Unsteady Boundary Layer Transition Measurements on a Rotor in Forward Flight

Abstract

A proof of concept test to measure the unsteady boundary layer transition locations on the lower surface of a Mach-scaled rotor in forward flight was performed during the Summer of 2017 in the NASA Langley 14- by 22-Foot Subsonic Tunnel. The transition locations were measured using high-speed infrared thermography with a rotating mirror assembly that could be remotely actuated to acquire data at several rotor azimuths. Data were acquired for eight unique rotor flight conditions for a range of advance ratios (mu=0.10 to 0.38), thrust coefficients (CT/sigma = 0.04 to 0.12) and rotor shaft angles (alpha = -6 deg to 0 deg). This paper presents the transition locations as a function of azimuth and radius for an advance ratio of, mu, of 0.30, and thrust coefficent, CT/sigma, of 0.08. At this condition, the lower surface is fully laminar on the retreating side and mostly turbulent on the advancing side except near the tip. The tip airfoils were greater than 60 percent laminar on the lower surface advancing side. Capturing the location of natural transition on a rotating blade in forward flight represents a new advancement toward understanding the boundary layer state and its important contribution to rotor aerodynamics. Documentation of the boundary layer transition location during testing is critical to understanding scaling model to full-scale performance data, validation of newly developed turbulence models, and the design of the next generation of high performance rotor blades.