Phoenix FD Dune Style Sand Motion

[leehenshall]

Hello,

I'm trying to achieve the best possible results using Phoenix FD to simulate a car driving on sand dunes.

I know PFD does not have a dedicated Sand Solver but I have been trying to get a good approximation using smoke.

I have also experimented using liquid particles to try and get that nice wave shape but not 100% sure how to tackle this. I have seen in a similar thread mentioning a hack to make liquid act more like sand using a hidden attribute called "reflprec" which should essentially over ride the default conservation to make the particles fall straight away.

https://forums.chaosgroup.com/forum/...h-an-hourglass

However I can't figure out how to access this hidden attribute in Maya to test this hack.

The results of my smoke tests look pretty decent on a still frame but I'm not 100% happy with the motion....it still feels a lot like Smoke.

I have tried:

Lowing my smoke temperature to below 300k to make it more "heavy" like sand as per this tutorial:
https://docs.chaosgroup.com/display/...ke+Smoke+Heavy

In Dynamics - setting the smoke buoyancy to minus amounts to stop the smoke from rising
Changing the Time Scale to values below 1.0 to help slow the motion to make the smoke feel heavier like sand plumes
Raising the Smoke Dissipation to values such as 0.85 so that the sand does not linger for too long as dune sand would usually fall fast and not leave a lot of sand in the air
Setting the cooling to 0.0 Not sure what to do with this value.....I assumed it would be best to keep this value locked at zero as I am already working at a temperature that is below 300k
Setting the conservation to PCG - 100
Increasing Material Transfer (Advection) to multi-pass with 3-6 subframes for the fast motion

I have also tried to get Motion Velocity working by adding dummy geo using the tire burnout example scene but I didn't notice much effect at high speeds.

It would be great if I could get some advice on best practices for this kind of shot. I have attached some reference photographs for the sort of look I'd like to achieve.

[Svetlin.Nikolov]

Ah, so "reflprec" is just the Conservation Quality option, but it's hidden for the Liquid Simulator in Max. Since in Maya the Liquid and Fire/Smoke simulators are combined, you still have access to it - you can check the online docs for the script name of the Conservation Quality in Maya and set it via script if it's greyed out.

However, I believe you should use a particle simulation to emit and carry the motion of a smoke sim for this kind of effect. This should create the perception of solid particles lying in the base of the movement, the smoke would represent the very fine dust, and the larger grains that are visible but don't drive the motion can be Drag particles produced by the source that also produces the smoke, and you can render them with the Particle Shader in a similar fashion with this tutorial - some settings in it are already outdated or renamed, but you should be able to follow it as far as rendering drag particles together with the smoke goes: https://www.youtube.com/watch?v=XpYJOIVSUx4

[leehenshall]

Hi Svetlin,

Thanks a lot of your response.

I used "eco all commands" in Maya script editor to find the "reflprec" (Conservation Quality) I believe in Maya to over ride this in liquid mode you'd use:

setAttr "PhoenixFDSimulator1.conservQuality" 1

I don't know Mel/MaxScript or Python so do correct me if I am doing it wrong.

I think a particle driven smoke simulation sounds like a good idea and I think perhaps using liquid particles would not be the best option here.

I only started learning Phoenix FD a few weeks ago so I'm unfamiliar with many aspects but have been reading the manual constantly.

Please can you confirm I am approaching this correctly:

1. Under phxSource check on the particles......this generates particles which will help spawn the smoke based on the particle motion
2. Keep the particles type set to drag. This will act as an influence on the smoke dynamics.
3. Set the temperature of the smoke below 300k to help the smoke feel more heavy

I am a little uncertain on the significance of Time Base, whether I should use Absolute or Particle Age. I am also uncertain about how I effect the particle dynamics such as mass, gravity and friction etc.

I am also not sure how many particles I need to generate and how to get them to render with my smoke.

[Svetlin.Nikolov]

Ah, let me correct points 1. and 2. - you would need and external particle system as the emitter from the source - such as nparticles. Then the source would emit smoke or Drag particles from the nparticle simulation. Drag particles are simply dragged along the velocity of the smoke, so they are rather a way to visualize the smoke and enrich it with a different look, but the driver behind the fluid motion must be an external system, such as nparticles.

[leehenshall]

Thanks for making me aware of this.

I have been putting off going into nParticles but yes I think the only way to get this motion is to create it with particles first. I will check back when I have more to show.

Would Phoenix FD be able to use BiForst Graph Particles as an input?

Seems to me that nParticles will soon be obsolete once BiFrost Graph is matured and better documented.

[georgi.zhekov]

Hey,

Bifrost Graph Particles are not supported at the moment but we will investigate if we could use them directly. For now nParticles are the way to go.

Cheers,

[squintnic]

I reckon tyflow would be perfect for this. i have been using tyflow to drive smoke sims that require more control than a simulation would allow for after trying to do a smoke ring explosion

[leehenshall]

Great suggestion. TYflow looks incredible.

I do have an active subscription of Max but I was mainly wanting to tackle most of this in Maya.

However after spending a day with nParticles yesterday I can get something workable but it was painful. Working at realworld scale with nParticles is a weird experience as the default unit setup expects you to be working ten times too small so out of the box. So that means if you work at real world scale nothing in nParticles works as you would expect as it thinks 100cm is 100m which makes things like gravity not effective unless you change the timescale factor to 0.01 the manual recommends this.....but it took me a couple of hours to figure it out and find the value in Maya. It still baffles me why Maya likes to work at crazy small scene scales rather than default to real world scale. Certain values that weren't in cm I could not figure out the correct scaling so would end up changing the values by trial and error.

Even when I got it all working properly after trying to figure out the unit setup for every element of the nParticle setup I would still get strange uniform stepping at high speeds in the viewport......it seems like nParticles can only do 1 step per frame for viewport preview but even when using nCache with high substeps i'd still get issues. I found a lot of people complaining about this sort of thing in nParticles online but that was from around 5 years ago and I couldn't figure out what had been fix and what had not and a lot of the fixes I could see required overly complex setups to create things like a kill plane or to make Maya solve motion arcs without creating stepping by default. I also couldn't get any turbulence working at the speeds I was simulating at even with substeps.....it seems like nParticles want you to work at small scale and perhaps work using oversampling to get a good solve.

I can't find any current information about using nPartciles that helps me in this situation and I'm not sure I can be bothered to learning as it seems quite obvious now why Autodesk decided to implement bi-frost graph to replace this very average particle system. I know I probably made all sorts of mistakes trying to get this setup but nParticles was fighting me every step of the way when trying to stay at real world scale.

I have a copy of xParticles for c4d so I might try that or I could start learning Tyflow. But I think I have discovered that nParticles is not a great particle system to learn unless you have extensive knowledge of the work arounds to work at real world scale.

[squintnic]

ah sorry i didnt see you were using maya!