How to make slow, steady flow

[josemendez]

Hmm... it kinda makes sense for water, but how about very sticky/viscous fluid, like honey? Honey can flow quite slowly in real life, right. Or something like a slime oozing out goo. But even with viscosity = 1 I still can't make it drip slowly like honey.

Viscosity is the ability of a fluid to flow against itself. You can think about it like how sticky particles are towards each other.
Adhesion is the ability of a fluid to flow against another surface. You can think about it like how sticky particles are towards objects.

Now, viscosity alone can't possibly affect flow rate at all since it doesn't change the speed at which particles are emitted. So no matter how much particles stick to each other, their relative velocity at the moment of emission (how much they move with respect to each other) will always be zero. In other words: if you launch water at 5 m/s and honey at m/s... well they both travel at 5 m/s, honey doesn't magically slow down mid-air.

However, if the fluid also has high adhesion it will stick to the walls of the tube/nozzle emitting it, slowing down regions of the fluid close to the walls of the tube. Then fluid towards the center of the tube -away from the walls- will also slow down due to viscosity though not as much. In literature, the "viscosity" they often refer to is a mishmash of actual viscosity, adhesion and friction, so you'll find that "flow rate is inversely proportional to viscosity".

Obi separates adhesion (called stickiness, a property of collision materials) from viscosity and friction. This is so you can have viscous fluids that don't stick to objects, inviscid fluids that stick to objects, fluids that stick to objects but flow tangential to their surface, and fluids that stick to objects and also can't flow along their surface, the whole palette.

So if you want to reproduce how viscous/sticky fluids flow out of a tube, then you need to:

- Increase viscosity in the blueprint, then adhesion (aka stickiness) and friction in the collision material. This will make it hard for the fluid to flow both against itself and other objects.
- Give the fluid some tube to flow out of. An emitter just creates fluid, but doesn't give it any surface to interact with.

Here's a comparison of a viscous fluid with an actual nozzle to flow out of (transparent walls so you can see the inside), and no nozzle. Same emitter (speed = 0.2 m/s), same fluid parameters.