Roughness Sensitivity Considerations for Thick Rotor

Transcription

Roughness Sensitivity
Considerations for Thick Rotor
Blade Airfoils
R. P. J. O. M. van Rooij
e-mail: [email protected]
W. A. Timmer
e-mail: [email protected]
Delft University Wind Energy Research Institute
Faculty of Civil Engineering and Geosciences
Stevinweg 1, 2628CN, Delft, the Netherlands
1
In modern wind turbine blades, airfoils of more than 25% thickness can be found at
mid-span and inboard locations. At mid-span, aerodynamic requirements dominate, demanding a high lift-to-drag ratio, moderate to high lift and low roughness sensitivity.
Towards the root, structural requirements become more important. In this paper, the
performance for the airfoil series DU, FFA, S8xx, AH, Risø and NACA are reviewed. For
the 25% and 30% thick airfoils, the best performing airfoils can be recognized by a
restricted upper-surface thickness and an S-shaped lower surface for aft-loading. Differences in performance of the DU 91-W2-250 (25%), S814 (24%) and Risø-A1-24 (24%)
airfoils are small. For a 30% thickness, the DU 97-W-300 meets the requirements best.
Reduction of roughness sensitivity can be achieved both by proper design and by application of vortex generators on the upper surface of the airfoil. Maximum lift and lift-todrag ratio are, in general, enhanced for the rough configuration when vortex generators
are used. At inboard locations, 2-D wind tunnel tests do not represent the performance
characteristics well because the influence of rotation is not included. The RFOIL code is
believed to be capable of approximating the rotational effect. Results from this code
indicate that rotational effects dramatically reduce roughness sensitivity effects at inboard
locations. In particular, the change in lift characteristics in the case of leading edge
roughness for the 35% and 40% thick DU airfoils, DU 00-W-350 and DU 00-W-401,
respectively, is remarkable. As a result of the strong reduction of roughness sensitivity, the
design for inboard airfoils can primarily focus on high lift and structural demands.
关DOI: 10.1115/1.1624614兴
1. DU xx-W-xxx from the D
គ elft U
គ niversity of Technology 共first
xx represents the year and xxx gives the thickness to chord
ratio兲
2. FFA-W-xxx, Fគ lygtekniska Fគ orsoks A
គ nstalten 共The Aeronautical Research Institute of Sweden兲
3. S8xx design from D. Somers 共xx is serial number兲
4. AH xx-W-xxx, D. A
គ lthគ aus from Institute for Aerodynamics
and Gasdynamics of the University of Stuttgart, Germany
5. Risø-A1-xx from Risø National Laboratory, Denmark 共xx
gives thickness to chord ratio兲
6. and NACA 63-4xx, designed by the N
គ ational A
គ dvisory
Cគ ommittee for A
គ eronautics predecessor of NASA, USA 共xx
represents the thickness ratio兲
Introduction
With increasing rotor diameters, blade designers tend to use
thick airfoil sections in a large part of the blade. Thick airfoils
provide more structural stiffness and enable the blade designer to
reduce weight, giving a reduction of fatigue loads and costs.
Airfoils with a thickness of 25% are already located at midspan sections, and thickness increases towards the root, ending in
airfoils of about 40% relative thickness. This thickness causes
increased pressure gradients over the aft part of the airfoil upper
surface that, in conjunction with leading edge contamination, may
lead to early turbulent separation and a severe reduction of the
maximum lift coefficient.
Contamination of the blade leading edge cannot be avoided,
and field measurements have demonstrated large power reductions
due to that contamination. Pollution of the airfoil nose can even
cause multiple stall levels of the rotor 关1兴. Both rated power and
maximum power coefficient will be reduced by leading-edge contamination. The actual loss in power coefficient depends on the
design of the rotor blade, i.e., the chords at the mid-span and
inboard segment, the airfoil choice and the design criteria with
respect to the optimal tip speed ratio. Severe nose contamination
at mid-span 共0.42⬍r/R⬍0.57兲 and inboard 共0.2⬍r/R⬍0.42兲 can
lead to a 共calculated兲 loss in power coefficient for each blade
segment of approximately 4%, resulting in a total loss of 8% for
the complete rotor running at a tip speed ratio of 8.
The amount of contamination differs strongly and simulation in
the wind tunnel is not always possible. The effect of roughness on
the performance of several special purpose wind turbines airfoils
will be addressed in this paper. In particular, the following airfoil
series will be discussed:
Contributed by the Solar Energy Division of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS for publication in the ASME JOURNAL OF SOLAR ENERGY
ENGINEERING. Manuscript received by the ASME Solar Energy Division January
24, 2003; final revision, July 12, 2003. Associate Editor: D. Berg.
468 Õ Vol. 125, NOVEMBER 2003
Measurement data from various wind tunnel experiments as well
as RFOIL predictions will be examined in this investigation.
On inboard sections, the influence due to rotation cannot be
neglected and the resultant delay of turbulent separation can partly
compensate for the negative effect of an increased adverse pressure gradient on thick airfoils. The RFOIL code—which has been
adopted to predict rotational effects—will be used to investigate
the change in aerodynamic performance for the mid-span and inboard locations in the clean and fixed-transition conditions.
2
Design Approach
The design of airfoils in rotor blades is a trade-off between
airfoil performance 共including rotational effects兲 and structural requirements. In particular, the latter dominates the choice with respect to the airfoil thickness. Proper tools like RFOIL are essential
and some of the features of this code will be highlighted next.
2.1 The RFOIL Code. The RFOIL code is a modification
of the XFOIL panel code 关2兴 in the area of boundary layer mod-
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eling. The standard design features in XFOIL, like the mixed inverse options, are unchanged and are indispensable during the
design process.
