INTERNATIONAL JOURNAL OF ENERGY RESEARCH
Int. J. Energy Res. 2012; 36:829 844
Published online 22 March 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.2912
REVIEW PAPER
Blade sections for wind turbine and tidal current turbine
applications current status and future challenges
*,
M. Rafiuddin Ahmed
Division of Mechanical Engineering, The University of the South Pacific, Laucala Campus, Suva, Fiji
SUMMARY
The designers of horizontal axis wind turbines and tidal current turbines are increasingly focusing their attention on the design
of blade sections appropriate for specific applications. In modern large wind turbines, the blade tip is designed using a thin
airfoil for high lift : drag ratio, and the root region is designed using a thick version of the same airfoil for structural support.
A high lift to drag ratio is a generally accepted requirement; however, although a reduction in the drag coefficient directly
contributes to a higher aerodynamic efficiency, an increase in the lift coefficient does not have a significant contribution to
the torque, as it is only a small component of lift that increases the tangential force while the larger component increases the
thrust, necessitating an optimization. An airfoil with a curvature close to the leading edge that contributes more to the rotation
will be a good choice; however, it is still a challenge to design such an airfoil. The design of special purpose airfoils started with
LS and S•RI airfoils, which are followed by many series of airfoils, including the new CAS airfoils. After nearly two decades of
extensive research, a number of airfoils are available; however, majority of them are thick airfoils as the strength is still a major
concern. Many of these still show deterioration in performance with leading edge contamination. Similarly, a change in the
freestream turbulence level affects the performance of the blade. A number of active and passive flow control devices have been
proposed and tested to improve the performance of blades/turbines. The structural requirements for tidal current turbines tend to
lead to thicker sections, particularly near the root, which will cause a higher drag coefficient. A bigger challenge in the design of
blades for these turbines is to avoid cavitation (which also leads to thicker sections) and still obtain an acceptably high lift
coefficient. Another challenge for the designers is to design blades that give consistent output at varying flow conditions with
a simple control system. The performance of a rotating blade may be significantly different from a non-rotating blade, which
requires that the design process should continue till the blade is tested under different operating conditions. Copyright ©
2012 John Wiley & Sons, Ltd.
KEY WORDS
wind turbine; tidal current turbine; blade section; lift; drag
Correspondence
*M. Rafiuddin Ahmed, Division of Mechanical Engineering, The University of the South Pacific, Laucala Campus, Suva, Fiji.

E-mail: [e-mail]
Received 8 August 2011; Revised 20 December 2011; Accepted 7 February 2012
1. lNTRODUCTlON at varying Reynolds numbers (Re) and poor performance due
to the roughness effect resulting from leading edge (L•) con-
Research efforts directed at maximizing the power output of tamination for these profiles. NACA airfoils are suitable mainly
horizontal axis wind turbines (WTs) and tidal current turbines for high Re and relatively small angles of attack (a) [2,3]. The
(TCTs) have increased significantly during the recent years magnitude, direction and turbulence levels of the atmospheric
providing impetus to extensive research on blades and blade wind are known to vary significantly with time, which
sections appropriate for specific applications. Advances in adversely affects the performance of WTs if NACA airfoils
the development of WTs and TCTs will have immense bene- are employed for the blades. Figure 1 shows the variation of
fits in providing solutions to the global energy requirements. the section lift coefficient (Cl) with a at different Re for
The rotor blade is one of the most important components of NACA23012 airfoil [2]. It can be seen that the Cl drops sharply
the WTs and TCTs, which is the primary energy conversion and significantly as a increases beyond the stall angle for differ-
device. For the turbine blade design, the selection of airfoils ent Re. Suchanabrupt dropinCl will significantly reduce the
for different sections and the distribution of chords and twists output of a WT or a TCT. The trend for special-purpose blade
are pivotal [1]. Most of the NACA airfoils are not appropriate sections started in the early 1980s with the design of LS and
for WTs and TCTs because of the poor stall characteristics, low Solar •nergy Research Institute (S•RI) airfoils after the experi-
structural efficiency near the root, inconsistent performance ence gained from employing aviation class NACA airfoils
Copyright © 2012 John Wiley & Sons, Ltd. 829
M. R. Ahmed Blade sections for wind and tidal current turbines status and future
Figure 2. Different a versus Cl behaviours.
consuming. Researchers are, at the moment, testing a number
of active and passive flow control devices to improve the
performance of the turbines and to control the load on the
rotor. The present paper discusses the different blade sections
that were used in wind turbines starting from the earliest
special-purpose airfoils that were designed by National
Renewable •nergy Laboratory (NR•L). The main perfor-
Figure 1. The section lift coefficient of NACA23012 airfoil at differ-
mance characteristics of the popular blade profiles used in
ent angles of attack. Ë%  Re = 3 106; Ä„%  Re = 6 106; Ę%  8.8
WTs and TCTs are discussed; many of these are also tested
106;  Re = 6 106 (standard roughness) [2].
for their performance under different operating conditions.
