Three-Component Reverse Flow Measurements on a Mach-Scale Rotor at High Advance Ratios

Reverse flow is the source of several unsteady effects that complicate load predictions for high advance ratio rotorcraft. As a way of improving load predictions, an ongoing series of experiments has been aimed at gaining a physical understanding of the unsteady aerodynamics of the reverse flow region. The current work contributes phase-averaged, three-component velocity field measurements collected on a rotor at high advance ratios. Stereoscopic particle image velocitmetry (PIV) was performed on a Mach-scale rotor across three advance ratios (0:6 ≤ μ ≤ 0:8), three radial stations (0:3 ≤ r/R ≤ 0:6), and one collective (θ0 = 10°). The present analysis focuses on how the reverse flow dynamic stall vortex, which results from flow separation about the sharp geometric trailing edge of a rotor blade in reverse flow, evolves over time in three dimensions. For a constant advance ratio, the size of the reverse flow dynamic stall vortex increases with decreasing radial station, creating a gradient in vorticity along the blade span. Tip-to-root radial flow, the amount of which increases for inboard radial stations, was also associated with the development of the reverse flow dynamic stall vortex. The current work postulates that due to an increase in radial flow, inboard radial stations are subject to a greater radial transport of vorticity than outboard sections and, in turn, produce less dimensional vorticity than outboard sections at the same convective time. Coupled with previous experimental models of reverse flow, the current work represents an important step in identifying the dominant unsteady characteristics of the reverse flow region and is designed to inform a low order model of rotors at high advance ratio.