Adjustments of the viscous flow calculations in RFOIL are performed in cooperation with the Energy Research Centre of the
Netherlands 共ECN兲 and the National Aerospace Laboratory NLR
to improve the post stall calculations and to study the influence of
rotation on the airfoil performance. The strong viscous/inviscid
interaction scheme in the program could be maintained because
the driving equations 共continuity, momentum and kinetic energy
equation兲 are of the same type. Modification of the 2-D boundary
layer equations focussed on improvement of the lift curve for
moderate Reynolds numbers between Re⫽1.0⫻106 and Re⫽3.0
⫻106 . Numerical stability has been enhanced and some of the
auxiliary closure relations for the turbulent boundary layer formulation are adjusted. The most important amelioration is in the closure relation for the shear lag coefficient, part of the Green’s lagentrainment equation. Now, deviation from the equilibrium flow is
coupled to the shape factor of the turbulent boundary layer. This
has led to a better prediction of the location of turbulent separation and results in improved lift values and pressure distributions
near and beyond the maximum lift coefficient in both the clean
and transition fixed conditions 关3兴.
The main feature of RFOIL is its capability of predicting airfoil
characteristics on rotating blades. The integral turbulent boundary
layer equations have been extended for radial flow based on the
Snel-Houwink model for blade rotation 关4,5兴. The three base
equations are formulated in cylindrical coordinates. The two momentum equations contain a number of ‘‘inertia force’’ type terms.
One equation includes the Coriolis force and the other holds the
centrifugal force. An additional cross flow velocity profile has to
be added and the closure relation for the radial dissipation coefficient for inner and outer turbulent boundary layer was modified to
enable the Green Lag formulation in radial flow. The 3-D velocity
profile of Johnston that was utilized has a triangular velocity
model and does not allow velocities in the negative direction;
therefore separation cannot be modeled properly. For attached
flow, however, the velocity model seems to be a good approximation 关6兴. Unfortunately, securing convergence restricted the radial
boundary layer terms to first and second orders 关7兴. No changes
are made to the laminar boundary layer equations because the
rotational influence was found to be small. A proper transition
model including cross flows was not available, and therefore the
original 2-D approach was adopted. It is, however, likely that
transition will occur sooner on a rotating blade than on a truly 2-D
blade. Also, other aspects which could influence the cross flow
over the blade have been investigated. Sensitivity analyses with
respect to tip speed ratio, radial gradients of the integral boundary
layer equations and a different wake model have been carried out
关8兴. In particular, these analyses showed that neglecting the radial
terms at low tip-speed ratios and high rotational frequencies could
lead to substantial errors. Most important, however, was the outcome that rotational effects scale with the local solidity—c/r—
alone, and for that reason, this term is used as input parameter for
the revised code. Comparison with pressure distributions and lift
curves from several wind turbine experiments demonstrated that
the lift values were over-predicted and adjustment to the physical
c/r value was necessary.
Fig. 1 Wind tunnel and rotating experiment compared with
calculated performance „solid line… both at the 30% section
Fig. 3 Comparison in the rotating configuration of measurements with calculations at rÕRÄ0.55 segment †8‡
Journal of Solar Energy Engineering
2.2 Effect of Rotation. The cross sections of rotating
blades are exposed to radial flow, and this may lead to deviation
from the 2-D airfoil characteristics. Various experiments on wind
turbine rotors have confirmed this and quite large amounts of
validation data are available. Some of the experiments were
carried out under IEA Annexes XIV and XVII ‘‘Field rotor
Aerodynamics.’’
To demonstrate the change in airfoil performance, the results
from the experimental wind turbine of the Delft University of
Technology 共TU Delft兲 will be used. The rotor blade comprises
the NLF共1兲-0416 airfoil and is equipped with pressure orifices
along the surface at several spanwise sections to determine the
flow characteristics; cn and ct. To obtain the associated inflow
angle, a flow direction probe is used close to the 30% inboard
section. In this investigation, the quasi-stationary results at anglesof-attack beyond 2-D stall are of interest, and therefore the data
obtained during strong wind fluctuations were removed. Unfortunately, some dynamic flow behavior still remains, and this causes
quite some scatter 共Fig. 1, 关9兴兲. In Fig. 1, the predicted characteristics from RFOIL and the experimental data from the field tests
have been plotted. A local solidity of 2/3 of the actual value for
this inboard segment is used in the RFOIL model. Two data points
close to the RFOIL cn⫺ct curve are selected to compare the associated experimental pressure distributions with the RFOIL results.
Fig. 2 Pressure distributions measured and calculated for the
rotating situation „ReÄ1.0Ã106 …
NOVEMBER 2003, Vol. 125 Õ 469
For these two data points, the corresponding lift values and pressure distributions compare fairly well with the measurements 共Fig.
2兲.
A similar comparison has been carried out for the NACA 44xx
airfoil series; the airfoils that are used in the Stork 5.0-WPX blade
during the FFA experiments in the 12⫻16 m low-speed wind tunnel of the Chinese Aerodynamic Research and Development Center 共CARDC兲关10兴. The lift performance and the pressure distributions for two selected data points at the mid-span segment
compare relatively well 共Figs. 3 and 4 关8兴兲. Here the angle-ofattack is derived from a blade element momentum method and
these values show quite some scatter. This difference in performance can not be observed in the cn⫺ct graph 共Fig. 3, left picture兲
and it seems obvious that the method used to determine the inflow
angle caused the errors.
Both validations indicate that RFOIL can match rotational effects quantitatively and could be of great help in approximating
the effects of rotation.
2.3 Design Considerations. The design of an airfoil for
mid-span or inboard sections is a mix between aerodynamic and
structural requirements. For the DU airfoils, the focus has been on
the aerodynamic performance; thickness distribution, location of
maximum thickness, airfoil tail restrictions and nose radius are
regarded as secondary requirements. The design of the 25% thick
airfoil was the start of a series of dedicated wind turbine airfoils
for several blade sections. This airfoil will be used to demonstrate
the main features of the design process for thick airfoils in
general.