Ä„romising flow control techniques and devices for impro-
vement of turbine performance are discussed in brief. The
highlighted the shortcomings of these airfoils for horizontal
performance characteristics of rotating turbine blades are com-
axis wind turbines (HAWTs) [4]. Stall-controlled HAWTs
pared with those of non-rotating turbine blades, and finally, the
produced excessive power in stronger winds, which caused
future challenges for blade designers are briefly discussed.
generator damage. The need to gain a better understanding
of the airfoil performance near stall was felt, as some stall-
controlled turbines were operating with some part of the
blade in deep stall a lot of time; the predicted loads were less 2. LlFT AND DRAG
than the measured loads, and the L• roughness was affecting
the turbine performance. The main focus in the design of blade sections has been to
Figure 2 shows the a versus Cl behavior of traditional maximize lift : drag ratio (L/D) mainly by increasing Cl. It
airfoils and some of the presently used airfoils that have a is still a generally accepted requirement. However, there are
gradual upstream movement of the point of separation with different preferences among blade aerodynamicists regarding
increasing angle of attack. A number of special-purpose air- the maximum Cl depending on the type of control the stall-
foils for WT applications have a gradual upstream movement controlled turbines restrict Cl,max to serve two purposes: (i) to
of the location of separation from the trailing edge so that Cl reduce the peak power generated; and (ii) to keep the thrust
does not drop sharply and the coefficient of drag (Cd) does on the system small. One of the earlier airfoils that were
not rise sharply with an increase in the angle of attack. The designed (in 1986) with a restricted Cl,max was S809 [6]. This
third type of behavior, in which the Cl value essentially airfoil was designed with another related objective of
remains constant over a wide range of angles of attack, is also reduced effect of L• roughness so that the value of Cl,max
shown. With a growing demand for a reduction in energy does not reduce due to L• contamination. On the other
costs, the designers are now forced to think of simple passive hand, the pitch-controlled turbines require a high Cl,max.
control techniques. An airfoil of this type of behavior will The pitch-controlled system adjusts a such that maximum
definitely contribute to a reduction in the cost, as its perfor- L/D is obtained for all the wind speeds up to the maximum
mance will not deteriorate with a change in the flow direction, power. A high value of maximum lift coefficient gives higher
and it will help achieve the challenge of  lean design [5], aerodynamic efficiency as long as the blade structure is satis-
which is especially important for offshore WTs and TCTs factory. One of the recent designs [7] focused on increasing
for which maintenance and repairs are expensive and time the tangential force coefficient (Ct) rather than Cl, as an
830 Int. J. Energy Res. 2012; 36:829 844 © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Blade sections for wind and tidal current turbines status and future M. R. Ahmed
increase in Cl contributes more to the thrust than to the rota-
tion. Having Ct as the design objective can allow for more
attention on insensitivity to L• roughness rather than on high
lift : drag ratio caused by low Cd. In most cases, it is desirable
that the design-a region is close to Cl,max because this enables
low rotor solidity and/or low rotational speed [7].
3. LOCATlON OF TRANSlTlON
POlNT
It is known that the location of transition of the boundary
layer (BL), especially on the upper surface, is a very impor-
tant factor that significantly influences the performance of
the airfoil. The distance from the L• to the point where
transition occurs, xtr, reduces with (i) increasing Re; and
(ii) increasing turbulence intensity (Tu). Figure 3 shows
the location of transition on the upper surface of SG6043
airfoil at a lower Re and 5% Tu. When Re is increased, a re-
duction in xtr can clearly be seen from Figure 4. A reduction
Figure 4. Location of transition on the upper surface of SG6043
in xtr also can be seen from Figure 5 when Tu is increased to
airfoil at high Re and 5% Tu.
