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Visualizing flow with particles can range from fog machines to watching fallen leaves blowing in the wind. What makes the difference is how dense the seeding is; whether individual particles can be distinguished, or if the particles are close enough together to look like a cloud. Lets consider the dense seeding case first.

There are three considerations when using particles to visualize flow:

1. The particles must to move accurately with the flow, must track the flow. This is not a problem with dyes; dye molecules generally move with the flow just fine, but particles are big enough that they may lag when the flow turns quickly.
2. We want the particles to NOT disturb the flow.
3. We want the particles to show up – to have high visibility.

1: When will particles track well and be good tracers?

Consider a particle in a curved streamline as shown in Figure 1. Assume the particle is small, but much denser than the fluid. Let’s say the curved flow is in the horizontal plane – in other words, don’t worry about gravity making the particle fall just yet. Now, what will the particle path look like compared to the fluid path?