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I've been starting to familiarize myself with the fluid domain. My initial goal was to drop a cube (rigid body physics, gravity drop) into a fluid domain and see if I could get a splash. I made the cube a Fluid Obstacle, etc.... and I never could get a splash. Quite frustrating.

However if I Animate the cube instead (still setting it up as a Fluid Obstacle), and create a couple of keyframes to drop it fairly fast into the fluid domain, I can get a splash. I did a lot of googling (unf the answers vary over the years as Blender features change with different releases) and I just became more confused... Is there in fact an incompatibility between fluid domain and rigid body objects? I was hoping to drop rain (particle system) into a fluid domain and get nice raindrop-ripples, but if dynamic objects won't interact with fluid domains then I'm snookered...

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Fluid simulations and rigid body simulations cannot interact.

Fluid simulations can react to rigid bodies, if the rigid body is a fluid obstacle. Set your falling objects both as fluid obstacles and rigid body active objects. A key setting is to check "Export Animated Mesh" in the fluid settings of the active rigid body object. This 2.68 tutorial should work fine in the newest 2.78.

The rigid body can be simulated beforehand or during playback and will not be affected by the fluid (the fluid can't push an active rigid body around).

However, in your case I wouldn't use this, because:

  1. The motion of falling raindrops is very simple, it can be easily done with keyframes
  2. As Duane Dibbley commented, the whole ripple-effect could be easier with dynamic paint
  3. Rain is not a rigid body... If you wanted maximal physical accuracy, the rain drops should be fluids as well.
  4. Particle systems are also not rigid bodies, it can be tricky to combine the two.
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  • $\begingroup$ Many thanks! Yup, I bet it was the Export Animated Mesh setting that I had overlooked. Will try again. BTW, while on the subject, can low-mass, high-volume objects (a cork sphere for example) float on the fluid surface with realistic physics? I'm thinking model ships here, in the long term... – $\endgroup$ – Tazling Dec 15 '16 at 19:37
  • $\begingroup$ As far as I know simulated floating is not possible, you would have to use keyframes or constraints for that. But in general if you have additional questions, it is better to ask them as a separate question so that more people see it. Also (if now it works), you should accept the answer, see here: stackoverflow.com/help/someone-answers $\endgroup$ – lbalazscs Dec 15 '16 at 20:07

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