10%. As xtr reduces, the region of laminar boundary layer
reduces, and that of turbulent BL increases; this results in
an increase in the skin friction drag. For higher a, the shift effect of varying the freestream turbulence intensity from
in the location of transition is normally less as the transition 0.25% to 9% for NACA0015 airfoil and reported an increase
anyway occurs close to the L•. However, for higher a, a in the peak lift coefficient because of delayed flow separation
reduction in xtr may shift the location of separation at higher a. Devinant et al. [9] varied the turbulence level
towards the T•, resulting in a reduction in the wake thick- from 0.5% to 16% and studied its effect on NACA
ness and a lower momentum loss. 654-421; they found that the flow was separating at higher
The freestream turbulence levels of the atmospheric wind angles of attack when the turbulence level was increased.
at heights at which wind turbines are normally installed are Most of the works on such studies are performed on NACA
generally higher compared with the levels achieved in stan- airfoils. Recently, Maeda et al. [10] studied the effects of
dard wind tunnels. Studies performed in the past have turbulence intensity on the static and dynamic characteristics
explored the effect of Tu on the airfoil characteristics of DU93-W-210 airfoil at Re = 350 000 and two turbulence
[8 10] below one million Re. Hoffmann [8] studied the levels of 0.15% and 11%. They found that the flow separa-
tion is delayed at higher turbulence levels, and the stall angle
increased.
Also, when the blade L• gathers dust, dirt, and so on, it
causes early transition of the BL. This roughness effect is
known to reduce the Cl,max [2]. Figure 6 shows the effect of
roughness on Cl,max at different Re. It can clearly be seen that
increasing roughness reduces Cl,max. Airfoils with a relative
thickness of 25% or more may suffer a significant deteriora-
tion in performance because of roughness effects. L• rough-
ness adds thickness to the BL and shifts the location of
transition very close to the L•. The resulting thicker BL leads
to increased drag, a reduction in the effective camber and an
earlier stall because of the weakening of the BL. The associ-
ated reduction in Cl,max depends on the airfoil geometry and
the degree of contamination of the airfoil L•. A study on the
effects of airfoil thickness and Cl,max on roughness sensitivity
concluded that the roughness sensitivity is directly propor-
tional to the airfoil thickness [11]. Some turbine blade
designers choose a profile and a design a, which has the tran-
sition point close to the L• so that for any reason, if the BL
transitions earlier, the performance of the blade is not much
Figure 3. Location of transition on the upper surface of SG6043 different from the case with free transition. The S•RI series
airfoil at low Re and 5% Tu. of airfoils (discussed in the next section) were designed to
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M. R. Ahmed Blade sections for wind and tidal current turbines status and future
families were designed for different rotor sizes [13] using
the •ppler code [14,15]. By minimizing the energy losses
because of roughness effects, by optimizing airfoils perfor-
mance characteristics for appropriate Re and thickness and
by limiting Cl,max, an annual energy improvement of up to
35% was estimated with S•RI series of airfoils. For small
to medium blade lengths, 11 15% thick airfoils were
designed, whereas for large lengths, 16 21% thick airfoils
were designed. Greater thicknesses were intended to provide
greater blade flap stiffness for tower clearance, lower blade
weight important for large machines, and to accommodate
aerodynamic overspeed control devices for stall-regulated
machines. Two of the airfoil families were thin and were suit-
able for downwind rotors of up to 10 m blade lengths and 20
to 100 kW power. Figure 7 shows the thin airfoil family
designed for pitch-controlled blades that did not have a
restrained Cl,max. The tip-region airfoil (S803) is 11.5%
thick, whereas the root-region airfoil (S804) is 18% thick.
The maximum Cl was found to be 1.5 at the respective Re.
For the stall-controlled blades of this size range, four thin air-
Figure 5. Location of transition on the upper surface of SG6043
foils, S806A, S805A, S807 and S808, were designed. The
airfoil at low Re and 10% Tu.
Cl,max value for the tip-region airfoil, S806A, was only 1.1.
Another thick airfoils family, S820, S819 and S821, was
have the transition point close to the L• just prior to reaching designed, which had similar characteristics; the tip-region
Cl,max. It is interesting to note that below a Re of about 105, blade maximum thickness was 16%, which was to accom-
the roughened or turbulated airfoils perform better because modate speed-control mechanism for stall-regulated turbines
of the transfer of energy from outside the BL to the low and has a greater stiffness at the cost of slightly higher drag.
energy region inside the BL. However, above this Re, the For blades of 10 15 m length and with ratings of
BL tends to become weaker for roughened or turbulated air- 100 400 kW, a thick-airfoil family consisting of S826,
foils and L/D drops [12]. S825, S814 and S815 was designed. The geometries and
the design specifications are shown in Figure 8. The tip-
region airfoil was 14% thick and was found to have a
4. SERl AlRFOlLS Cl,max of 1.6. The root region (40% radius) airfoil, S814, is
24% thick and was designed with two primary objectives:
The development of special-purpose airfoils for HAWTs (i) to achieve a Cl,max of at least 1.30 for Re = 1.5 106,
began in 1984 as a joint effort between the NR•L, formerly which should not reduce with transition fixed near the L•
the S•RI, and Airfoils Incorporated. In all, nine airfoil on both the surface; and (ii) to obtain low profile drag
Figure 6. Effect of surface roughness on the maximum lift coefficient for NACA 63(420)-422 at different Re [2].
