Rotating grid with a ball of fire like from catapult?

[japetus]

What's the best way to simulate something like a ball of fire arcing through the air? I tried to use Motion Inertia and just parent my grid to the "ball" as it arced through the air, which worked fine until the ball started arcing, then the whole sim began to rotate, which is obviously undesirable. I then turned off those sorts of things but obviously things became worse. I'm on Max 2021, Phoenix 4.3. Here you can see it's supposed to follow the red arc, but instead we get the whole simulation rotating.

[Svetlin.Nikolov]

Hey,

Not sure why rotation is undesirable. It might blur the grid somewhat, but same would happen with translation.

However, if you don't want it, then use adaptive grid and make sure to uncheck "No smaller than initial grid". This way the grid will follow the object anywhere. If your object is moving quickly, you might have to iterate on the Extra Margin and the Threshold so the grid could keep up.

Cheers

[Ivaylo Katev]

The rotation is part of the max mechanism for nested coordinate systems, not sure if it's possible to be turned off (somehow it mustbe possible), but nevertheless the simulation must be correct in both the cases, internal wind must be produced and the smoke must remain on the trajectory.
As i see this is not happening, the smoke is straight, if these red dots are particles, they are looking to behave as expected, but not the smoke. Would be good if you provide viewport screenshot showing the velocity

[japetus]

I'm on nightly build 2021030330635... thanks Svetlin and Ivaylo.... i was already using adaptive grid with those settings, I've attached more screenshots and an animated gif so you can see the strangeness. The red dots are a pflow, and curving like expected, but you can see the smoke just rotates as a whole

[Svetlin.Nikolov]

Hey, if possible, can you send that scene over?

[japetus]

Sure thing, here it is

[japetus]

Hmm well after more testing it appears that my Vortex space warp was causing the smoke to go straight out.....when I exclude it from the sim it bends along the spline. What I was ATTEMPTING to do was to keep the smoke more tapered at the end, instead of spreading out. I'd like to have a large start and then the smoke trail tapers as it trails away...so I guess more stylized comet trail thing.

[slavina.nikolova]

Hey,

Thanks for the scene! We just went through it and it looks like that using the Vortex Warp is not the best approach in this case.

You could try the following setup, which will help you have more control over the process:


First step, select the Follow Path and choose Line001 as a Spline, then set the Pull Speed to a higher value (something around 200-300 or even more), this is the value that controls how much the smoke will be attracted by the selected spline.

Set the Follow Speed and Rotation Speed to 0 and leave the other options to their default values.



For better visibility of the Force behavior, go to the Preview rollout -> Voxel Preview -> at the bottom of the menu enable Forces and select the Follow Path Force.

Furthermore, you could use Draw Just a Slice and choose the Axis that has a better visibility. Now you could see how the Force is "pushing" the Smoke to the spline.



In the next step we are going to use a Voxel Tuner in order to make the comet trail more stylized as you described.

Crate a Voxel Tuner and press Edit Condition.

In the Edit Condition window, click on Age, then the Edit Value Expression window shows up.

Switch the condition from Channel - Age to Distance To. Press the None button and choose the GeoSphere001 geometry.

Then set the Distance value (50-100-200 for example) - this will tell the Tuner at what distance the Follow Path will start to affect the Smoke with the increased Multiplier value.



Now we have our condition set, close the Edit Condition window.

Disable the Then options for the affected channels.

Enable Affect by Forces and pick Follow Path1 Force, and increase the With Multiplier value to 2-3.

You could experiment with the values and see which is the best combination.


Hope this helps!

[japetus]

Thanks Slavina! That seems to be just the thing I was looking for. I've never used the Voxel tuner before, seems like it's very powerful! Another question I had about this,

1.) Is there a way to have some of the fire at the top of the fire emitter, have some wisps or flame effect? Right now it's sorta being all pulled down as the ball moves through space and sorta exposes the rocky emitting objects.
2.) If you look at my fire rendered i'm still getting values of over 100 for my fire, even though my fire graph and multiplier are ridiculously low. The size of the fireball is supposed to match the size of a rather large ship so things are in Meters. Is the crazy brightness mostly a factor that I'm using meters? In my latest version of the scene I turned on VRay Exposure Control and that seems like a good way to go.
3.) I seem to remember at some point, maybe I imagined it, that you could add modifieres like Bend on Phoenix sims? Maybe that only applied to liquid? The idea being, I could simulate a "straight" fireball then add a few VRayVolumegrids and have fireballs arcing through the air at a few different angles. However in practice it doesn't seem to work at all, so maybe I did imagine it?

Thanks for the help!

[slavina.nikolova]

Hey,

1. You could experiment with the Noise option, located under the Mask slot of the Source. Adding some Noise will make the fluid more interesting and not so uniform.


2. In the Rendering rollout -> Volumetric Options -> Fire tab you could find the option named Physically Based. This option is enabled by default - this means the Fire will be as bright as in nature, so maybe try different values to see which value matches better to your idea.

Some more information about the Fire Rendering Options could be found in our Documentation page - https://docs.chaosgroup.com/display/...IT/Render+Fire


3. Bend will work only when the Simulator's Render Mode is set to "Mesh". This could be changed from Rendering rollout -> Mode.


Cheers!