Colored emissions at the blade tips show the cycloidal shape of the vortices as well as their lengthwise distribution. In this example of unsteady three-dimensional flow, the knowledge of vortex paths and of their evolution as a function of various parameters (rotor tilt angle, number of blades, etc.) sheds some liqht on the unwanted interaction effects of the vortices on the following blades.

Image created by Henri Werlé. Reproduced with permission from his spectacular collection Courants et Couleurs published in 1974 by ONERA, the French Aerospace Lab.