BL3.138 WIND TUNNEL STUDY ON SURFACE PRESSURE MEASUREMENT ON A

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					BL3.138
       WIND TUNNEL STUDY ON SURFACE PRESSURE MEASUREMENT ON A BLADE OF HAWT
                                Takao Maeda, Yasunari Kamada, Hideyasu Fujioka,
                          Mie University, 1577 Kurimamachiya, Tsu, Mie 514-8507, Japan
                                           Email maeda@mach.mie-u.ac.jp
                                    TEL +81-59-231-9382 FAX +81-59-231-1572


Summary
Experimental results of the surface pressure distribution on a blade of a horizontal axis model rotor for the rotating
and non-rotating status are shown. The experiments were carried out in a wind tunnel with a 2.4m diameter

three-bladed rotor. The pressure distribution at r/R>0.7 showed good agreement between the rotating and the
non-rotating situations. The pressure distribution for the inboard section of r/R<0.5 showed a stall delay. The method
of determining the effective angle of attack for a rotating blade from LDV measurements is also discussed.


1.   INTRODUCTION
     Although in recent years there has been substantial progress in aerodynamic modeling and design of horizontal
axis wind turbines, the prediction of high aerodynamic loads has still failed. Many of the rotors found on current
generation HAWT systems are designed using a combination of a 2D aerofoil design and blade element momentum
theory. Consequently, rotors are frequently designed on the basis of a 2-dimensional verification of the characteristics
of the airfoil sections which experience larger loads than estimated. In addition, stalling has not occurred, even when
the angle of attack has exceeded that of the stall angle for the two dimensional flow. This phenomenon, known as stall
delay (Himmelskamp), has been investigated by measuring the pressure distribution on the surface of the blade [1-3].
However, the relationships between the pressure on the blade surface and the flow around the blade have not yet been
fully investigated and clarified. Based on this background, the authors undertook this study in order to evaluate
experimentally in a wind tunnel, the aerodynamic behavior of wind turbines. By a previous report (4), the velocity
distribution around rotating blade was measured by LDV and quantity of circulation distribution of blade was
discussed.   In this paper, pressure measurements on the blade surface were made with a focus on the relationship
between the pressure at each radial section and the angle of attack.


2. EXPERIMENTAL SETUP AND METHODS
2.1 Experimental setup
Figure1 shows the schematic drawing of the experimental setup. The wind tunnel is a close circuit type with diameter

of 3.6m. The maximum wind speed reaches 30m/s. The test three-bladed upwind HAWT was set in the test section,
1D downstream from the outlet of the wind tunnel. The wind turbine was equipped with a torque meter, a tachometer,
an encoder for detecting the rotating angle (azimuth angle), a speed-up gear box, and a variable-speed generator. The
rotor turned counterclockwise (viewed from upwind) at a variable rpm depending on the operating conditions. A
maximum of 600rpm could be set. The azimuth angle could be measured with a resolution of 0.4 degree. The wind
speed was measured by a pitot tube set in the test section. The goal of the experiment was to compare the surface
pressure distributions acquired in the cases of rotating and non-rotating operating conditions. For rotating conditions,
the experimental wind velocity was set at 7 m/s and the blade pitch angle was locked at -2°. The operating speed of

the rotor was changed from 100rpm to 450rpm (λ =1.8- 8.1) in accordance with discrete tip speed ratios obtained
during the performance test.          At the optimum tip speed ratio, λ = 5.20, the Reynolds number was about 2.1×105. In
the case of the non-rotating blade, the undisturbed wind speed was set at 15m/s and the blade pitch angle was
changed incrementally by 1 degree. The Reynolds number during the stationary blade test was 8.5×104.
     Figure 2 shows the pressure acquisition system. Pressure sensors were of a semiconductor type, having 32
measuring and standard ports and a rated pressure of ±7.65kPa. In order to avoid the error due to centrifugal forces,
pressure sensors were installed in the hub and the pressure membrane was parallel to the rotating surface.            The
pressures captured by the pressure sensors were entered as analog signals in the A/D board in the rotating system, and
after being converted into digital signals were transmitted through the slip rings in the main axis to a personal
computer for further processing. The sampling time for one channel was 0.1ms, and the sampling time of one group
composed of 32 channels was 3.2ms, which for the optimum tip speed ratio of 5.2 was converted to an azimuth angle
of 5.6 degrees.           The pressure data were averaged over 100 revolutions of the rotor.


