Experimental Studies on Velocity Field around Wind Turbine Rotor

7/28/2019 Experimental Studies on Velocity Field around Wind Turbine Rotor

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Abstract--This paper describes the behavior of the flow field

around rotor blades. A three-bladed upwind rotor was tested in a

single return type wind tunnel. The rotor had a diameter of 2.4

m. The flow field around the rotor blades was measured using

two-dimensional LDV. The three velocity components of flow

field were measured in the x-y plane. The velocity vectors at

optimum operation showed a smooth flow around the blade and

the bound vortex around blade cross-section seemed persistent.

On the other hand, the velocity vectors in stall condition

demonstrated significant fluctuations in the near wake and

separation on the blade suction side was observed. The

circulation around a blade span-wise section was calculated at a

certain control volume. From observations of the flow field and

the calculated results of circulation, it seems that the flow is

separated at the blade from the middle-span region to the tip

region in stall condition. No separation was observed at the

blades root region.

Index Terms-- Wind energy, Aerodynamics, Velocity

distribution, LDA measurement.

I. NOMENCLATURE

r : Span-wise position m

CP : Power coefficient=T/ (1/2R2U0

3) -

R : Radius of rotor=1.2 mT : Rotor torque Nm

U0 : Undisturbed wind speed m/s

u, v, w : Velocity components by x,y and axis m/s

: Circulation m2/s

: Tip speed ratio=R/U0 -

: Air density kg/m3

: Azimuth angle deg

: Angular velocity 1/s

II. INTRODUCTION

IND turbine efficiency has been considerably improved

due to recent advances in fundamental research andwith the adoption of new technologies. The aerodynamic

efficiency of a wind turbine largely depends on the efficiency

of the blade itself. In order to stabilize power output vis--vis

wind velocity fluctuations, by developing new thicker aerofoil

[1] or by applying vortex generators [2], improvements in

blade efficiency are achieved due to boundary layer control on

Associate Professor Yasunari. Kamada and Professor Takao Maeda are

with the Division of Mechanical Engineering, Graduate School of Mie

University, Tsu, Mie, Japan. (e-mail: [email protected];

[email protected]).

the surface of the blade. Numerous efforts utilizing wind

tunnel experiments or field testing have been undertaken to

investigate flow conditions around a rotating blade or in its

near wake. Ebert and Wood measured the near wake of an

experimental wind turbine in a wind tunnel using the hot-wire

anemometry method [3-5]. Velocity measurements around the

wind turbine have been reported [6], and visualization of the

flow on the blade surface performed [7]. Significant

achievements have been reported with regard to the

investigation of the helicopter-rotor wake. A detailed review

of the developments during the past two decades on thehelicopter rotor research is given from Bhagwat and Leishman

[8]. Measurements of bound and wake circulation on a

helicopter-rotor have been reported and it is proven that the

closed-loop contour integration method used for estimating

blade-bound circulation on rotor blades is sensitive to the

choice of the integration path. Wood and Boersma studied the

self-induced velocity of equally spaced multiple vortices in

case of multi-bladed wind turbines, propellers, rotors in

ascending, descending, or hovering flight [9]. However, the

current state of knowledge of the flow around a blade of a

wind turbine or in its wake remains incomplete.

In this experimental research work, the authors attempt to

clarify the flow around the rotating blade of a three-bladed

upwind turbine by measuring the velocity distribution with a

Laser Doppler Velocimetry method. Results for 3 different

experimental conditions, optimum operation, stall and

overrunning conditions, were compared. In addition, the

circulation around the blade was calculated from the velocity

distribution. The properties of the flow around a rotating blade

were identified and quantified.

III. EXPERIMENTAL SETUP AND METHODOLOGY

A. Experimental Devices

The experimental arrangement is depicted in Fig. 1. Thewind tunnel is a single return type, with an open test section,

outlet diameter of 3.6m, and maximum wind speed of 30m/s.

