apr 29 '15 at 19:04
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You cannot visualize vortex with fluid simulation voxel data, because there is nothing moving inside the fluid mesh, it all happens on the fluid's surface.

You would have to use 2 interacting domains and form the vortex with their interface by mixing them. This cannot be done in Blender since multiple domains cannot interact. You can simulate only 1 domain in scene, although you can playback multiple domains from baked files in single scene.

There are Trace particles for visualizing the movement of voxel liquid. Add some dummy mesh object (plane) into scene, enable fluid and set type to Particle and check Tracer. Under Domain > Fluid Particles set their count. But these particles are not always behaving nice (they stick to walls, play dead).

So you might want to use SPH Fluid Particle simulation instead and get a very nice vortex. If you mesh these particles with the CubeSurface addon you could also color the opposing streams differently so the render will be nice.

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  • $\begingroup$ Mostly right, just a couple of corrections: the add-on is called CubeSurfer, and it won't work with fluid particles, just straight newtonian particles. However, with the add-on Molecular (made by the same guy) it is possible to get these newtonian particles to behave as a fluid. $\endgroup$ – Jbergman Apr 12 '16 at 19:22

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