Fluid-Cloth Simulation

Jenee Hughes

Grad Graphics, Spring 2010

Dr. Zoe Wood

Imagine wearing a waterfall as a cape. Imagine that it moves and swishes around like a cape, but that the water droplets are still falling and behaving like water droplets should.

That's what this is. A fluid-like particle system constrained to an invisible triangle-mesh cloth simulation.

Why's it special?

A normal cloth simulation generates forces that manipulate vertices in a triangle mesh as if they were particles fixed in uv space, in order to deform the polygon faces on the mesh. In this case, the particle-vertices don't move with respect to the cloth, only with respect to the world.

RECAP:

Triangle mesh, with polygon faces

Vertex == Particle, fixed in uv

The Specialness:

My fluid-cloth simulation builds upon a normal cloth simulation by redefining the vertices as "Nodes", which are fixed in uv space and influence the behavior of particles swarming around the cloth. The particles are fixed in w, but not in u and v. We don't draw the faces on the triangle mesh, instead allowing the particles to fill in the area.

Recap:

INVISIBLE TRIANGLE MESH, with particles that move around on it.

Vertex == Node: fixed in uv, exerts forces on particles

Types of Nodes:

An Emitter Node emits particles into the simulation.

A Neutral Node does nothing.

An Attractive Node pulls a particle toward it, and ceases to have an effect on the particle once the particle has passed over it in u or v.

A Repulsive Node pushes a particle away from it, but ceases to have an effect on the particle once the particle has passed over it in u or v.

An Evaporator node allows a particle to rise off the cloth plane in w, and makes the particle begin to decay.

A Boundry Node lets no Particles pass it, by applying a force slightly greater than and opposite to the sum of forces on the particle.

I didn't manage to get the last two types of nodes working fully, but am planning on implementing them this summer.

Particles

The particles can be used to represent anything you'd use a particle system to represent: Fire, Water, Sand, Iron Shavings.

For the purposes of illustration, think of the particles as iron shavings, and the Attractor and Repulsor nodes as magnets of different polarity.

To break that down into individual forces for one particle:

Test Cases:

Emitter Node surrounded by Neutral nodes:

The particles all continue in whatever direction they were sent in originally by the emitter Node. The emitted particles have some built-in variation of direction and magnitude.

Diagram:

Implementation:

Emitter Node With One Attractor Node:

The particles all move towards the attractor node in addition to whatever direction they were sent in originally by the emitter Node. Once they pass the attractor node, it ceases to have an effect on them. The forces applied on the particles are attenuated by distance (i.e. a farther away Attractor Node will have less effect than a closer one will).

Diagram:

Implementation:

Emitter Node With One Repulsor Node:

The particles all want to move away from the Repulsor node in addition to whatever direction they were sent in originally by the emitter Node. If they pass the Repulsor node, it ceases to have an effect on them. The forces applied on the particles are attenuated by distance (i.e. a farther-away Repulsor Node will have less effect than a closer one will).

Diagram:

Implementation:

Emitter Node With A Combination of Node Types:

These are just some of my favorite test cases. Though they don't look half as good when they're not animated, these should give you an idea.