Twenty-five percent thick airfoils are generally located at span
positions between approximately r/R⫽0.4 and 0.6. Stall or pitch
control of the wind turbine hardly affects the aerodynamic demands. The design targets are a mix of specific inboard and midspan requirements. This means that the airfoil characteristics
should demonstrate:
1. A high lift-to-drag ratio for a high power coefficient to maximize the energy yield of the turbine. The corresponding design lift should be moderate to high to restrict the blade
chord. This lift value should, however, have sufficient margin with respect to the maximum lift coefficient of approximately cl⫽0.2. 共A moderate design lift also guarantees that
the associated angle-of-attack is sufficiently high to make
running at the optimal design point feasible without a strong
increase of the blade twist.兲
2. A moderate to high maximum lift coefficient to reduce the
blade area 共and reduce standstill loads兲. Additional lift due
to rotational effects is generally small for the mid-span locations. Stall should be of the trailing edge type.
3. Low roughness sensitivity with respect to maximum cl /cd
and cl-max. This allows acceptable tolerance for production
and ensures that the airfoil leading edge is insensitive to
imperfections.
These demands lead to the design approach that limits the uppersurface thickness to reduce upper-surface velocities. The lower
adverse pressure gradient can now more easily withstand disturbances of e.g. roughness, and a strong reduction of the maximum
lift can be avoided. The maximum lift-to-drag ratio is largely determined by the location of the maximum thickness, in combination with the nose radius. Deviances in the nose radius could have
a great effect on the 共overall兲 performance of the airfoil and a
sensitivity analysis with respect to variation in the nose radius
should therefore always be part of the design process.
Limiting the upper-surface thickness means that the lowersurface thickness must increase to obtain the required thickness.
To achieve sufficient lift, an S-shape tail 共or an under camber tail兲
can be applied to produce aft-loading. The design of this shape is
a trade-off between increasing the lift and preventing turbulent
separation at low inflow angles. An aft location of the maximum
lower-surface thickness reduces the aft-loading capabilities. The
airfoil shapes in Fig. 5 demonstrate the result of this design approach. The depicted NACA airfoil was obtained by up scaling the
coordinates of an airfoil with smaller thickness, a common procedure for these airfoil series. The DU 91-W2-250 共t/c⫽0.25兲 and
S814 共t/c⫽0.24兲 airfoils are, however, designed with the objectives described above. The S814 airfoil has the smallest upper
surface thickness. In order to prevent early transition and turbulent
separation on the lower surface, the location of the maximum
thickness for the S814 airfoil was moved forward by stretching
the S-shape. The maximum airfoil thickness is at x/c⫽0.27 for the
S814, while it is at x/c⫽0.31 for the DU airfoil. The latter simplifies the structural design in view of, e.g., cross-section stiffness.
3
Experimental Set-up
3.1 Wind Tunnels. The 2-D wind tunnel measurements investigated in this paper are from different tunnel set-ups, and this
complicates a proper evaluation of the airfoil characteristics.
Fig. 4 Pressure distributions of two selected data points from
experiment and calculations at the rÕRÄ0.55 segment †8‡
Fig. 5 The shape of three approximately 25% thick airfoils
Fig. 6 Comparison in the clean configuration for DU 91-W2250 measured by Delft
The most important tunnel characteristic is the free-stream turbulence level, and that is similar for the test facilities at Delft 关11兴
and Stuttgart 关12兴. Both are low-turbulence tunnels having a turbulence level well below 0.1% during the tests. The experiments
carried out by Risø in the VELUX wind tunnel deal with a much
higher free-stream turbulence level of at least 1% 关13兴. A comparison with calculations could indicate the difference in airfoil
performance due to the tunnel characteristics. Figure 6 shows the
difference in measured and calculated characteristics for the DU
91-W2-250 airfoil. The Delft measurements give a lower maximum lift-to-drag ratio of 11% but an equal value for the maximum
lift.
A similar comparison of the measurements in the VELUX tunnel with calculations for the FFA-W3-241 airfoil shows less
agreement 共Fig. 7兲. The calculations give a 33% higher lift-to-drag
ratio and a 19% higher maximum lift coefficient (⌬cl⫽0.32) than
what was measured experimentally. The high free-stream turbulence level could cause early transition, resulting in increased drag
and a reduced maximum lift coefficient.
RFOIL sometimes tends to over predict the airfoil characteristics for a certain class of airfoils; therefore calculations have been
carried out for a similar design—FFA-W3-211 共relative thickness
Fig. 7 Comparison for FFA-W3-241 measured by Risø †13‡
Fig. 8 Comparison for FFA-W3-211 measured by FFA †14‡
21.1%兲—measured in the low-turbulence tunnel of the Royal Institute of Technology 共KTH兲 in Stockholm 关14兴 at a Reynolds
number of 1.8⫻106 共Fig. 8兲. Now, the RFOIL characteristics are
in better agreement with the measurements, resulting in a difference of the maximum lift-to-drag ratio of 9.5% and of 5% for the
maximum lift (⌬cl⫽0.08).
It appears that the high turbulence intensity in the VELUX
wind tunnel adversely affects the maximum lift-to-drag ratio and
maximum lift measured for airfoils in the clean configuration. The
effect of the high turbulence level is expected to be much smaller
in the configuration with transition fixed.
3.2 Roughness Simulations. Contamination at the leading
edge of the blade nose will, in general, lead to premature transition of the laminar boundary layer and result in early turbulent
separation. This is especially important when the pollution is on
the upper surface, because early separation there could affect the
maximum lift capacity.
To simulate this in the wind tunnel, transition is usually fixed
by putting a roughness strip or zigzag tape on one or both sides of
the airfoil. The critical roughness height Reynolds number based
on distributed roughness should be at least 600 according to the
method of Braslow 关15兴. The applied grit roughness causes only
transition and will hardly increase the momentum thickness of the
starting turbulent boundary layer; therefore the smallest increase
in drag and smallest reduction in maximum lift will be achieved.
