8.1. Trajectories, streamlines and streaklines8.1.1. Trajectories in a moving fluid8.1.2. Streamlines of a moving fluid8.1.3. Streak lines of a moving fluid8.1.4. The case of steady flow: stream surface and stream tube8.2. Vorticity, circulation, stream function and velocity potential8.2.1. Vorticity and Crocco’s theorem8.2.2. Circulation: Kelvin, Lagrange and Helmholtz theorems8.2.3. Stream function8.2.4. Potential function for the velocity or velocity potential