                                          LDV Probe
  Wind tunnel outlet
                                                                                                     Nacelle
                 Wind turbine                                            Hub
                                          2000




                                                                           Pressure      A/D
                                                                           Sensors       Board        Slip Rings
   Φ3600



                  Φ2400




           1D                                                                                                   Rotor Axis
                                                               Reference Port

                                                                                                           PC
                                                                         Blade
                                   4500
                                                                                                  Pressure Taps

           Fig. 1 Experimental apparatus                                    Fig. 2    Measurement system



2.2 Test blade
     Figure 3 shows the test blade and the arrangement of the pressure taps. The test blade was a taper and twisted
blade. The chord length of blade tip was 85mm, the twist angle was 18.3°. At certain radial positions, starting from

the blade root, cross sections of the blade were composed of the following airfoil sections: DU91-W2-250、
DU93-W-210、NACA63-618 and NACA63-215. The pressure taps, with diameter 0.4mm, were mounted on the
blade surface. From the taps, copper tubes each with an inner diameter of 0.6mm, transmitted the surface pressures to
the pressure transducers in the hub of the wind turbine. A total of 31 pressure taps were mounted on the suction
surface and the pressure surface of each radial section under study. Pressure data were acquired successively at four
radial positions of 0.3R, 0.5R, 0.7R and 0.85R. respectively.
2.3 Fluid dynamics components acting on the rotating blade
      In this study the local aerodynamic properties were normalized and non-dimensionalized based on the pressure
coefficient. The pressure coefficient was defined as the ratio between the total pressure measured on the blade surface
and the dynamic pressure of the inflow wind, as indicated in eq.(1).

        p
 cp =                    (1)
        pd

      Figure 4 shows various components of fluid dynamic forces acting on blade cross section. Local aerodynamic

properties have been discussed focusing on the normal force coefficient, Cn, and tangential force coefficient, Ct. A
positive normal force is considered to be oriented from the pressure surface to the suction surface and perpendicular
to the chord line, while a tangential force is oriented from the leading edge towards the trailing edge. Forces acting on
the blade have been calculated from the pressures measured on the blade surface. Friction forces on the blade have
been ignored.


                   r/R=0.3           0.5    0.7    0.85

                                                                                           Normal force
                                                                                            Cn                Chord line
      Rotor Axis
                                                                               Lift              Drag
                                           Pressure Taps                              Cl            Cd


                                                                               α                    Tangential force
                                                                                                         Ct
  0                            0.5                     1.0                 Inflow direction

           Non-dimensional Chord Position x/c                      Fig.4 Definition of Normal Force and Tangential Force
        Fig. 3 Pressure taps on the measurement blade


3. RESULTS AND DISCUSSION
3.1 Pressure distribution
      In this section, the pressure distribution on the blade surface in cases of the non-rotating and rotating blades have
been compared, and details of the flow at sections where remarkable pressure changes were apparent, have been

discussed. Figure 5(a) shows the pressure distribution measured at radial position r/R=0.85. The vertical axis shows
the pressure coefficient, cp, while the horizontal axis shows the dimensionless chordwise position, x/c. Data acquired
for an angle of attack of 10 degrees for the non-rotating blade and for the optimum tip speed ratio of λ=5.19 in the
case of the rotating blade, have been compared. Figure 5(b) shows the relationship between the normal force

coefficient, Cn, and tangential force coefficient, Ct, for the rotating and non-rotating blades. Note that there is the
same tendency in the relationships between Cn and Ct in both cases. The same relationships were found for the radial
position r/R=0.7. Figure 6(a) shows the chordwise pressure distribution for a radial position r/R=0.5. Data acquired
for a tip speed ratio of λ=5.15 for the rotating blade shows good agreement with that for an angle of attack of α=12
degrees in the case of the non-rotating blade.
                                          Section Pressure Coefficient Cp
                                                                    -6
                                                                                                          Rotating (λ=5.19)
                                                                                                                  系列3
                                                                                                           Rotational(λ=5.19)