A more standard approach is the application of zigzag tape with
a height of 0.35 mm 共Fig. 9, ttape /c⫽0.00058) having a critical
Reynolds number of approximately 200. This means that a much
lower tape height 共compared to grit roughness兲 would be sufficient to trigger transition. The very effective zigzag tape will now
lead to a significant increase in momentum thickness of the turbulent boundary layer and the influence on the airfoil characteristics will be more severe than it would be with grit roughness. This
complicates the comparison of some of the airfoil performance
data, even when transition is fixed at the same chord location.
The effect of roughness on the upper surface at x/c⫽5% could
give a fair indication of the ability of the airfoil to cope with a
thick turbulent boundary layer on the suction side. Experiments
with zigzag tape at the blade nose carried out on a NEG Micon
700/44 turbine showed that the drop in power level was similar to
previous measured power curves 关1兴. However, since the roughness configuration differs 共the tape on the wind turbine is fixed at
x/c⫽0.0% and applied between 0.55R blade span and the tip兲, it is
still difficult to make a proper comparison between a full-scale
wind turbine and wind tunnel experiments. Leading edge contamination on wind turbine blades also affects the flow on the pressure
side of the airfoil. In general, the favorable pressure distribution
will diminish the effect on the airfoil characteristics. Nose roughness on thick airfoils with huge aft-loading tails could, however,
lead to early turbulent separation at moderate angles-of-attack,
effectively decambering the airfoil. Decambering of the aft part of
the airfoil sometimes reduces the 共maximum兲 lift.
The importance of the critical roughness Reynolds number implies that the airflow velocity plays a role as well. Increasing the
undisturbed air velocity results in a reduction of the required
roughness height. Applying the same zigzag tape means that the
negative effect on the performance could be even larger and
evaluating the roughness sensitivity of different airfoils will become more difficult.
4 Overview of 2-D Airfoil Performance in Clean and
Rough Condition
Fig. 9 The shape of the ZZ-tape applied at the Delft measurements „thicknessÄ0.35 mm…
4.1 Test Results for 25% Thick Airfoil Class. The design
approach described in paragraph 2.3 is in contrast to the traditional way of deriving thick airfoils and the consequences can be
observed in the performance for the DU 91-W2-250, S814 and
NACA 63( 421) -425 airfoils. The Reynolds number in the experiNOVEMBER 2003, Vol. 125 Õ 471
ment was 3.0⫻106 , which is well below that of 100-m wind turbine rotors where Reynolds numbers close to Re⫽7.0⫻106 occur.
4.1.1 Clean Configuration. All measurements on the DU 91W2-250 关16,17兴, S814 关18兴 and NACA 63( 421) -425 关19兴 airfoils
were carried out in the Delft wind tunnel. In the clean configuration, only a small difference in performance between the airfoils
can be observed 共Fig. 10兲. The lift-to-drag ratio is almost the same
for these airfoils and the effect of any differences here on the
turbine power coefficient can be neglected. The associated design
lift coefficient is the largest for the DU airfoil, and this will lead to
slightly smaller blade chords on an optimally designed blade with
the same tip-speed ratio. The maximum lift coefficient of the special purpose designs—DU and S—is about 0.1 higher than the
NACA airfoil; this can be attributed, in part, to the aft-loading
design approach used for these airfoils. Other special purpose airfoils in this class are AH 93-W-257, FFA-W3-241 and Risø-A124. The shape and the performance of the AH 93-W-257 is quite
similar to the DU 91-W2-250 airfoil, but the maximum lift-todrag ratio is a bit lower. The FFA and Risø airfoils are approximately 24% thick, similar to the S814 airfoil. Both airfoils were
tested in the VELUX tunnel at a relatively low Reynolds number.
The shapes of the airfoils are plotted in Fig. 11. The main difference is at the nose, where the FFA airfoil is thicker and has a
much larger nose radius. This could cause a reduction of the pressure peak at the nose and consequently lead to a significant difference in the post stall region. The maximum lift, however, is not
affected and is the same for both airfoils. The larger lift-to-drag
ratio is in favor of the Risø airfoil 共Fig. 12兲. The lift-to-drag ratio
obtained from the experiments in the VELUX tunnel is relatively
low, compared to the measurements in the Delft and Stuttgart
wind tunnels, and could be caused by an increase in drag as a
result of the higher turbulence level.
lift curve, while this is no longer the case for the NACA airfoil
共Fig. 13兲. For the NACA airfoil, turbulent separation at the trailing
edge already starts near 5.0 deg angle-of-attack and moves gradually to the leading edge, resulting in a strong reduction of the lift
in stall. This poor performance for the fixed transition case is not
that surprising because the designs of the NACA airfoils were
never aimed at their use in wind turbine rotors. A fair comparison
between the DU 91-W2-250 and the S814 airfoils is difficult due
to differences in roughness type. The zigzag tape applied to the
DU airfoil is much thicker than the grit roughness on the S814
airfoil and, therefore, an additional increase of the momentum
thickness can be expected on the DU airfoil. This leads to early
turbulent separation and may account for the lower maximum
lift for the DU airfoil. Table 1 gives an overview of the
performance for the 25% thick airfoil class in the clean and rough
configuration.
In the Risø experiments 关20兴, transition was fixed on the upper
and lower surface at 5% and 10%, respectively. The loss in maximum lift compared with the clean configuration for the FFA-W3241 and Risø-A1-24 airfoil is 15% 共Compare Figs. 12 and 14兲.
The degradation in maximum lift-to-drag ratio is the smallest for
the Risø airfoil 共36%兲 and is 40% for the FFA airfoil. These reductions are small compared to the other airfoils and they are a bit
strange because a considerable roughness height was applied on
both sides of the airfoil. Keeping in mind the fact that the high
turbulence level attributes to a high drag in the clean configuration, the outcome is less surprising. Overall, the 24% thick RisøA1-24 performs very well despite the fact that the measurements
were carried out at a lower Reynolds number and at a much higher
turbulence level; the maximum lift coefficient in the rough configuration is equal to that of the DU airfoil.
