In a perfect world, FLIP particles will never cross the collision boundary. The pressure projection is supposed to create a velocity field that moves around it, and they are supposed to follow that field. Of course, this doesn't happen, so the particle collision is needed for these wayward particles.

The “Particle” method does particle to polygon collisions. This lets you hit thin plates and things that didn't get rasterized by the collision SDF. So it is our default. But Move-Outside should usually be faster as it just need a single SDF lookup to do the detection. The kill umoveable is greyed out in particle mode because the particles are never tested against the sdf, so we won't know to kill them if they get on the wrong side for some reason.

The rotation method I refer to is when you have *no* SOP level rotation on the VDB. At the SOP level, you have your gear be non-deforming. Instead, at the object level you rotate the gears. When you bring this object in as a static collider, you don't have to turn on the use deforming geometry as the geometry isn't deforming. But you will pick up all the rotation & velocities from the object motion for free. One catch is to not use any object scales. They may work, but object-level scales will bite you eventually, so just avoid.

Hi again;

I reran my sim last night using all your recommended optimizations and adjustments, including breaking the colliders into their own groups, rotating them at the obj level, and bringing them in individually without marking them as deforming objects. My sim is about 700 frames in and seems to be remaining incredibly stable, with zero leakage and minimal volume loss. Thank you for this!

I notice that in previous versions, the central spokes/cogs caused some nice turbulence and berming around those parts (which is why I include them). With your corrections, the sim seems somehow ‘calmer’; there's less turbulence and almost no berming, which makes the sim overall less interesting. I tried increasing velocityScale to 1.5 under the volumeMotion tab of the solver, but this didn't seem to have much effect. I presume I must be getting collider velocities despite not using deforming geometry with trail sops to manually generate their velocity, right? Would the less-energetic sim be the result of switching to APIC? I'm wondering if it's possible to preserve the relative stability while still getting some nice splashing happening?

Thanks!
–a

I think you are using stick-on-collisions that will lock the surface particles to the collider's speed. I'd play with those settings or disable it. Likewise, FLIP rather than APIC might give you more splash. APIC is better for preserving vortices which is what this sort of cross-directional flow should be inducing.

Very glad to hear it is stable, though. A lot easier to add splash to something stable than the opposite.

One thing that helped me a bit with a long sim and volume loss, was to increase the birth threshold slightly on the Reseeding tab. Although it might have been better if I had thought to use Gas Equalize.. Need to test this sometime.

Hi @jlait;

I've spent today playing with varying stickOnCollision values (including disabling it completely) as well as switching back to the splashy kernel for the flipSolver. While I can still seem to get some very lovely vorticular motion, I can't seem to get any ‘loft’ or splashes happening; it's as if the particles are aggressively constrained to the original surface at the start of the sim.

I've tried a few things to change this: I tried eliminating the gasEqualizeVolume, but this doesn't seem to get me any splashes happening. I've tried lower the magnitude of my popVOP force, but this just makes things move slower overall, rather than seeing any berming or splashing.

You made a comment about the scaling of obj-level colliders; I've indeed avoided that, but I DO animate them by way of a few stacked upstream transform nodes; could this be problematic?

I've gone so far as replacing my gear geometry with a much-cruder block shape, which I feel should definitely offer some large splashing, but still I see only vorticular stuff. Might you have any further advice about what might be happening? The water doesn't seem to be being ‘pushed’ the way I would expect it to, especially looking at some of the in-help examples for flip sims.

https://www.dropbox.com/s/fzgt9pha5a61845/waterWheel_08.hiplc?dl=0 [www.dropbox.com]

Attached, in comparison, is the level of berming/swell I was getting back when I started this adventure; it looked great, but lost volume over time, which is what sent me down this path to begin with.


cheers;
–a

How far did you lower the magnitude? I ran at 0.02 separation & turned off equalize for testing how the gravity waves propagate. With the slow motion of a single cog + 9.8, things stay very flat as you experienced. Dropping two orders of magnitude to 0.098 and I got waves forming in front of the moving spoke.

Hey sir;

Ah, I indeed see that. I guess I was hung up on 1) the fact that a magnitude of 9.8 gave me some nice shaping when using sourceVolume colliders, and 2) I tried to model this whole thing in a reasonable space (a few meters), thinking that using real-world values would yield results that looked appropriate to scale. The latter is, unfortunately, a habit I repeatedly have to shake doing water stuff, I've noticed.

In any case, mostly I wanted to confirm I wasn't still doing anything ‘wrong’, and that this force magnitude knob was indeed the right one to be turning. Lowering it an order of magnitude does indeed give me lots of nice splashing, which looks totally great. This sim is also running fast enough for me to iterate in a more artistic way now, which is great. And, the best bit is, I've learned more about the FLIP solver in the past week with your help than I managed to over the course of several years working in a large studio. So, a thousand thanks for all your superb help! I truly appreciate it so much!

–a