STROBED LASER-SHEET VISUALIZATION OF A HELICOPTER ROTOR WAKE

The individual wakes trailing behind each blade of a helicopter rotor form turbulent vortex sheets that quickly roll into concentrated tip vortex filaments. A phase-locked, strobed laser light-sheet illuminating a seeded flow has been shown to provide both a quantitative and qualitative method to study the evolution of this turbulent wake. The tip vortex cores can be identified by a well-defined seed void. The shear layer produced by the blade wake appears as a line of discontinuity in the streaklines. Because the vortex core is a rolled-up portion of the original trailed wake, the vortex core and the vortex sheet were studied as interdependent flow structures. Some outboard portions of the vortex sheet were found to descend axially through the flow at a greater rate than the tip vortex core. While maintaining some connection to the original vortex core, the vortex sheet interacts with the tip vortex trailed from another blade. During the interaction, three-component laser Doppler velocimetry (LDV) measurements show a substantial increase in the angular velocity of the fluid within the vortex core. A three-dimensional reconstruction of the wake geometry in terms of the core trajectories agrees with results measured by LDV.