4.1.2 Rough Configuration. In the fixed-transition configuration, the difference in airfoil performance becomes quite substantial and the special purpose airfoils achieve better performance. The DU and S814 airfoil clearly show a maximum in the
Fig. 12 Performance for two 24% thick airfoils „clean configuration, VELUX tunnel…
Fig. 10 Performance for 25% thick airfoils in the clean configuration „S814 is 24% thick…
Fig. 11 The airfoil shapes of two 24% thick airfoils
Fig. 13 Airfoil performance for three ‘‘25%’’ thick airfoils with
simulated roughness
Table 1 The effect of roughness on the airfoil performance for
„about… 25% thick airfoils
clean
Configuration
airfoil
Re⫽3.0e6
DU 91-W2-250共1兲
NACA 63421-425共1兲
AH 93-W-257共2兲
S814共1兲
Re⫽2.0e6
DU 91-W2-250共1兲
Re⫽1.6e6
FFA-W3-241共3兲
Risø-A1-24共3兲
Re⫽1.5e6
DU 91-W2-250共1兲
AH 93-W-257共2兲
‘‘rough’’
L/Dmax
cl-max
L/Dmax
cl-max
127.6
120
120.7
114.1
1.37
1.277
1.41
1.408
61.8
39
55
61.4
1.16
0.803*
1.04
1.357**
121.6
1.375
81
90.5
1.37
1.36
48.5
57
1.16***
1.17***
113.8
113
1.4
1.46
51.8
46.4
1.06
1.03
共1兲
Delft 关16,17兴.
Stuttgart 关12兴.
共3兲
Risø 关13,20兴.
*At kink in lift curve.
**Grit roughness at upper surface x/c⫽0.02 and lower surface x/c⫽0.10.
***ZZ-tape at x/c⫽0.05 upper surface and at x/c⫽0.10 on lower surface.
共2兲
4.2 Test Results for 30% Thick Airfoil Class. Airfoils of
30% thickness are located at the inner 40% of the blade. The
design considerations here are driven by both aerodynamic optimization and structural requirements. In particular, the location
of the maximum thickness and, to a lesser extent, the thickness of
the upper surface depend on the structural design requirements.
Figure 15 shows three dedicated wind turbine airfoils. The shape
of the DU airfoil represents the design approach described in section 2.3. The upper-surface thickness is slightly increased com-
pared to the DU 91-W2-250 in order to obtain a higher maximum
lift coefficient at the cost of increased roughness sensitivity. The
location of the maximum thickness and the lower-surface pressure
distribution are optimized such that the flow on the pressure side
stays attached for positive lift values.
4.2.1 Clean Configuration. The number of measurements
for airfoils with Reynolds number close to Re⫽3.0⫻106 is limited. Only characteristics of the DU 97-W-300 and AH 94-W-301
airfoils are available for a Reynolds number of 2.5⫻106 ; while
the characteristics of the FFA-W-301 airfoil are available at a
maximum Reynolds number of Re⫽1.6⫻106 共Figs. 16a and 16b兲.
The performance for the DU-97 and AH-94 airfoils is rather similar except for the maximum lift coefficient, which is 0.11 higher
for the DU airfoil. The aft-loading tail of the DU airfoil is probably responsible for this 共Fig. 15兲. The characteristics of the FFAW3-301 airfoil show a considerably lower maximum lift coefficient and a large reduction in lift-to-drag ratio caused by a
relatively high drag. The higher turbulence level at the VELUX
wind tunnel experiment may be responsible for this.
4.2.2 Comparison With Rough Configuration. The difference in Reynolds number, variation in roughness location and
wind tunnel set-up makes a fair comparison of the characteristics
difficult 共Table 2兲.
The performance of the DU 97-W-300, AH 94-W-301 and FFAW3-301 airfoils with fixed transition but at different Reynolds
numbers is demonstrated in Fig. 17. The huge differences in drag
polar are partly due to the variation in test set-up. A fair comparison for the degradation in maximum lift-to-drag ratio with the
clean configuration for the DU 97-W-300 and AH 94-W-301
airfoils—having the same roughness conditions—is still realistic
because the influence of the Reynolds number is expected to be
small. For the DU airfoil, the transition-fixed condition leads to a
Fig. 14 Measured data for two 24% thick airfoils with transition fixed at upper „xÕcÄ0.05… and lower surface „xÕcÄ0.1…
Fig. 15 The airfoil shapes of three 30% thick airfoils
Fig. 16 „a… Characteristics for three 30% thick airfoils in the
clean configuration „FFA data is at ReÄ1.6eÃ106 …. „b… The liftto-drag ratio and the lift for three 30% thick airfoils in the clean
configuration „FFA data is at ReÄ1.6eÃ106 ….
Table 2 2-D performance for 30% thick airfoils
clean
Configuration
airfoil
Re⫽3.0e6
DU 97-W-300共1兲
Re⫽2.5e6
DU 97-W-300共1兲
AH 94-W-301共2兲
AH 93-W-300共2兲
Re⫽2.0e6
DU 97-W-300共1兲
Re⫽1.6e6 **
NACA 63-430共3兲
FFA-W3-301共3兲
Re⫽1.5e6
AH 93-W-300共2兲
AH 94-W-301共2兲
‘‘rough’’
L/Dmax
cl-max
L/Dmax
cl-max
98.1
1.56
53.2
1.17
95.6
98.2
65.8
1.547
1.43
1.16
28.6
0.67*
91.7
1.546
48.4
1.09
50.6
43
1.05
1.31
21.7
29.8
0.53*
0.834
62.1
92.5
1.25
1.46
24.5
28.7
0.71*
0.69*
共1兲
Delft 关21兴.
Stuttgart 关12兴.
Risø 关13兴.
*At kink in lift curve.
**zz-tape at x/c⫽0.05 upper surface and at x/c⫽0.10 on lower surface.
共2兲
共3兲
degradation in L/D-max of 47% at Re⫽2.0⫻106 compared to a
degradation of 69% for the AH 94-W-301 at Re⫽1.5⫻106 .