                                                                                                                                                                                     Normal Force Coefficient Cn
                                                                    -5                                    Non-rotating (α=10°)                                                                                     3.0
                                                                                                           Static (α=10°)
                                                                                                                  系列1
                                                                                                                                                                                                                                            Rotating
                                                                    -4                                                                                                                                             2.5
                                                                                                                                                                                                                                            Non-rotating       Separation
                                                                    -3                                                                                                                                             2.0
                                                                    -2                                                                                                                                             1.5
                                                                    -1                                                                                                                                             1.0
                                                                            0                                                                                                                                      0.5            α=6~13°

                                                                            1                                                                                                                                       0
                                                                                0         0.2       0.4       0.6           0.8           1                                                                         -0.4         -0.3     -0.2     -0.1      0             0.1
                                                                                                 Chord Station x/c                                                                                                                Tangential Force Coefficient Ct
                                                                                            (a) Pressure distribution                                                                                                       (b) Relationship between Cn and Ct

                                                                                                           Fig. 5 Measurement results at blade tip section of r/R=0.85


                                  Figure 6(b) shows the relationship between Cn and Ct referred to radial position r/R=0.5. Note that for non-rotating
                   angles of attack of 6-14 degrees, there is good agreement with the data obtained for the rotating blade. For Ct in the
                   range -0.3<Ct<-0.2, the rotating Cn increases with decreasing Ct, but the non-rotating Ct does not reach the value -0.3.
                   The flow is thought to be separated. In addition, when the flow is separated, normal force coefficients in the case of
                   the rotating blade become larger than in the case of the non-rotating blade. .

                                      -6
        Section Pressure Coefficient Cp




                                                                                                       Rotating (λ=5.15)
                                                                                                              Rotational
                                                                                                             系列1                                                    3.0
                                                                                                                                      Normal Force Coefficient Cn




                                      -5
                                                                                                       Non-rotating(α=12°)
                                                                                                              Static
                                                                                                             系列2
                                                                                                                                                                    2.5                                                             Rotating     Separation
                                      -4
                                                                                                                                                                                                                                    Non-rotating
                                                                                                                                                                    2.0
                                      -3
                                      -2                                                                                                                            1.5

                                      -1                                                                                                                            1.0

                                          0                                                                                                                         0.5                                                      α=6~14°
                                          1                                                                                                                                           0
                                                               0                    0.2         0.4       0.6         0.8         1                                                    -0.4                               -0.3       -0.2      -0.1        0         0.1

                                                                                              Chord Station x/c                                                                                                              Tangential Force Coefficient
                                                                                          (a) Pressure distribution                                                                                                       (b) Relationship between Cn and Ct
                                  -6                                                                                                                                             -6
                                                                                                       Rotating (λ=3.86)                                                                                                                      Rotating (λ=2.88)
Section Pressure Coefficient Cp




                                                                                                              Rotational
                                                                                                                                                   Section Pressure Coefficient Cp




                                                                                                             系列1
                                                                                                                                                                                                                                                     Rotational
                                                                                                                                                                                                                                                   系列3
                                  -5                                                                                                                                                 -5
                                                                                                        Non-rotating (α=15°)
                                                                                                              Static
                                                                                                             系列2                                                                                                                              Non-rotating (α=18°)
                                                                                                                                                                                                                                                     Static
                                                                                                                                                                                                                                                   系列2
                                  -4                                                                                                                                                 -4
                                  -3                                                                                                                                                 -3
                                  -2                                                                                                                                                 -2
                                  -1                                                                                                                                                 -1
                                          0                                                                                                                                              0
                                          1                                                                                                                                              1
                                                               0                    0.2  0.4       0.6      0.8                   1                                                                         0               0.2       0.4       0.6        0.8        1
                                                                                      Chord Station x/c                                                                                                                            Chord Station x/c
                                                                        (c) Pressure distribution with minimum Cp                                                                                                        (d) Pressure distribution on stall region

                                                                                                      Fig. 6 Measurement results at blade middle section of r/R=0.5
Figure 6(c) shows the pressure distribution at radial position r/R=0.5 in the case when the minimum pressure
coefficient was achieved. For the rotating blade, the data were acquired for a tip speed ratio λ=3.86, while for the
parked rotor the data were acquired for an angle of attack of 15 degrees. The minimum pressure coefficient achieved
on the suction surface for the rotating blade was -4.34, and for the non-rotating blade about -3.47.
Figure 6(d) shows the pressure distribution for the stall condition. Note that for both cases, the rotating and the
non-rotating blades, there is no significant suction on the suction surface near the leading edge and the flow has
completely stalled. The pressure coefficient is almost the same from leading edge to trailing edge; about -1.1 for the
rotating blade and -0.7 for the non-rotating blade.

