Measurements on a yawed model rotor blade pitching in reverse flow

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Smith, L.R.
Lind, A.H.
Jones, A.R.
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In forward flight, high advance ratio rotors encounter a large region of reverse flow on their retreating side. The reverse flow region is dominated by the reverse flow dynamic stall vortex (RFDSV), which incurs large unsteady torsional loads that are not well-predicted by modern comprehensive rotorcraft codes. To gain a physical understanding of the impact of yaw on the RFDSV, a sub-scale, NACA0012 model rotor blade has been experimentally tested at two reverse flow yaw angles, _rev = 0_ and _rev = 30_, over a range of reduced frequencies, 0.160< k <0.450. Pressure time histories were obtained from surface-mounted unsteady pressure transducers, and three-component velocity fields were obtained using time-resolved stereoscopic particle image velocimetry at the midspan. The present work focuses on the impact of yaw on general flow morphology, dynamic stall vortex behavior, and spanwise flow. The presence of yaw was found to lessen the magnitude of the pressure wave induced by the RFDSV and hinder the development of secondary flow structures. The presence of yaw did not lead to a noticeable difference in the convection speed of the RFDSV in the direction of the freestream, but did lead to a substantially higher magnitude of spanwise flow. Ultimately, this experimental study lays the groundwork for understanding three-dimensional effects on the formation and convection of the reverse flow dynamic stall vortex.