The DU airfoil, and, to a lesser extent the FFA airfoil, show a
clear 共local兲 maximum lift coefficient, while this has 共almost兲 vanished for the AH airfoil. This trend is comparable with the lift
curves in Fig. 18 for the AH 93-W-300 and the NACA 63-430
airfoil.
The FFA-W3-301 airfoil has additional roughness on the pressure side, and this will affect the performance of the airfoil as
well. In the Delft tunnel, measurements have been performed on a
quite similar configuration 共Fig. 19兲, and the additional degradation of the airfoil performance appears to be negligible. Therefore,
the comparison between the DU, AH and FFA airfoil for this
rough configuration is regarded to be reasonably fair.
The DU 97-W-300 airfoil combines a high maximum lift in the
clean configuration with an acceptable degradation in lift when
the airfoil nose is contaminated. The drop in maximum lift is
considerable (⌬cl⫽.39 and ⌬cl⫽.46 at Re⫽3.0⫻106 and 2.0
⫻106 , respectively兲 but acceptable in view of the large airfoil
thickness.
4.3 Test Results for 35% and 40% Thick DU Airfoil. Extremely thick airfoils, with a relative thickness of 35% or more,
are located near the root of the rotor blade and structural requirements strongly affect the aerodynamic design of these airfoils.
The upper- and lower-surface thickness become more or less the
same and the maximum thickness location moves to a more aft
chord location. The thick suction side will now increase the airfoils’ susceptibility to roughness effects considerably, while the
lower surface has to be redesigned with respect to the aft-loading
tail, resulting in a reduction of the lift contribution. At the inboard
Fig. 17 Characteristics for 30% thick airfoils in the ‘‘rough’’
configuration „DU at ReÄ2.0Ã106 , FFA at ReÄ1.6Ã106 and AH
at ReÄ1.5Ã106 …
Fig. 19 The effect of additional transition „at xÕcÄ0.2… on the
lower surface of the DU 97-W-300 airfoil
Fig. 18 The lift performance two for 30% thick airfoils in the
‘‘rough’’ configuration „AH at ReÄ1.5Ã106 , NACA at ReÄ1.6
Ã106 …
Fig. 20 Measured and calculated lift performance for DU 00W-350 and DU 00-W-401 at ReÄ3.0Ã106 „clean and transition
fixed condition, solid lineÄcalculations…
locations, aerodynamic requirements are, in general, limited to a
high maximum lift coefficient at a relatively high angle-of-attack.
Another issue that cannot be neglected at these locations is the
influence of rotation and this should be incorporated in the design
phase.
4.3.1 Measurements. Figure 20 presents the performance of
the clean and transition-fixed configurations for two DU airfoils
with a maximum thickness-to-chord ratio of 35% and 40.1%.
These tests were performed in the Stuttgart wind tunnel. The 35%
thick airfoil shows a relatively high maximum lift coefficient in
the clean configuration, in contrast to the thicker DU 00-W-401
airfoil, where increased thickness at the suction side now leads to
early turbulent separation. Up to maximum lift and in the nearstall region, RFOIL calculations with free transition are in fair
agreement with the measurements. In the transition-fixed configuration, the differences between calculations and measurements are
larger, but the trend in lift performance is still rather good. Especially striking is the kink in the lift curve for the 40.1% thick
airfoil. In the measurements, transition has been fixed by application of bump tape 共h⫽0.5 mm兲 at 2% of chord on the upper
surface and at 10% of chord on the lower surface. The same
transition locations are adopted in the calculations. In this condition, the flow around the DU 00-W-401 airfoil is massively separated on both the lower and upper surfaces, and this effectively
results in a change in the airfoil camber. At angles-of-attack left of
the kink in the lift curve, the airfoil resembles a blunt streamline
body with a horizontal lower surface 共due to the separated flow on
the pressure side兲 and a cambered upper surface. This results in
positive lift values. On the right hand side of the kink, at e.g. 5.0
deg incidence, the opposite appears; here early separation prevents the suction side from contributing to the lift.
For the 2-D configuration, the huge airfoil thickness limits the
aerodynamic design opportunities with respect to roughness sensitivity. The kink in the lift curve can be avoided by removing aft
loading, but the downside is a strong reduction of the maximum
lift in the clean configuration. More important at these inboard
sections, however, is the influence of rotation. Delay of turbulent
separation will automatically reduce the huge separated areas near
the kink and will change the lift curve. This issue will be addressed in Section 6.
Table 3 2-D performance with vortex generators on the upper
surface
Configuration
airfoil
Re⫽2.0e6*
DU 91-W2-250共1兲
⬙
DU 97-W-300共2兲
Re⫽1.6e6**
Risø-A1-24共4兲
FFA-W3-241共4兲
⬙
FFA-W3-301共3兲
⬙
NACA 63-430共3兲
⬙
clean
position VG
共x/c兲
L/Dmax
cl-max
20%
30%
20%
30%
66.4
73.9
63.2
69.1
1.9
1.89
1.97
1.88
20%
20%
30%
20%
30%
20%
30%
47.1
45.6
59.1
38.7
39.2
31.9
45.6
1.805
1.543
1.54
1.636
1.36
1.3
1.37
‘‘rough’’
L/Dmax
cl-max
53.2
1.93
41.9
44.4
51.6
1.77
1.38
1.43
32.2
1.02
36.5
1.13
共1兲
Delft 关22,16兴.
Delft 关22兴.
Risø 关13兴.
共4兲
Risø 关20兴.
*zz-tape at x/c⫽0.05 upper surface.
**zz-tape at x/c⫽0.05 upper surface and at x/c⫽0.10 on lower surface.
共2兲
共3兲
the vg and mixing with the outer flow can no longer be performed.
The delay of turbulent separation leads to higher maximum lift
values and increased stall angles.
