Tip vortex measurements on a cycloidal rotor blade at ultralow Reynolds numbers
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Copyright 2017 by AHS International, Inc. All rights reserved. This study provides the first in-depth analysis of the formation, strength, and convection of cycloidal rotor tip vortices. The blade force and PIV-based tip-vortex measurements were conducted for different blade aspect ratios and pitch kinematics in water at a chord Reynolds number of 18,000. Two phase-locked PIV configurations were utilized to investigate the flow field induced by the cyclorotor blade: (1) a laboratory-fixed field of view to enable investigation of vortex development at increasing vortex ages; and (2) a blade-fixed field of view to investigate the early development of the wingtip vortex at fixed 2 vortex age for varying azimuthal locations. The instantaneous blade force measurements on the cycloidal rotor showed a decrease in lift coefficient with decreasing blade aspect ratio. This is due to the higher peak swirl velocity of the tip vortex produced by the low AR blade, thereby resulting in higher induced downwash along the blade span. The aspect ratio of the blade did not affect the shape of the vortex convection trajectory, however, the rate of downward convection increased with increasing aspect ratio due to the higher thrust produced. The tip vortices showed self-similarity in both the velocity and the circulation profiles. The measurements indicate that the core-radius of the vortex experiences a logarithmic growth and the swirl velocity experiences a logarithmic decay, with vortex age due to viscous diffusion. When compared to previous helicopter rotor studies, the observed vortex dynamics from the present study exhibit increased viscous diffusion, likely due to the significantly lower Reynolds number. The tip vortex strength varied cyclically with blade azimuthal location due to the cyclic variation of blade pitch angle and the dynamic virtual camber effects. The periodic variation in tip vortex strength leads to a periodic variation in the induced flow velocity on the blade.