                                  Figure 7(a) shows the pressure distribution for the radial section r/R=0.3 in the case of the minimum pressure
coefficient achieved for that section. The data corresponding to a tip speed ratio of λ=2.88 for the rotating blade and
for an attack angle of 17 degrees for non-rotating blade have been compared, with minimum pressure coefficients of

cp=-5.28 and -2.64, respectively.                                Figure 7(b) shows the pressure distribution for a radial section r/R=0.3 in the case
of the stall condition. Pressure coefficients in the case of the non-rotating blade are larger than for the rotating blade,
having an average of about -1 for all ranges from leading edge to trailing edge. In the case of the rotating blade, a

pressure coefficient of about -2.7 for chordwise sections 0≤x/c≤0.7 is evident.
     -6                                                         -6
                                  Rotating (λ=3.93)
                                                                                            Section Pressure Coefficient Cp




                                                                                                                                                             Rotating (λ=2.88)
Section Pressure Coefficient Cp




                                         Rotational
                                         系列1                                                                                                                       Rotational
                                                                                                                                                                   系列3
     -5                                                         -5
                                  Non-rotating (α=17°)
                                         系列3                                                                                                                 Non-rotating (α=21°)
                                                                                                                                                                    Static
                                                                                                                                                                   系列1
     -4                                                         -4
                                  -3                                                                                          -3
                                  -2                                                                                          -2
                                  -1                                                                                          -1
                                   0                                                                                          0
                                   1                                                                                          1
                                        0     0.2       0.4       0.6        0.8        1                                           0        0.2      0.4        0.6       0.8        1
                                                     Chord Station x/c                                                                             Chord Station x/c
                                               Fig. 7 (a) Pressure distribution                                                    Fig. 7 (b) Pressure distribution at on stall region

                                                                 Fig. 7 Measurement results at blade root section of r/R=0.3


                                  3.0
Normal Force Coefficient Cn




                                                                                                                                                                          Chord line
                                  2.5                                                                                                     Inflow direction
                                  2.0
                                                              Separation
                                                                                                                                    β+θ
                                  1.5
                                                                                                                                    αL                         u
                                  1.0                                                                                                                                    Rotor plane
                                              Rotating
                                  0.5         Non-rotating                                                                                   w+rω
                                   0                                                                                                        Fig.8 Definition of Angle of attack αL
                                    -0.4     -0.3      -0.2      -0.1       0       0.1
                                                    Tangential Force Coefficient Ct
                                            Fig. 7 (c) Relationship between Cn and Ct

Large differences in the pressure distributions are noted at section r/R=0.3 for rotating and non-rotating blades. The
relationships between Cn-Ct for radial section r/R=0.3 are shown in Figure 7(c). Significant disagreements between
rotational and stationery conditions are found. Although the blade tested was identical in both conditions, it can be

seen that the performances of the blades at section r/R=0.3 are quite different.


3.2 Calculation of effective angle of attack based on LDV velocity measurements
     In previous sections, the angle of attack for the rotating blade was acquired by comparing the pressure
distribution for the rotating and the non-rotating blades. In this section, the discussion is focused on the effective

angle of attack calculated from the velocity components measured by LDV and defined as αL.           Different authors
apply different methods in order to modify the inflow angle, φ, to an effective angle of attack in case of a
three-dimensional situation.       In this study , the inflow angle has been calculated based on the measured axial

velocity component, u, the peripheral velocity component, w, and radial distance of the measured section, r, as shown
in eq.2. The effective angle of attack, is then derived from the flow angle by eq.3.