An overview of the airfoils and several vg options are listed in
Table 3. The maximum lift in the Delft experiments is close to 2.0
5 The Effect of Vortex Generators on the 2-D Airfoil
Performance
Thick airfoils in rotor blades are often equipped with vortex
generators 共vg兲, in particular on stall-controlled turbines 共Fig. 21兲.
For these turbines, additional lift at large inflow angles on the
inboard and mid-span locations is required to achieve a reasonably low rated wind speed. This performance should, however, be
maintained when the blade is polluted. Vortex generators extend
the lift curve by suppressing turbulent separation through mixing
of the outer flow with the boundary layer by means of small
vortices. Stall is reached when turbulent separation is in front of
Fig. 21 A sketch of the vortex generators applied at the wind
tunnel tests. Dimensions are in mm
Fig. 22 The effect of vortex generators „xÕcÄ0.2… on the performance of the DU 97-W-300 airfoil
Fig. 23 The effect of vortex generators „xÕcÄ0.2… on the performance of the Risø-A1-24 airfoil
and considerably higher than in the clean configuration 共compare
with Tables 1 and 2兲. The increase in maximum lift with vortex
generators at x/c⫽0.2 is more than 20% for the DU 97-W-300
airfoil 共Fig. 22兲. The downside, however, is an increase in drag
caused by the wake from the vanes of the vortex generator. In the
Delft data, the presented drag is an average value of this regular
wavy pattern. The higher drag reduces the maximum lift-to-drag
ratio by some 30%, and this will result in a reduction of the power
coefficient. The same trend can be observed from the Risø experiments at a slightly lower Reynolds number. In case of the RisøA1-24 airfoil with vortex generators at x/c⫽0.2, the percentages
for maximum lift increment and the reduction of lift-to-drag ratio
are 25% and 48%, respectively 共Fig. 23兲.
The increment in maximum lift and the reduction in lift-to-drag
ratio depend very much on the airfoil shape and the location of the
vortex generators. A more aft location of the vg will result in a
smaller increase of the maximum lift and a smaller reduction in
(L/D) max compared to the clean configuration. The fact that the
parameters 共maximum Cl and maximum L/D兲 are higher in the
Delft experiments could indicate that the tunnel performance characteristics have an effect on the airfoil performance with vortex
generators as well.
In the transition-fixed condition, the lift performance of the DU
97-W-300 is hardly affected 共Fig. 24兲. The thicker boundary layer
results in a higher drag and, therefore, a reduction of the lift-todrag ratio 共i.e. 16%兲. The performance with the transition fixed by
zig-zag tape at x/c⫽5% is even better than in the condition without vortex generators; both maximum lift and lift-to-drag ratio are
higher 共77% and 10%, respectively兲. For the Risø measurements,
fixing the transition 共on both upper and lower surfaces兲 always
leads to a considerably lower maximum lift value. This is a bit
surprising, but it may be that the height of the vg that was used is
Fig. 24 The effect of ZZ-tape „xÕcÄ0.05… on the performance
with vortex generators at xÕcÄ0.2
Fig. 25 The effect of ZZ-tape on the airfoil performance with
vortex generators
not sufficient under these test conditions and mixing with the
outer flow was not fully successful. The 30% thick FFA-W3-301
airfoil with vg’s at x/c⫽0.3 gives a reduction in maximum lift of
⌬cl⫽0.34 and a lift-to-drag ratio of only 32.2 共Fig. 25兲.
Except for the Risø-A1-24 airfoil, all the airfoils perform better
共for both maximum lift and maximum lift-to-drag ratio兲 in the
‘‘rough’’ configurations with vortex generators attached than without them.
6
Performance Due to Rotation
The influence of rotation depends on the spanwise location and
can be modeled in RFOIL by variation of the local solidity factor.
Fig. 26 The influence of the spanwise position for the clean
configuration „ReÄ3.0Ã106 , solid linesÄcalculations…
Fig. 27 The influence of rotation for the configuration with
fixed transition at ReÄ3.0Ã106 „solid linesÄcalculations…
Fig. 28 Calculated difference in 3-D lift between clean and
fixed-transition for 2 airfoils at the inboard blade position for
ReÄ3.0Ã106
A blade geometry for a 750 kW to 1.0 MW rotor with geometrical
c/r of 0.29 共representing an inboard location兲 and c/r of 0.12 共representing a mid-span location兲 is used to investigate the sensitivity
with respect to the span position. 共For larger rotors the local solidities tend to be slightly smaller.兲 Figure 26 demonstrates the
effect of rotation on the lift curve for the 30% thick DU 97-W-300
and 35% thick DU 00-W-350 airfoils in the clean configuration.
The increase in maximum lift beyond 2-D stall for the mid-span
section is small and is rather similar for both airfoils. At the inboard location, the increase in maximum lift coefficient is significant, resulting in an additional lift coefficient due to rotation for
the 30% and 35% thick airfoils of 0.45 and 0.75, respectively.
This is an increase in lift of 28% and 54%, respectively. The same
trends in lift performance can be observed near the blade root for
the configuration with fixed transition 共Fig. 27兲. The change in lift
curve for the thick 35% airfoil at the inboard section is remarkable. Turbulent separation on lower and upper surface is strongly
delayed, and the resulting lift curve has a more familiar shape.
This longer attached flow also results in a large reduction of the
drag for inflow angles beyond 2-D stall 共not shown兲. At the inboard section with a geometrical c/r of 0.29, the degradation in lift
performance in the ‘‘rough’’ condition is strongly reduced due to
blade rotation. The difference in lift curve between the clean and
transition-fixed condition for the DU 97-W-300 airfoil has almost
disappeared and the maximum lift of the airfoil becomes nearly
insensitive to contamination at the nose 共Fig. 28兲. A comparable
behavior was observed in Fig. 24, demonstrating that suppression
of trailing edge separation due to rotation is similar to suppression
of trailing edge separation with vortex generators located at x/c
⫽0.2. The delay of turbulent separation due to rotation also results
in a considerable lift increase for the DU 00-W-350 airfoil.