The uniformity turbulence levels of wind tunnel are less than

1.3% and less than 1.1% respectively. The three-bladed

upwind turbine with diameter of 2.4 m to be tested is installed

downstream at a distance of 1 diameter from the wind tunnel

outlet. The nacelle of the wind turbine is equipped with a

speed-up gearbox, variable-speed generator and torque meter,

a revolution indicator and an encoder for detection of the

rotation angle (azimuth angle). The rotor turned

Experimental Studies on Velocity Field around

Wind Turbine RotorY. Kamada and T. Maeda

W


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counterclockwise (viewed from upwind) at a variable rpm

depending on the operating conditions. The azimuth angle

could be measured with a resolution of 0.4 degrees. The

velocity distribution was measured by a two-dimensional

Laser Doppler Velocimeter. The probe, with a focal length of

about 2m, was mounted on a three dimensional traversing

system on top of the test section. The probe measured 0.15mm

in diameter and 4.3mm in length. The tracer was seeded from

the inlet port of the wind tunnel. The undisturbed windvelocity was measured by a Pitot tube mounted at the edge of

the outlet section of the wind tunnel. The experimental wind

velocity was set at 7 m/s, blade pitch angle was locked at -2.

The operating speed of the rotor was variable from 100rpm to

450rpm ( =1.8- 8.1). At an optimum tip speed ratio, = 5.20,

the Reynolds number at the blade tip was about 2.1105.

The measurement mesh consisted of 48 points in a radial

direction, 40mm equidistance from each-other, fromy=0.24m

to y=1.32m; and 13 points in an axial direction, 15mm

equidistance from each other, from x=-0.9m to x=0.9m. All

the measuring points were in the main diametric horizontal

plane. First, velocity components, u and v, on the x-y plane

were measured. Changing the direction of the probe, the

velocity components u, w, on the same plane, was measured.

Therefore, a three dimensional velocity field around the

rotating blade was determined. The zero azimuth angles were

triggered when the instrumented blade was at a horizontal

position (3 oclock position). Data were acquired when the

instrumented blade was passing from azimuth angle =-30

to azimuth angle =90, with a bin azimuth angle of 1.2

degrees. Sampling time for one measuring point (for 2000

samples to be acquired) was between 2 and 20 seconds. The

averaging was performed for revolutions of the rotor

corresponding to the above mentioned sampling time, being in

the interval between 9 and 90 revolutions.

B. Test Blade

Blades used throughout this experiment were twisted and

tapered. Blade taper and twist distributions are shown in Fig.

2. The blade chord at the tip was 85mm and the blade twist

was reduced from 18.3 degrees at 0.2R to 0.0deg at the tip.

Blade cross sections were composed of DU91-W2-250,

DU93-W-210, NACA63-618 and NACA63-215 airfoil

sections at locations starting from the blade root. Intermediate

sections were found by interpolation.

IV. EXPERIMENTAL RESULTS AND CONSIDERATIONS

A. Performance Test Results

The performance test results of the test wind turbine are

shown in Fig. 3. The data were acquired for a setting blade

pitch angle of -2 degrees. Note that the maximum power

coefficient reaches a value of 0.435 at the optimum tip speed

ratio =5.20. When the tip speed ratio decreases there is a

stall state, and there is an increase of the torque ratio with the

change of rpm. Investigation of the flow around the rotating

blade in case of stalling conditions were performed for a tip

speed ratio of =3.72 at which certain stall occurred. The

power coefficient achieved in this state was CP=0.202. An

overrunning state of operation was observed at high tip speedratios. To achieve a continuous operation, for flow

measurements around the blade at an overrunning state, the tip

speed ratio of =6.50 was chosen as appropriate. The power

coefficient at this overrunning state was CP=0.373.

The test blade used in this study was designed based on

BEM theory. Power coefficient obtained from the blade

design computer program is compared with the measured data.

Fig. 3 shows that the power coefficient for the optimum or

higher tip speed ratio is slightly over predicted.


