Fluidsim is a free and easy to use 2D fluid simulator written in the Java programming language.

Just click and drag to swirl fluid around. Left-clicking adds ink while doing so, whereas right-clicking simply swirls the fluid without doing so.

Explanation of the sliders:
· Red / Green / Blue - Controls the color of the ink added when clicking on the fluid. Initially white.
· Splat Size - Size of the ink splat added when you click on the fluid.
·  Diffusion - Rate at which the ink diffuses and spreads out in the fluid.
· Viscosity - Rate at which fluid motions damp out over time.
· Vorticity - Controls the fine-scale flows and vortices of the fluid. To really see the effect of this slider, set it to zero, then start swirling colors into the fluid, then slide this up to maximum and notice what happens.
· Timestep - Controls the time-increment per frame of the simulation. Note that large timesteps will lead to inaccurate fluid behavior, and large timesteps will also cause the simulation to blow up at high vorticities.
· Iterations - Controls the number of iterations used in the iterative linear solver that calculates the fluid. Low values will result in a fast simulation, but the fluid will behave inaccurately, whereas high values will give more accurate behavior, but slow down the simulation. The primary effect is to make fluids more incompressable (as they should be) to display more accurate swirling effects. The default position for this slider (just a little bit above minimum) is usually good.