                         æ u ö                                                                                     (2)
              j = tan -1 ç        ÷
                         è w + rw ø

              a L = f - (q + b )                                                                                   (3)
     From figure 5(a), it was found that, for a radial position r/R=0.85, there was very good agreement between the
pressure distribution for the rotating blade at λ=5.19 and the non-rotating blade having the angle of attack of 10
degrees. When the pressure distribution for the rotating and the non-rotating blades are identical, it is presumed that
the angle of attack should be the same. Therefore, in case of optimum operation, at radial section r/R=0.85, an angle
of attack of about 10 degrees was thought to be relevant.

     Figure 9 shows the relationships between the azimuth angle and the effective angle of attack, αL, calculated in
case of the optimum tip speed ratio and for the velocity components acquired at a radial position r/R=0.87. As the
azimuth angle Ψ=0 degrees corresponds to the moment when the blade passes the measuring plane, the laser fringe
volume was blocked and no data were acquired at that moment. Therefore, the flow angle near azimuth angle 0
degrees is not certain. The upwash in front of the blade (for the negative azimuth angle) and the downwash behind the

blade are clearly visible. At Ψ=0° (at blade passage) the upwash is suddenly turned into downwas as expected. From
the figure it is seen that an effective angle of attack of about 10 degrees is found to be for azimuth angle Ψ=-30~-25°
and Ψ=70~90°. For a three-bladed wind turbine, an azimuth angle of 90 degrees for the tested blade corresponds to
an azimuth angle of -30 degrees for the successive blade. Therefore, a range of effective   angle of attack of about 10

degrees can be achieved in the range of azimuth angle Ψ=-50~-25°. Using the same approach for radial positions
r/R=0.7, r/R=0.5, for optimum tip speed operation with λ=5.15, average effective angles of attack of about 10 degrees
and 13 degrees respectively, were found. Relationships between the azimuth angle and the effective angle of attack,

showed that in the case of radial position r/R=0.7, a constant flow angle of 10 degrees was found in the range of
Ψ=-37~-23°, and in the case of radial position r/R=0.5, a constant flow angle of 13 degrees was found to be in the
interval Ψ=-52~-26°. From the above discussion it can be concluded that the effective angle of attack calculated
from LDV measurements and referred to azimuth angle Ψ=-30° can be representative of the angle of attack of the
rotating blade.
     .
                                30                                                                                                 30




                                                                                                       Angle of attack αL [deg]
Angle of attack αL [deg]


                                20                                                                                                 20


                                10                                                                                                 10


                                             0                                                                                               0


                        -10-60           0     30      60 -30 90      120     -10
                                                                                  -60    -30      0     30     60     90       120
                                        Azimuth angle ψ [deg]                                   Azimuth angle ψ [deg]
                     Fig.9 Relation between Azimuth angle and αL at r/R=0.85 Fig.10 Relation between Azimuth angle and αL at r/R=0.3



                                                  Figure 10 shows the relationships between the azimuth angle and the angle of attack calculated from LDV

              measurements for radial position r/R=0.3. Based on the above mentioned finding, the angle of attack representative of
              the radial position under discussion, is presumed to be 17.5 degrees.

                                                  Figure 11 shows    presents a comparison of the pressure distribution at radial position r/R=0.3 for the rotating
              blade, with data acquired at λ=5.17, and for the non-rotating blade with data acquired at angle of attack of 17 degrees
              Though it is presumed that the angle of attack is the same in both cases, there is a disagreement in the pressure
              coefficients. Comparing three other sections under study, the flow near the blade root in the case of the rotating blade
              differs substantially from the non-rotating blade. A big difference is noted, especially in the pressure distribution
              curve on the suction surface.


