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Research Papers

Experimental Investigation of the Effects of Winglets on the Tip Vortex Behavior of a Model Horizontal Axis Wind Turbine Using Particle Image Velocimetry

[+] Author and Article Information
Yaşar Ostovan

Department of Aerospace Engineering,
METU Center for Wind Energy,
Middle East Technical University (METU),
Ankara 06800, Turkey
e-mail: yasar.ostovan@centralelille.fr

M. Tuğrul Akpolat

Department of Aerospace Engineering,
METU Center for Wind Energy,
Middle East Technical University (METU),
Ankara 06800, Turkey
e-mail: tugrul.akpolat@metu.edu.tr

Oğuz Uzol

Department of Aerospace Engineering,
METU Center for Wind Energy,
Middle East Technical University (METU),
Ankara 06800, Turkey
e-mail: uzol@metu.edu.tr

1Present address: Research Engineer at Laboratoire des Fluides de Lille Kampe de Feriet (LMFL), Villeneuve d'Ascq 59650, France,

2Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received September 21, 2017; final manuscript received August 4, 2018; published online September 14, 2018. Assoc. Editor: Yves Gagnon.

J. Sol. Energy Eng 141(1), 011006 (Sep 14, 2018) (13 pages) Paper No: SOL-17-1391; doi: 10.1115/1.4041154 History: Received September 21, 2017; Revised August 04, 2018

This study presents an experimental investigation on the effects of winglets on the near wake flow around the tip region and on the tip vortex characteristics downstream of a 0.94 m diameter three-bladed horizontal axis wind turbine (HAWT) rotor. Phase-locked 2D particle image velocimetry (PIV) measurements are performed with and without winglets covering 120 deg of azimuthal progression of the rotor. The impact of using winglets on the flow field near the wake boundary as well as on the tip vortex characteristics such as the vortex convection, vortex core size, and core expansion as well as the resultant induced drag on the rotor are investigated. Results show that winglets initially generate an asymmetric co-rotating vortex pair, which eventually merge together after about ten tip chords downstream to create a single but nonuniform vortex structure. Mutual induction of the initial double vortex structure causes a faster downstream convection and a radially outward motion of tip vortices compared to the baseline case. The wake boundary is shifted radially outward, velocity gradients are diffused, and vorticity and turbulent kinetic energy levels are significantly reduced across the wake boundary. The tip vortex core sizes are three times as big compared to those of the baseline case, and within the vortex core, vorticity and turbulent kinetic energy levels are reduced more than 50%. Results show consistency with various vortex core and expansion models albeit with adjusted model coefficients for the winglet case. The estimated induced drag reduction is about 15% when winglets are implemented.

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Figures

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Fig. 1

(a) Model wind turbine located at the jet exit of the open jet wind tunnel and (b) the winglets attached to blade tips of the turbine

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Fig. 2

Inlet velocity and turbulence intensity distributions 0.5D downstream of the open jet tunnel. Here R is the rotor radius. The rotor tip and the tunnel boundary positions are indicated on the figure.

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Fig. 3

Definitions of winglet design variables

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Fig. 4

(a) Phase definitions and sample three phases and (b) PIV measurement domain

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Fig. 5

Convergence plot for two measured velocity components (left) as well as for turbulent kinetic energy (right) using samples from a grid point in window 1, for phase 90 and near the vortex core

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Fig. 6

Picture of the facility while performing phase-locked PIV measurements

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Fig. 7

(a) A sample PIV raw image for the baseline case at phase Φ = 36 deg, (b) and (c) corresponding phase averaged u¯ velocity and out-of-plane vorticity distributions near the blade tip, respectively, superimposed by vortex-induced velocity vectors. The free stream flow is from left to right. The dashed rectangular region represents the blade position at phase Φ = 0 deg. (x′/R)=(y′/R)=0 is the position of the blade tip at phase Φ = 0 deg.

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Fig. 8

Phase-averaged axial velocity (u¯) distributions for rotor phases 0–120 deg for baseline (left) and winglet (right) cases

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Fig. 9

Phase-averaged out-of-plane vorticity (ΩZ¯) distributions for rotor phases 0–120 deg for baseline (left) and winglet (right) cases. Vortex-induced velocity vectors are superimposed on the distributions.

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Fig. 10

Phase-averaged turbulent kinetic energy distributions for rotor phases 0–120 deg for baseline (left) and winglet (right) cases

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Fig. 11

Overall average of 21 phases (0–120 deg with 6 deg intervals) for (a) axial velocity, (b) vorticity, and (c) turbulent kinetic energy for baseline (left) and winglet cases (right)

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Fig. 12

Data extracted from a vertical line at (x′/R)=0.8 from the overall averages of 21 phases (0–120 deg with 6 deg intervals) for (a) axial velocity, (b) out-of-plane vorticity, and (c) turbulent kinetic energy for baseline and winglet cases

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Fig. 13

Vortex center positions from vortex age 30 deg to 330 deg with 30 deg intervals for the baseline and winglet cases

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Fig. 14

(a) Vortex-induced swirl (tangential) velocity magnitude, (b) out-of-plane vorticity, and (c) turbulent kinetic energy distributions along a horizontal line intersecting the center of second vortex core at rotor phase 60 deg (vortex age 180 deg). Horizontal axis is the nondimensional distance from the vortex core center normalized by the tip chord length of the rotor blade.

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Fig. 15

Comparison of vortex core models with current experimental data for three different phases and ages. Vortex ages 60 deg, 180 deg, and 300 deg correspond to phase 60 deg of blades 1–3, respectively, (a) baseline and (b) winglet cases.

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Fig. 16

Vortex core growth for the baseline case from vortex age 12 deg to 348 deg with 6 deg intervals compared to winglet case. The vortex core expansion model of Sant et al. [32] is also presented.

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Fig. 17

Estimated rotor induced drag for baseline and winglet cases, calculated for rotor phases 24–114 deg

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