Hey guys,

I've got a general question about the sim scale of flip fluids. I've notived that houdinis flips work better in larger scale sims. There is just less mesh popping, I think because there are more particles to build even small droplets/looks averaged out better.

But at the same time large scale sims look kind of sluggish/slow. Now I want for example simulate chocolate on bisquit and don't want it to look like 1000 L of fluid.

What would be the best way of making large scale sims look like small scales in Houdini? In Realflow you just have to increase the daemons force scale in order to make the fluids behave more the way you like it. Simming in pretty small scales isn't recommented there either.
Is it just a matter of increasing the gravity forve or is there a more elegant way available?

Thanks!

Yeah, playing with forces is a good start. I also would guess that Velocity Smoothing would be helpful, but not because I've done lots of small-scale fluids, but because I reduce that smoothing when I want to get more splash/slosh/action.

Hope that helps!

There's really no “fudge factors” in Houdini FLIP fluids, as you might be used to with Realflow. You should be aiming to simulate all fluid scenes at their true, realistic metric scale, otherwise gravity and other forces simply won't add up properly. If the nature of the sim feels off to you, then it's likely a problem elsewhere, not because large scale sims inherantly look better somehow.

I mean, large scale sims *do* tend to be more forgiving, by their very nature of being relatively slow moving compared to their grid size/particle separation, but there's really nothing about that look which you can translate to a small sim without just turning it into something it isn't.

A good starting point is to consider this: At a smaller scale, gravity doesn't change, so it will naturally be accelerating fluids across far more grid cells per frame, due to the grid cells being smaller. FLIP breaks down if you try to have it travel through too many grid cells in a single timestep… so typically even just for a glass of water sloshing around, you'll need at least 3-4 substeps set on the FLIP Solver… where a larger sim can happily get away with 1 in most cases. For an animating glass, I've had to push it up to 12-16 substeps to keep it under control.

You should also try to avoid low particle counts in general. You may think a smaller scale fluid will get away with less particles than a large one, but you're usually viewing it from a much more critical point of view too, so you need to push the particle separation right down until you're using at least a few hundred-thousand particles, if not a couple of million. The more the better really… it's a trade off against solve speed obviously, but in general, you can never have too much resolution in a fluid sim… certainly not within the tight bounds of current PC hardware. Max the resolution as much as possible within your project's time constraints.

Thanks for your replies. But to be honest: I don't believe that small scale sims look and work similar good as big scales. At the moment I'm trying to do everything in the “right scale” but that brings problem after problem. Particles disappear, they explode, need more substeps, collisions don't work particularlly well…..I just have to search for too much errors.

By just increasing the scale and reducing the separation at the same time many problems disappeared. In the example down there the skeleton is about 20m high, I only increased the gravity force to about 30-50 (could be even higher). The fluid almost behaves like in small scales but doesn't produce that much problems. At the moment I'm trying this setup with lava in a smal scale sim and almost lost my mind already

By the way, another strange problem appeared here. Probably not due to the sim scale but nevertheless annoying. The particles which get pushed away kind of slow down in the air. no drag force enabled.