Paused
Tool
Ink colour: Rainbow
Start with
How to play
- Drag through the tank to stir. Use two or three fingers for two or three streams.
- Pick Wall and draw a wall. Stir again and watch the fluid flow round it.
- Try Wind + ball and watch swirls peel off behind the ball.
| Do this | Phone | Keyboard |
|---|---|---|
| Stir | Stir tool, drag (more fingers = more streams) | 1, then arrows (on the tank) |
| Drop a blob of ink | Stir tool, tap | Space (on the tank) |
| Draw a wall | Wall tool, drag | 2, Space pen down, arrows draw |
| Rub out a wall | Eraser tool, drag | 3, Space pen down, arrows rub |
| Next ink colour | Colour buttons | C |
| Velocity arrows / pressure view | The two toggles | V / H |
| Pause / Play | Pause button | P |
| Clear ink / Clear all | Buttons | I / X |
What's happening?
- The tank is a grid of tiny boxes. Each box knows how fast the fluid in it is moving, and how much ink it holds.
- Advection: everything is carried along by the flow, even the flow itself. That's why a push keeps going after you let go.
- Pressure: water can't be squashed. Push it one way and it has to get out of the way somewhere else. Every step, the sim finds the pressure that makes each box have exactly as much flowing in as out. In the pressure view, pink is high (squeezed) and blue is low.
- Viscosity: how thick the fluid is. Thick fluids like honey smear every swirl out.
- Swirl (vorticity): finds the spinning bits and spins them a little harder, so small curls don't blur away.
- Together these are the Navier-Stokes equations: the same maths behind weather forecasts, aircraft design and the smoke in films.
Try this: load Wind + ball. Do the swirls behind the ball peel off one side, then the other? Now turn viscosity up to honey. What happens to them?