832 Int. J. Energy Res. 2012; 36:829 844 © 2012 John Wiley & Sons, Ltd.
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Blade sections for wind and tidal current turbines status and future M. R. Ahmed
Figure 7. Thin NREL airfoil family for medium blades (high tip Cl,max) [13].
coefficients over the range of Cl from 0.6 to 1.2 for the same the L• with increasing (or decreasing) Cl. This feature results
Re. With these objectives, the drag polar was plotted; the Cd in a L• that produces a suction peak at higher Cl values,
for the Cl range of 0.5 to 1.2 increased only slightly because which ensures that transition on the upper surface occurs
of the elimination of significant laminar separation bubbles very close to the L•, and hence, the value of Cl,max is insen-
on the upper surface that are found on many laminar-flow sitive to roughness at the L• [16]. Based on these aspects, the
airfoils. The drag increases very rapidly outside the low-drag pressure distributions along the polar were deduced. The
range because the BL transition point moves quickly toward desired pressure distribution for the higher Cl angle is shown
Figure 8. Thick NREL airfoil family for large blades (high tip Cl,max) [13].
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M. R. Ahmed Blade sections for wind and tidal current turbines status and future
in Figure 9. As can be seen from the figure, the suction peak trip, Cl,max dropped from 1.37 to 1.16, as shown in Figure 11.
occurs just aft of the L•, which indicates that the transition However, after the stall, the Cl value rose more sharply com-
point moves very quickly towards the L• with increasing pared with the case without the zigzag tape. To serve as an
Cl leading to roughness insensitivity of Cl,max. intermediate airfoil between DU 91-W2-250 and outboard air-
foils with rather high camber, the 21% thick DU 93-W-210
was designed. The airfoil had a maximum L/D of 143 at
Re = 3 106 andaCl,max of 1.35. The airfoil model was exten-
5. DU AlRFOlLS sively used to experimentally verify the effect of vortex genera-
tors (VGs), Gurney flaps and trip wires. The VGs are solid tabs
The design objectives of the DU series of airfoils were to mounted on the airfoil surface to promote mixing and avoid/
keep sensitivity due to L• contamination and contour delay BL separation; the Gurney flaps are small tabs attached
imperfections of the nose as low as possible. The maxi- to the lower surface to improve the lift, and trip wires are used
mum lift capacity was held at moderate levels, to keep to trip the BL to turbulent.
the loss of lift due to surface contamination as small
as possible. It was found that the thicker versions of the
NACA airfoils that were employed for the root region were 6. RlSØ AlRFOlLS
suffering from a severe degradation of the performance
because of premature transition. It was felt that thick The RisØ A1 series of airfoils were designed by RisØ
airfoils in use that time had a restrained upper surface National Laboratory for wind turbines with stall, active stall
thickness to avoid early separation. To compensate for or pitch regulation, for rotor sizes of about 600 kW [18].
the resulting loss of lift, certain amount of lower surface Detailed wind tunnel testing was performed on three of these
aft-loading was incorporated, giving the typical S-shape airfoils: RisØ-A1-18, RisØ-A1-21 and RisØ-A1-24 [19].
to the DU series of airfoils [17]. Two of the airfoils The following were the characteristics of the airfoils:
DU-91-W2-250 (25% thick) and DU-93-W-210 (21% thick) (i) maximum lift coefficient around 1.5 in natural conditions
are shown in Figure 10. The measured Cl,max was found to be for all airfoils; (ii) high lift : drag ratio also at high angles of
1.37 for the DU-91-W2-250 airfoil, which was slightly less attack just below maximum lift; (iii) insensitivity to leading
than the design value. The maximum L/D was 128. The edge roughness for the maximum lift coefficient; and
sensitivity of the airfoil to distortion of the boundary layer (iv) trailing edge stall, smooth post-stall behaviour. Tests
at the nose was investigated by applying zigzag tape of conducted on all the airfoil sections with zigzag tape showed
0.35 mm thickness at the 5% chord station. Because of the that the airfoils were reasonably insensitive to L• roughness;
the value of Cl,max reduced from about 1.4 to about 1.2
for Re = 1.6 106. The combination of vortex generators
and Gurney flaps increased the Cl,max to about 2.0. The
RisØ-A1-24 airfoil is also a popular choice for TCTs.
