Challenges in Understanding the Vortex Dynamics of Helicopter Rotor Wakes

Many experimental challenges are encountered when attempting to understand the complex vortical wakes generated by helicopter rotors. The problems are illustrated by means of several examples, including blade/vortex interactions, vortex perturbations, wake instabilities, and vortex/surface interaction phenomena. Specific emphasis is placed on flow visualization and quantitative measurements using laser Doppler velocimetry