pyro FX explosion?

[divi]

I have been trying to learn pyro fx lately and I have a question:

How do i make an initial explosion ? You know when explosion occurs it isn't anything like the default fireball shelf tool , it's more violent and fast.

Thank you in advance.

[lukeiamyourfather]

The shelf tool is just a starting point or an example. Depending on many factors (type of explosion, amount of frame it occupies, art directed or not) it could take as long as weeks/months to create a realistic explosion. Start by exploring the parameters of the solver and sources. The faster the fluid moves the more steps it will require to solve without breaking.

[old school]

Exactly what luke said. Tweak the default Pyro. With just some basic tweaking you can get a nice expanding initial explosion which is what I think you are after.

Have a look at the attached file. Just a box and the Fireball Pyro preset, added initial velocities on the source and tweaked a few parameters in the Pyro Solver DOP.

I always add initial velocity attributes on ALL my sources and enable point velocity in the Pyro Solver. Have a look at the source geometry SOPs in the attached scene file. You can sculpt these further with the Brush SOP if you wish to direct the initial blast direction.

Then start tweaking away on the Pyro Solver combustion parameters like burn rate, heat output and more.

Read the Note comments I added to the dop network in the attached file to get you started.

I have found that when sculpting initial source geometry point velocities like this, increasing the resolution of the sim will give this initial geometry velocity more effect. As you increase the sim resolution, you have to cut back the point velocities to suit. After some point (trial and error) increasing the sim will result in consistent source velocity effect. I believe this has to do with the inaccuracy in the lower grid resolution containers and lack of sub-stepping with the huge velocities involved.

I would sim everything in slow motion and then speed up either in comp or in SOPs using the Time Shift SOP. I posted a file a couple days ago on the Side Effects forums for re-timing an RBD sim that applies directly to fluid simulations as well.

[old school]

Here's the file.

rapid_fireball_explosion.001.hip.zip

[divi]

Thanks everyone, I will look into the file.

[divi]

Thanks for your file. I looked at it and was playing around. I think i would do this better in comp. My problem is that it need very high velocity like over 1000 , and if i do that i get like a first emission out of nowhere and then the real one (fireball). I know this because if i put burn rate to 0 the first emission still emits.

[old school]

For initial huge velocities, you need to sub-step the fluid simulation. Increase the substeps at the top level DOP Network node. With 1000, you may need 10 sub-steps or more to resolve those huge velocities.

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It is good to understand why you need to sub-step with large instantaneous velocity changes.

All fluid simulations are aware of the current frame rate and read/write velocities in units per second. Houdini defaults to 24fps. With a velocity of 1000 houdini units per second (this is how all velocity values are to be written and read), that turns in to 1000/24fps or 41 houdini units per frame. That's a LOT of houdini units to span in the space of a frame with a single time step for a fluid simulation. The results will be very inaccurate and difficult to control with parameter tweaks in a predictable manner. Increasing the resolution of the container will also see completely different results.

You can cut the large instantaneous velocity changes down to a manageable scale by increasing the time steps on the solver. With 10 time steps, you end up with (1000/24fps)/10 steps_per_frame = 4 houdini units per frame. At least you now have a chance at getting something more reasonable but this is still a very large distance to push a fluid in a frame.

You may want to consider knocking down the initial velocity of 1000 down a lot once you start sub-stepping the simulation as it will get more accurate and you don't need such high velocities to get your effect.