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B. Comparison of the Velocity Distributions

Fig. 4 shows the velocity vector distribution for every

operational state studied; Fig. 4(a) shows the velocity vectors

in the x-y plane for a stalling condition ( = 3.72), Fig. 4(b)

for optimum operation ( = 5.20), and Fig. 4(c) for an

overrunning operation ( = 6.50). The left side of every figure

shows data averaged for the azimuth angle = 6, where

remarkable differences in the flow around the blade were

apparent; and the right side of every figure shows dataaveraged over one revolution of the rotor. Furthermore, in the

case of averaging over the azimuth angle = 6, the area near

the trailing edge at the blade root is included in the

measurement plane. From Fig. 4(a), with regard to data

averaged for the azimuth angle = 6 , it is found that as soon

as the blade passes the measuring plane, a disorder of the flow

developed in a radial direction 0.3


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radial direction, and at 0


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C. Comparison of Bound Circulation

In previous sections we discussed qualitatively the flow

behavior around the rotating blade by focusing on velocity

vector distributions. In this section, to quantitatively compare

the operational states under study, the circulation around the

airfoil section is calculated and discussed.

Considering the circulation area as a summation of the

elementary areas having nodes at the measuring points, as

shown in Fig. 6, the integral over the closed peripheralrectangular path will give the circulation strength, , at a

particular radial section.

Fig. 7(a) schematically shows the areas for the circulation

calculations. The integration contour was chosen a rectangular

one having the width between -0.075x/R0.075.

Fig. 6. Definition of circulation calculation.

Fig. 8 shows the circulation change versus azimuth angle for

the blade section at 0.8R. The calculation territory is fixed in

space, corresponding to the position of an azimuth angle of

=0 where the rotating blade passes at its center. In the case of

optimum and overrunning operations, the circulation reaches

its maximum for an azimuth angle -18


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Fig. 9. Circulation distribution along the blade span.

V. CONCLUSIONS

This paper quantitatively and qualitatively discusses and

compares flow behavior around the rotating blade of a three-

bladed upwind experimental turbine, under three operating

conditions; stall, optimal operation, and an overrunning state.Velocity data were acquired using the LDV method. The main

findings of this study being:

1. When stalling, the turbulent flow area in the wake is

larger than under optimal or overrunning operational

conditions. A remarkable stall is noted near the blade tip in the

case of small tip speed ratios. No stall is found near the blade

root, where lift is thought to be generated. Flow seems to be

separated from the middle-span region to the tip.

2. Radial distribution of the circulation strength around the

blade showed that a larger circulation was found near the

blade root in the case of stalling conditions.

VI. REFERENCES

[1] Timmer, W.A., Rooy, R.P.J.O.M. van, Wind tunnel results for a 25%thick wind turbine blade aerofoil, in Proc. European Union Wind

Energy Conference '93, pp. 416-419.

[2] Timmer, W.A., Rooy, R.P.J.O.M. van, DU 94-W-280, a thick aerofoilwith a divergent trailing edge, inProc. European. Union Wind EnergyConference 96, pp. 737-740.

[3] Ebert, P.R., Wood, D.H., The near wake of a model horizontal-axiswind turbine-II. General features of the three-dimensional flowfield,

Renewable Energy, Vol. 18, pp. 513-534, 1999.

[4] Ebert, P.R., Wood, D.H., The near wake of a model horizontal-axiswind turbine. Part 3: properties of the tip and hub vortices,Renewable

Energy, vol. 22, pp. 461-472, 2001.

[5] Ebert, P.R., Wood, D.H., The near wake of a model horizontal-axiswind turbine at runaway,Renewable Energy, Vol.25, pp.41-54, 2002.

[6] Vermeer, N.J, Bussel, G.J.W. van, Velocity measurements in the nearwake of a model rotor and comparison with theoretical results, in Proc.

1990 European Wind Energy Conf., pp.218-222.

[7] Vermeer, L.J., Timmer, W.A., Identification of operational aerofoilstate by means of velocity measurements, in Proc. 1999 European

Wind Energy Conf., pp. 168-171.

[8] Bhagwat, M.J., Leishman, J.G., Measurements of bound and wakecirculation on a helicopter rotor,Journal of AIRCRAFT, vol. 37, No. 2,

pp.227-234, 2000.

[9] Wood, D.H., Boersma, J., On the motion of multiple helical vortices,Journal of FLUID MECHANICS, vol. 447, pp.149-171, 2001.

e generation of the tip vortex. The circulation strength in th

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Experimental Studies on Velocity Field around Wind Turbine Rotor