Hello;

I would like to create an explosion effect(Pyro) between two walls(boxes). The problem is some smokes travels through walls. I increased simulation substemps, but it didn't help.


Thanks for helping.

You can use a gasenforceboundary node for that. Then you can lower your substeps.

Hi “Enivob”;
Thank you for reply.
The problem is my walls are not entirely even in all areas.
I think I need another way to creating a collision surface.

Hi,
and thanks for the sample-scene. I extended the scene using two more collision-boxes, one on the top and one on the back, but now the collision is´nt working properly again?!

Subdivide your walls. Large flat quads just don't make good colliders. I experienced this in Maya and Houdini, with voxels and flips. My rule of thumb, is no polygon in the collider is to be larger then 1 unit of grid in scene. Houdini DOPs are 1 meter based. Maya Dynamics are 1cm. Maya BiFrost is 1 meter. In Houdini I use VDB volume collision. For fast moving fluids, it may help to have a repulsion (a negative attractor) defined by collision geometry, to push FLIP pops and pyro voxels away, as well as collide.

Hi Dave,
and thx for your reply :-) Subdivision and higher substebs for the DOP-network seems to work!

But out of curiosity, how exactly do I use VDB volume collision? I´m fairly new to houdini and still trying to understand the mechanics. Would it be very time consuming to rebuild the attached test-scene?

Thanks in advance :-)

In DOP's Static Object DOP which is upstream of staticSolver DOP, there are options for this.
Set Display Geometry to OFF.
In RBD Solver tab Volume sub-tab;
Set Collision Guide to ON. Choose your color. I use either blue or yellow.
Set Mode to ‘Volume Sample’
Set Division Method to `By Size'
Set Division Size to a desired value (in meters). In my example it is .225
Set Fix Signs to ON
Set Force Bounds to OFF.
Set Proxy Volume to desired location. In my example it is set to ‘/obj/OgreBotFX/ogreBot/VDB’. H17 should set this automatically for u.

Experiment with other settings in volume sub-tab. For complex collider , you may want to cache the VDB and read back using File Mode setting.
Link below to YT vid showing VDB collision volumes sampled from arbitrary animated geometry.
Good luck and cheers.

Houdini used was V17 Apprentice. 17.0.459

https://www.youtube.com/watch?v=-atR755StoM [www.youtube.com]

There is further approach that I use, as a matter of doctrine, which may or may not be the best way. I avoid , if possible, using substeps higher then 1. If the simulation needs it, I rather simulate with 1 substep, but at higher FPS.
You are probably aware that FLIPs loose their performance advantage over SPH or fluid solvers, if substeps are higher then 1. I find this to be the case in Houdini (up to version 16.5), Maya biFrost (all versions), and Realflow. It is better to sim at 48, or 60 FPS for 24 film/HD, and HD 30FPS. Then to use substeps while keeping sim at 24 or 30 FPS. Granted, this is not a universal ROT . To me sims made FPS 2X of content delivery FPS , looks smoother, more lifelike, organic. Content retains that smoothness when rendered elements are retimed in NUKE for comping at 24FPS. But I am one the few who likes film and HD programming at 48FPS or at 60. Love feature film in Real3D with HFS 48FPS digital projection. However the industry seems to have rejected 3D as a goto standard. Turns out that content that matches FPS to digital display Mhz is optimal for quality. Most digital displays and projectors are at 60Mhz, with prosumer ,pro displays , and venue projectors going to 120, 144, and 240Mhz. My two main panels , on my home rig, are 2560X1440 144Mhz, with third panel being 1920X1080 HD at 60Mhz. Playing content at 60fps or slightly slower 48, on my panels, looks much more organic then 24 or 30 fps.

Example of boiling and churning water simulated at 60FPS.
https://www.youtube.com/watch?v=ypsxzVJy_9M [www.youtube.com]

Hi Dave,
and thanks a lot for your detailed reply and insight in your workflow! I wasn´t even aware that the static object uses the VDB as Proxy Volume by default in H17. This makes things so much clearer!

And I actually do all sims with 60FPS, but without at least 3 substeps fast moving FLIPs still penetrates my collision objects. I´ll test a few more things, like using a single collision object instead of 4 individuals, even higher subdivisions for the mesh and of course the new H17.5!

Btw…absolutely incredible showreel! Kudos