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I need some time to figure it out, but the little blue dots are the "air molecules". They bounce against the sail!

I first thought that they were the sea waves, of a nice visual effect, but they are a part of the simulation.

Yes, every time one of those hit the 'sail' the direction and magnitude of the force it imparts on the sail is calculated. It is not really needed, as one could just calculate it from the strength and direction of wind in relation to the sail, and setting that they hit the sail at some constant rate. I decided to render it so that it gives some intuition regarding the forces that are acting on the sail.

I like that it shows the air which has hit the sail and then spilled off, or "dirty" air. It's a good indication of why you don't want to be sailing behind a boat you are trying to catch!

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