Characterizing Low Frequency Velocity Fluctuations in the Wake of a Coaxial Rotor in Compressed Air

Low-frequency fluctuations within the wake of a coaxial rotor system were consistently observed during wake surveys of a scale model of the NASA Dragonfly coaxial rotor system conducted in the Penn State Compressed Air Wind Tunnel (CAWT) at high Reynolds numbers. While wake meandering is a well-documented phenomenon in wind turbine flows, typically characterized by a Strouhal number of approximately 0.3 and associated with bluff-body vortex shedding, such analogies are less applicable to rotorcraft in hover due to the absence of freestream flow. In this study, wake meandering was most prominent in hover conditions, exhibiting the highest amplitude fluctuations. In axial climb, the amplitude of these low-frequency components diminished, suggesting a dependence on blade loading and background flow convection. These findings parallel observations from single rotor studies, where tip vortex core meandering within the wake have been correlated with blade loading and lift coefficient. However, this study presents the first known evidence of such behavior in the coaxial rotor configuration. Results provide new insight into the unsteady aerodynamic environment of coaxial systems and motivate future in-depth parametric studies on the phenomenon.