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Publication Title | Radial Flow Rotating Blade Retreating Blade Stall

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ao wind deficit factor c chord length, m
k reduced frequency R rotor radius, m
JOURNAL OF THE AMERICAN HELICOPTER SOCIETY 58, 022005 (2013)
An Exploration of Radial Flow on a Rotating Blade in Retreating Blade Stall
Vrishank Raghav Narayanan Komerath∗ Graduate Research Assistant Professor
Daniel Guggenheim School of Aerospace Engineering Georgia Institute of Technology, Atlanta, GA
The nature of radial flow during retreating blade stall on a two-bladed teetering rotor with cyclic pitch variation is investigated using laser sheet visualization and particle image velocimetry in a low-speed wind tunnel. The velocity field above the retreating blade at 270◦ azimuth shows the expected development of a radially directed jet layer close to the blade surface in the otherwise separated flow region. This jet is observed to break up into discrete structures, limiting the spanwise growth of the radial velocity in the jet layer. The discrete structures are shown to derive their vorticity from the “radial jet” layer near the surface, rather than from the freestream at the edge of the separated region. The separation line determined using velocity data shows the expected spanwise variation. The results of this study are also correlated in a limited range of extrapolation to the phenomena encountered on a full-scale horizontal axis wind turbine in yaw.
Nomenclature
􏲓t time interval between pulses, s γ wind turbine yaw angle, deg ψ azimuthal position, deg
􏲔 rotation rate, rad/s
ω vorticity, 1/s
ωf flapping frequency, rad
Introduction
Retreating blade stall of a helicopter rotor is a primary limitation of the flight envelope. Blade stall is accompanied by sharp changes in pitch link loads and vibrations. Flow reattachment is delayed in a hysteresis loop which persists well into the fourth quadrant of the rotor disk. Although this phenomenon has been studied extensively, predicting the precise timing, extent, and hence the rotor phase of dynamic stall and reattachment remains elusive. Centrifugal effects on the boundary layer are expected to drive a strong radial flow, but experiments have not delineated the expected magnitude of these effects. The question “what is the role of radial flow on postdynamic stall lift and pitching moment evolution?” remains unanswered.
This work explores the radial velocity field during retreating blade dynamic stall. Experiments on a rotor in forward flight in a low-speed wind tunnel are used to derive conclusions that are relevant to the phe- nomena occurring on the retreating blades of a helicopter in high-speed forward flight. Particle image velocimetry (PIV) is used to investigate the detailed physics of the radial flow field on the retreating blades. In addition, the spanwise variation of the separation line is determined us- ing velocity data. This work also discusses the correspondence between the wind tunnel test case and the relevant operating conditions on a full-scale horizontal axis wind turbine (HAWT) in yaw. It is shown that
Re Reynolds number
r radial/spanwise location, m
Vc velocity component in the plane of the wind turbine, m/s
Vi induced velocity for the rotor setup, m/s
Vn velocity component normal to the wind turbine, m/s
Vr radial velocity, m/s
Vtw tangential component of velocity to wind turbine blade, m/s Vw freestream velocity for wind turbine, m/s
V∝ freestream velocity for rotor setup, m/s
x chordwise location, m
z axial location, m
αir induced angle of attack on rotor setup, deg
αiw induced angle of attack on wind turbine, deg
αpr blade pitch of rotor setup, deg
αpw blade pitch of wind turbine, deg
αr angle of attack of rotor setup, deg
αw angle of attack of wind turbine, deg
∗Corresponding author; email: Komerath@gatech.edu.
Presented at the 26th AIAA Applied Aerodynamics Conference, Honolulu, HI, August 18–21, 2008, and 2nd International Forum on Rotorcraft Multidisciplinary Technology, Seoul, Republic of Korea, October 19–20, 2009. Manuscript received October 2011; accepted January 2013.
Most figures in this paper are in color in the electronic version.
DOI: 10.4050/JAHS.58.022005 022005-1 ⃝C 2013 The American Helicopter Society

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An Exploration of Radial Flow on a Rotating Blade in Retreating Blade Stall: The nature of radial flow during retreating blade stall on a two-bladed teetering rotor with cyclic pitch variation is investigated using laser sheet visualization and particle image velocimetry in a low-speed wind tunnel. The velocity field above the retreating blade at 270◦ azimuth shows the expected development of a radially directed jet layer close to the blade surface in the otherwise separated flow region. This jet is observed to break up into discrete structures, limiting the spanwise growth of the radial velocity in the jet layer. The discrete structures are shown to derive their vorticity from the “radial jet” layer near the surface, rather than from the freestream at the edge of the separated region. The separation line determined using velocity data shows the expected spanwise variation. The results of this study are also correlated in a limited range of extrapolation to the phenomena encountered on a full-scale horizontal axis wind turbine in yaw.

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