The bullet scene is a shamefully simple setup. Note all I write in regard to the bullet in this text is about the model and the sim, not the bullet solver in Houdini.
So in regard to FLIP fluids, this scene is based on what I learned watching this FLIP fluids tutorial -
vimeo.com/122217238 [
vimeo.com] - and here's how I set this up.
First I created the bullet and turned it to a point cloud. Then I used an attribute create SOP to create the point attribute I named “tip” which I later use for driving viscosity. That value is created by multiplying the bounding box in Y and remapping, so this means the top (the tip) will become softer than the bottom (the back) of the point cloud.
Then I used the shelf tools to create the FLIP from that SOP and made the necessary adjustments to fit the input object (as that is points, not a mesh). Next I activated the fluid viscosity thingie and set it to use the “tip” attribute.
Now I set up the second bullet as an ordinary RBD object and also create all the settings for the velocities and what not to get the sim started. Another important thing is to increase the fluid solver feedback value to “1”, else your fluid bullet will not calculate the collision forces correctly.
And now it's all tweaking and tweaking to get it looking good. Change the values, resim, change values, resim etc… And you gotta dig into the solver and object settings, check the help references for the operators and just generally get an idea on how all this stuff “ticks”, else you'll be tweaking aimlessly forever.
(this is the first FLIP setup I've done in Houdini and it took me in total a couple of hours to get it looking ok)So this is your first sim and when you get a result you are happy with (and take into account you are going to use it as a collision object), you use the ROP output driver to cache the simulation to disk. When that's done you re-import it to the scene, at object level, the whole thing 180 degrees and as a last step, make it a static RBD object.
The next step is to disable the RBD bullet object and adjust everything for the next simulation to collide with your imported bullet sim. As before this is all a matter of tweaking and I can say as much as I didn't spend much time on that for the second bullet, which is apparent in the video, the left bullet is the second sim. This was just RnD, a proof of concept setup - and it worked out really great.
[i.imgur.com]
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