Dave Stewart

Easy fix! Just go into your FLIP Volume Source node, Particles tab, and set Time Offset Scale to 0 (zero). --Dave

Here's a .hip with a sample setup, I'm sure there's a more efficient way to do this but it does work! I'd also recommend building your own meshing setup VDB from particles/particle fluid but since you can output VDB's from the PFS HDA this works too. --Dave DStewart_FLIP_Meshing_Blend2Fuilds_01.hiplc

Howard, here's how I would do it. Take your 2 VDBs, dilate them, VDB Combine using SDF Intersection, then convert the result to a Fog VDB and plug it into the 2nd input of the final VDB Smooth node. Should mask the regions that you don't want smoothed. Throwing together a quick'n'dirty example... --Dave

Thanks for the info Ryan, makes sense. SideFX confirmed the issue a while back, and my current solution is still the same though I haven't opened this test file in H17. Reducing surface extrapolation, lowering grid size and adding a bit of viscous slip helps, but doesn't eliminate the problem. Curious to hear more about your possible solution! --Dave

Great!

Scaling your lines so they're contained inside the box, adding normals and using normal direction for the ray node seems to work better? Is this what you're going for? --Dave test_DS.hipnc

Ekan, there are a bunch of issues with your setup, but I don't think scene scale is the main issue. I'd try matching the pour motion and speed with a real bottle, my guess is you'll make a big mess! You'll need a lot of substeps as your bottle/glass are very thin, and the fluid will still go flying when the bottle flips back. Your sim is also really slow as it's rebuilding very high res collision volumes for the bottle/cup on every frame (deforming is on). I'd apply your motion at object level so that you only need to build the collision SDF once at the beginning. Cup is static as well, so no need for the deforming flag to be enabled as that really slows things down! This is an instance where using the collision shelf tools is great for getting something that works well for most scenes with minimal tweaking. Will post a tweaked version of your scene with more notes if you still need help! --Dave

What a goldmine of techniques and ideas, so glad I found this!! Wish I'd known about the independent resolution trick a few weeks ago, that one will really come in handy in the future. Thanks SO MUCH for taking the time to put these together and sharing!! --Dave

Cool, clever use of foreach and minpos! Does seem like that could come in handy for other stuff. Looks like both links point to the same file though? --Dave

Thanks for taking a look at this guys! It seems that it may be a limitation of the viscosity solver, as it's decoupled from the pressure solve. Sounds like this may get addressed in a future version though! https://cs.uwaterloo.ca/~elariono/stokes/index.html I'll go ahead and submit the file to SFX to see what they say about it. Nigel, increasing substeps doesn't seem to have any affect on the artifacts. Atom, it could be that or just a velocity matching issue relative to the grid scale. Got some interesting thoughts on this in the Houdini Artists FB group. Anthony, I did try that and it helps with the fluid surface/collision interface, but not when the artifacts appear on the outside surface of the fluid as it gets very thin. Cheers, --Dave

Hi All! I'm wondering if anyone has run into this and found a good solution, it's been popping up in my low-to-mid viscosity sims for a while. Perhaps I'm missing something stupidly obvious? Collision geo/field are fine and the interface between fluid and collision object looks great before turning on viscosity, then I get these grid artifacts that are tough to get rid of. Reducing Surface Extrapolation way down in Volume Collisions helps but doesn't completely remove the artifacts. Also, using a high slip-on-collision value helps, but I usually don't want the sliding behavior with sticky viscous fluids. Reducing Grid Scale reduces the relative grid size and helps too, but the artifacts are still there, just smaller. Attached file is somewhat of a worst-cast example, just curious if other folks have dealt with this before. I know I can run some post-sim tweaks to clean up the collision surface intersection, but I've also noticed that the artifacts reveal themselves on the outer surface of the fluid if it gets thin enough, which is not so easy to fix... 01 - viscosity disabled 02 - viscosity enabled 03 - viscosity enabled, surface extrapolation 0.01 Thanks in advance for any help/ideas!! --Dave DStewart_FLIP_ViscousCollisionStepping.hip

Here's a simpler (maybe? Maybe not...) setup sourcing the viscosity point attribute directly into the GasFieldVop using PCOpen/filter. I still think the other solution is better, but this avoids the memory overhead of adding an additional field I guess... --Dave pp_st_ds_PopVOPs_Simpler.hiplc

Here's another version using PopVop/GasFieldVop instead of wrangles... hope it helps clarify the process! Set this up *very* quickly so let me know if I missed anything. Should be able to drive the surfacepressure field directly from the viscosity field, but the values would probably need adjusting in the 'fit' node. --Dave pp_st_ds_PopVOPs.hiplc

Pazuzu's file, slightly modified, would let you drive the surface tension field values based on a point viscosity attribute or whatever. Makes for some interesting shaping by the end! See attached modded file... --Dave pp_st_ds.hip

Love these explorations! Looking forward to seeing an example of a CHOPs solution, seems ideal for this task. Need to learn more CHOPs, not enough time in the day...