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Velocity measurements on a retreating blade in dynamic stall

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Abstract

The velocity field over a moderately low aspect ratio retreating rotor blade is investigated using particle image velocimetry as the two-bladed teetering rotor with cyclic pitch operates in a low-speed wind tunnel. The phase-averaged velocity field shows that the stall vortex circulation is comparable to the section dynamic lift, confirming it as a dynamic stall vortex (DSV). Strong tip vortex effects suppress stall at outboard locations. The DSV on the rotating blade after liftoff is elongated and remains in close proximity to the blade surface in contrast to classical two-dimensional DSV on non-rotating blades. The phase-averaged DSV is roughly 40 % weaker and spatially diffused than those observed in individual instantaneous velocity fields. Spanwise variations of the vortex are also discussed. Cycle-to-cycle variations of the instantaneous velocity fields suggest a radial-flow-induced stabilization in the strength, but not in the spatial location of the DSV.

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Abbreviations

\(\varGamma\) :

Circulation (m2/s)

\(\varGamma_1\) :

Scalar function

μ :

Advance ratio

ν :

Kinematic viscosity of air (m2/s)

\(\varOmega\) :

Angular velocity of rotor (rad/s)

ω :

Vorticity (1/s)

ψ :

Rotor azimuth angle (°)

b :

Blade span (m)

c :

Blade chord (m)

C L :

Coefficient of lift

k :

Reduced frequency

R :

Radius of the rotor disc (m)

r :

Radial location (m)

Re :

Reynolds number based on chord length

U ∞ :

Freestream velocity (m/s)

U e :

Effective freestream velocity (m/s)

U T :

Tip speed of the rotating blade (m/s)

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Acknowledgments

This work was funded by the Army Research Office (ARO). Grant No.: W911NF1010398. The authors would like to thank Dustin Teuscher and Laura Hershberger for help with conducting the experiments.

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  1. School of Aerospace Engineering, Georgia Institute of Technology, 270 Ferst Drive, Atlanta, GA, 30332, USA

    Vrishank Raghav & Narayanan Komerath

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  1. Vrishank Raghav
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  2. Narayanan Komerath
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Correspondence to Vrishank Raghav.

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Raghav, V., Komerath, N. Velocity measurements on a retreating blade in dynamic stall. Exp Fluids 55, 1669 (2014). https://doi.org/10.1007/s00348-014-1669-9

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