In the transition-fixed condition, the lift is about ⌬cl⫽0.25
smaller over almost the entire range of inflow angles compared to
the clean condition but the drop is much less than in the
2-D configuration.
As a result of this very strong reduction of roughness sensitivity
due to rotation effects, the design of inboard airfoils can focus
on high lift and structural demands only. An increased thickness
of the upper surface can be applied, simplifying the blade shape
near the root and possibly leading to a reduction in stress forces.
7
Conclusions
This paper compares the performance for the DU, FFA S8xx,
AH, Risø, and NACA airfoil series for both clean and transitionfixed configurations. The impact of vortex generators on the airfoil performance is examined, and the RFOIL code is used to
estimate the influence of rotational effects on the airfoil performance. The following conclusions have been reached:
• To meet the aerodynamic requirements for a mid-span blade
section, the airfoil shape tends to a relatively small uppersurface thickness and an aft-loading lower surface tail.
• The measurements indicate that, in the 25% thick airfoil
class, the special purpose airfoils: DU 91-W2-250, S814 and
Risø-A1-24 have the best overall performance and lowest
roughness sensitivity.
These airfoils have:
- High lift-to-drag ratios 共127.6 for DU 91-W2-250兲 for a
high power coefficient.
- High maximum lift, close to cl⫽1.4 共necessary for a relatively high power output兲.
- A limited reduction in maximum lift due to leading edge
pollution (⌬cl⫽⫺0.05⇔⫺0.21 depending on the kind of
roughness兲
• There are various measurements of 30% thick airfoils, but a
proper comparison is complicated by differences in tunnel
characteristics, Reynolds number and applied roughness. The
DU 97-W-300 seems to offer the best compromise with respect to the aerodynamic and structural requirements.
• Structural demands for 35% and 40% thick airfoils lead to
increased upper surface thickness. Low roughness sensitivity
in the 2-D configuration becomes impossible for these airfoils.
• Vortex generators on the airfoil reduce the sensitivity to
roughness and lead to better overall rough performance of the
airfoil. Application of vortex generators is favorable, in particular, on stall-regulated turbines on inboard and mid-span
sections.
• At inboard locations, the influence of rotation may change the
airfoil characteristics strongly, compared to 2-D performance,
and should therefore be included in the design process. The
RFOIL code is believed to serve as a valuable tool to investigate this effect.
• Rotational effects may reduce roughness sensitivity, depending on spanwise locations. The requirements for roughness
sensitivity can be alleviated and structural demands become
the primary design driver.
Acknowledgment
We thank GE-Wind Energy for providing some of the airfoil
test data and Mr. Dan Somers for providing the S814 characteristics.
References
关1兴 Corten, G. P., and Veldkamp, H. F., 2001, ‘‘Insects Cause Double Stall,’’ 2001
European
Wind
Energy
Conference,
Copenhagen,
Denmark,
pp. 470– 474.
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Technology, Boston, MA, USA.
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Delft University of Technology, Delft, the Netherlands. http://
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P. J. O. M., and Timmer, W. A., 2000, ‘‘Voorstudie Radiale Stroming in Overtrek’’ 共Preparatory study on the effects of radial flow in stall兲, Report ECN-C00-002 (in Dutch), Energy Research Center of the Netherlands, Petten, the
Netherlands.
关9兴 Rooij, R. P. J. O. M. van, 2001, ‘‘Experiments on Wind Turbine Airfoils and
Wind Turbine Rotors,’’ International Wind Tunnel Symposium—Memorial
Ceremony of Mie University Satellite Venture Business Laboratory, Tsu, Japan.
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Wind. Eng. Ind. Aerodyn., 39, pp. 105–118, Amsterdam, the Netherlands.
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University Wind Turbine Dedicated Airfoils,’’ 41st Aerospace Sciences Meeting, Paper no. AIAA-2003-0352, Reno, NV, USA, pp. 22–31.
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Wind Tunnel of the Institut für Aerodynamik und Gasdynamik der Universität
Stuttgart,’’ ISBN 3-528-03820-9, Stuttgart, Germany.
关13兴 Fuglsang, P., Antoniou, I., Dahl, K., and Madsen, H., 1998, ‘‘Wind Tunnel
Tests of the FFA-W3-241, FFA-W3-301 and NACA 63-430 Airfoils,’’ Risø-R1041(EN), Roskilde, Denmark.
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IEA 7th Symposium on Aerodynamics of Wind Turbines, TU-Denmark, Copenhagen, Denmark.
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of Critical Height of Distributed Roughness Particles for Boundary-Layer
Transition at Mach Numbers From 0 to 5,’’ NACA technical note 4363, USA.
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a 25% Thick Wind Turbine Blade Airfoil,’’ Proceedings European Community
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关17兴 Timmer, W. A., 1993, ‘‘The Design and Testing of Airfoil DU 91-W2250,’’Proceedings of the 6th IEA Symposium on the Aerodynamics of Wind
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Root Airfoil,’’ Proceedings of ASME 1995, and SED-Vol. 16, Wind Energy—
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pp. 243–246.
关20兴 Fuglsang, P., and Bak, C., 1999, ‘‘Wind Tunnel Tests of the Risø-A1-18, RisøA1-21 and Risø-A1-24 Airfoils,’’ Report Risø-R-1112(EN), Risø National
Laboratory, Roskilde, Denmark.
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the DU 97-W-300 Airfoil兲, Report IW-98003R (In Dutch), Delft University of
Technology, Delft, the Netherlands.
关22兴 Timmer, W. A., van Rooij, R. P. J. O. M., and Bruining, A., 1999, ‘‘Measured
Section Performance of Rotating Blades as Input to the Design of Inboard
Airfoils,’’ Proceedings European Wind Energy Conference, Nice, France, pp.
679– 682.

Roughness Sensitivity Considerations for Thick Rotor

Transcription

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