First, one needs to learn what the flow past streamlined and not streamlined (fluid dynamists would say “bluff”) mirrors are. If one could see the air, one would just sit in a moving car and watch. As it is, however, one has to use specialised software. In this lesson an online service called Flow Illustrator will be used. It is located at www.flowillustrator.com. The service makes videos of fluid flows past objects in the pictures uploaded by the user. Here are two such videos:
In the videos the small white dots, called “snowflakes”, are marking fluid particles and are moving together with them. The particles are marked at random, and the snowflakes, after a period of time, disappear: “melt”. The coloured areas show those parts of the fluid that have noticeable rotation, with the red colour marking clockwise rotation and the green colour marking counter-clockwise rotation. In fluid dynamics a quantity called vorticity, equal to twice the rotational speed of the small fluid volumes, is often used. Vorticity is being carried by the flow almost in the same way as the snowflake, but in addition it is diffused (spread out, dispersed) by the action of viscosity. Snowflakes and vorticity are a convenient way to observe the pattern of fluid motion.