What This Is

This is a fluid simulation written in C++ and made available on the web via Google's Native Client. I originally wrote up an implementation of the paper "Particle-based viscoelastic fluid simulation" just to get my feet a little wet with Native Client. I ended up revisiting it with the intention of just working on optimizing that implementation but ended up rewriting it and going with a different methodology after going through the fluid simulation rabbit hole a bit.

Rather than produce yet another explanation of Navier-Stokes and all the bits of math behind fluid simulation I will just link to a few different resources I found useful:

My simulation is to a large extent based off the work in the last few links; I started down this path after seeing Grant Kot's fantastic demo and fluid sim videos (which does a bit more than what I am doing here). The simulation works by tracking individual particles within the context of a fixed grid and "spreading" the particle values into the grid using a biquadratic interpolation scheme. From the grid I can then easily calculate my fluid forces and use these to then update the individual particles in the simulation. It is much faster than what I was doing before and works pretty well - although the fluid does end up compressing more than it should and the gridding seems to end up making things a bit more viscous. To be sure, I am not really going so much for accuracy here as something believeable and that could be dropped into a game to have some fun with.

As for the actual simulation steps they proceed roughly as follows:

Clear all grid cells

For each particle

Determine grid cell and the 8 surrounding cells

Calculate weights for biquadratic interpolation at particle position

Add particle mass and velocity to surrounding grid cells based on interpolation weights

For each particle

Calculate interpolated mass and velocity gradients at particle position

Use calculated mass and velocity gradient to determine pressure and viscosity forces

Add these forces back into the grid cells

For each particle

Calculate interpolated acceleration at particle position

Add acceleration due to gravity

Update particle velocity

Add particle velocity back into the grid cells

For each particle

Calculate interpolated velocity at particle position

Update particle position and velocity with interpolated velocity

Beyond that I also "cheat" a little and introduce a bit of a force near collision surfaces to push particles away.

For more detail you can check out the source code I have made available here.