A question for OLDSCHOOL (Jeff) about pyroclastic dust

[vfxhound]

Hello Jeff,

First, I'd like to say I'm very inspired by your technical abilities and I thank you so much for the heaps of information you generously share on this forum.

I've been reading your posts on this topic:

http://forums.odforce.net/topic/18397-get-rid-of-mushroom-effect-in-explosion-and-add-details-in-the-opening-frames/page-2

I'd like to know how to apply these concepts on a pyroclastic dust effect.

Here's my problem:

I'm working on a destruction shot for a skyscraper. I did the fracturing in Maya and brought in the sim to houdini to add trailing dust with a pyroclastic cloud at the center of the destruction. No fire/fireball.

I isolated some fragments which will emit the trailing dust. At the beginning of the sim, the fragments are still in place close together at the center of destruction, so when they start to emit dust they create the cloud of density there, then as they move through the sim the dust trails behind them.

My problem is, I just can't get those pyroclastic swirls/eddies. I always get a "hairy" wispy look.

Here's what I did:

I scattered points on the fragments based on uvs, then emitted particles out of them with a low life expectancy. I used those particles as a fluid source, with curl noise vel on the emitter. I'm using smoke solver not pyro. Now I get streaks of density moving opposite to the fragments direction with a wispy-like shape that don't expand outward. I tried adding disturbance, turbulence and confinement in DOPS (hoping to get some rolling eddies) and set disturbance to affect vel not temperature. I got that hairy look instead.

I then tried taking the particles I used as emitters, and added a VOP SOP before the source fluid node and added some curl noise to the point velocities of the particles. Thought maybe if I had more random direction in the particles velocities I might get those swirls. Now I get very small almost spherical type shapes and still kind of hairy, but not that pyroclastic looking dust either.

Then I read some of your posts about divergence, I don't quite understand it but thought I should give it a try because I wanted the dust trails to also expand outwards not just move in the same line they were emitted. So I took the same particles I emitted fluid from, created divergence from their velocities, blurred them and piped them into DOPS (using a reference file you posted before on this forum to set it up). That either blew up the sim if it was dialed high, or did nothing much if dialed low.

A side note, the whole sim happens over 17 frames. The fragments are moving pretty fast.

I'm beat and I need professional help. Unfortunately I can't share the file online because it is for a well-known client, not just a personal test/project.

[Sierra62]

First off I would recommend not using the smoke solver and use the pyro solver.  If it's moving that fast I hope you have a bunch of substeps.  You could also try increasing the Source volume amount in the "Source volume" node. 

[vfxhound]

First off I would recommend not using the smoke solver and use the pyro solver.  If it's moving that fast I hope you have a bunch of substeps.  You could also try increasing the Source volume amount in the "Source volume" node.

Hello Sierra, thanks for your reply. Can you tell me why I should use the Pyro solver? Because I see all the extra attributes in the pyro solver are available as micro-solvers to use with the smoke solver.

About the substeps, something weird happens when I increase the Max Substeps on the smoke solver. I suddenly get point of density appearing out of nowhere in the container where there is no emitter to emit them.

Regarding increasing the source volume, would that give me more billowy dust rather than wispy? I'll try it out anyway.