                                                 -6
                                                                                                                                                  35
                                                                             Rotating (λ=5.17)
               Section Pressure Coefficient Cp




                                                                                    Rotational
                                                                             Rotational(λ=5.16)                                                        BEMT
                                                                                                                                                       BEM
                                                 -5                                                                                               30
                                                                             Non-rotating (α=17°)
                                                                                                                       Angle of attack αL [deg]




                                                                                                                                                       Measurement
                                                                                                                                                       Measurement
                                                 -4                                                                                               25
                                                 -3                                                                                               20
                                                 -2                                                                                               15
                                                 -1                                                                                               10
                                                 0                                                                                                 5
                                                 1                                                          0
                                                      0     0.2       0.4       0.6       0.8      1          0      0.2    0.4     0.6         0.8    1.0      1.2
                                                                  Chord Station x/c                                  Non-dimensional Radius r/R
                                                  Fig.11 Pressure distribution at r/R=0.3 on optimum λ Fig.12 Relation between angle of attack and radial position

              3.3 Angle of attack- BEM versus measured values
              The test blade used in this study was designed based on Blade Element Momentum theory (BEM). Driving the wind
              turbine at aligned flow in the wind tunnel, has premises that the conditions in applying BEM are met. In this section,
              the effective angle of attack, calculated from measured velocity components from LDV discussed in previous section
              is verified with the BEM method. Tip speed ratio entered in the BEM was that corresponding to optimum operation

              of the wind turbine, λ=5.20. Entering in the BEM the calculated inflow angle φ, the twist distribution and the blade
              pitch angle, the angle of attack was obtained. This is presented and compared with the measured values in Fig.12.
              Note that the effective angles of attack at radial sections 0.7R and 0.85R match perfectly in both cases of BEM and
measured values. Closer to the blade root, the measured values of the effective angle of attack exceed those calculated

from the BEM. At radial position r/R=0.5 a difference of about 1.5 degrees is observed, while at radial position
r/R=0.3 the difference stands at about 5.8 degrees. The main reason for the disagreement between the BEM and
measured values at blade root is that, in the BEM the rotor is modeled as a rotary disk and the complexity of the flow
resulting from the existence of the nacelle and the hub is not considered.


4. CONCLUSIONS
The pressure distribution on the blade surface was acquired by testing a wind turbine in rotating and non-rotating
conditions in a wind tunnel. Comparing the flow characteristics at every radial position, the following conclusions
can be drawn:
1.     The fact that the pressure distributions on the rotating and non-rotating blades are the same near the blade tip,

       namely at radial positions r/R=0.7 and r/R=0.85, and that it matches perfectly with the theory of the blade design,
       proves that it is effective to design the blade tip by making use of two-dimensional data.

2.     For the area near the blade root, r/R=0.3, a clear disagreement between the rotating and non-rotating blades was
       in evidence at a broad range of peripheral tip speed ratios, from λ=6.90 to λ=1.9. In addition, the Cn experienced
       at both cases, compared to Ct, proves that in the case of the rotating blade it exceeds substantially that of the
       non-rotating blade. The blade efficiency has greatly changed in both cases.
3.     Good agreement was found between the angles of attack estimated from the pressure distribution and the

       two-dimensional BEM method in the area near the blade tip, r/R=0.7 and r/R=0.85, during operation at an
       optimum tip speed ratio λ=5.2. A significant difference was found for the intermediate area and near the blade
       root, r/R=0.5 and r/R=0.3, respectively. The difference near the blade root is considered to be because of the
       stall delay due to the presence of the nacelle and hub, resulting in the complex flow exhibited near the blade
       root.
4.     It was found that the effective angle of attack estimated from the LDV velocity components, during operation at
       the optimum tip speed ratio λ=5.2 and azimuth angle -30 degrees, represents satisfactorily the effective angle of
       attack at every radial position.


References
1. A. Björck,S-E Thor, S.P.Fiddes, A.J.Brand, F.Rasmussen , DYNAMIC STALL AND 3D EFFECTS, Proc. of Euro.
     Union Wind Energy Conf. 94 1994; 768.
2. A. Björck, S-E Thor, Dynamic Stall and 3D Effects, Proc. of Euro. Union Wind Energy Conf. 96 1996; 683.
3. R.J.H. Paqynter, J.M.R. Graham, BLADE SURFACE PRESSURE MEASUREMENT ON AN OPERATING

     WIND TURBINE, Proc. of Euro. Union Wind Energy Conf. 96 1996; 687.
4. Takao MAEDA, Yasunari KAMADA, Yusaku SAKAI, Naoki TAKAHARA, Experimental study on flow around

     blades of horizontal axis wind turbine in wind tunnel, Trans. of JSME 2005; 71-701(B):171.