The RisØ B1 family of airfoils was designed to maxi-
mize the tangential component of the force (rather than
the lift) so that the contribution in the direction of rotation
is high. As described earlier, an increase in lift mainly
increases the rotor thrust, whereas the tangential compo-
nent increases only a little. The RisØ-B1 airfoils were
designed with the objective of maximizing Ct rather than
Cl. However, it was found that for most of the airfoils, Cl
and the Ct peak at the same a. Another design objective
was good geometric compatibility between the different
airfoil sections and good geometric properties for inboard
airfoils. The design point region was ar (a - a0) of the order
of 9o 14o. The design a-region was chosen close to Cl,max
Figure 9. Desired pressure distribution for the higher Cl angle for because it enables low rotor solidity and/or low rotational
the S814 airfoil [16]. speed. For the root region, a high Cl,max allows a reduction
in solidity. However, the contribution to the overall torque
Figure 10. Profiles of DU-91-W2-250 and DU-93-W-210 airfoils [17].
834 Int. J. Energy Res. 2012; 36:829 844 © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Blade sections for wind and tidal current turbines status and future M. R. Ahmed
Figure 11. Measured airfoil performance of DU-91-W2-250 [17].
from the root region is limited, and aerodynamic shap- manufacturers such as Vestas, DeWind, G•-Wind,
ing for high Cl should not happen at the expense of Suzlon and R•-Ä„ower.
the cross-section structure. They found that the applica-
tion of vortex generators in combination with Gurney
flaps can increase Cl,max. The RisØ B1 family of air- 7. CAS-W1 AlRFOlLS
foils with thickness greater than 24% were designed to
have a high Cl,max, by ensuring the desirable cross- The CAS-W1 family of airfoils was designed by the Chinese
section structure. Figure 12 shows the measured Cl Academy of Sciences. The airfoils have thicknesses of
and Cd for the 24% thick airfoil with vortex generators 15 25%. The CAS-W1-250 airfoil has a maximum thickness
at 20% chord alone and vortex generators in combina- of 25% and a T• thickness of 0.6%. At Re = 3 106, the air-
tion with triangular Gurney flaps of 1% height. The foil has very good aerodynamic characteristics with a Cl,max
combination of vortex generators and Gurney flaps led of 1.7 at 15o in clean conditions and of 1.66 at the same a
to an increase of 34% in Cl,max to a value of 2.17. This with L• roughness. The maximum L/D is 157.6 at a = 6o
could provide an attractive choice for the root part of a [20]. Thus, it has better aerodynamics characteristics and
wind turbine blade where reduction of solidity is a key reduced sensitivity to L• roughness compared with the
issue to reduce blade costs [7]. The airfoils discussed in RisØ-A1/B1-24 and DU-91-W2-250 airfoils. The airfoil is
the above sections are employed in WTs of a number of also found to have stable stall characteristics.
Figure 12. The measured Cl and Cd values with vortex generators at 20% chord and a combination of vortex generators and Gurney
flaps [7].
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M. R. Ahmed Blade sections for wind and tidal current turbines status and future
8. OTHER THlCK AlRFOlLS wind tunnel test results and actual turbine blade results are
significant because of the effects of rotation. Table I presents
Airfoils of 30% thickness are located at the inner 40% of the a comparison of the effect of roughness on the performance
blade, where both aerodynamics performance and structural of different 25% thick airfoils. Although there was a small
considerations are important factors. The location of the difference in the roughness simulation for different airfoils,
maximum thickness and the thickness of the upper surface it can be seen from the table that the RisØ-A1-24, S814
depend on the structural design requirements. The airfoil and DU-91-W2-250 airfoils performed quite well with their
shapes of three dedicated 30% airfoils, FFA-W3-301, Cl,max for rough conditions dropping only a little compared
DU-97-W-300 and AH-94-W-301, are shown in Figure 13. with the clean conditions. The CAS-W1-250 airfoil,
The performance characteristics of DU-97-W-300 and discussed in section 7, performed very well with simulated
AH-94-W-301 airfoils were found to be similar except for roughness, with its Cl,max dropping from 1.71 to 1.66 at
Cl,max, which is slightly higher for the DU airfoil [21]. The a = 15o [20]. For the RisØ-B1 airfoils, insensitivity to L•
FFA-W3-301 airfoil had a lower Cl,max recorded. roughness was ensured by two additional design objectives:
Two airfoils, one 18% thick and the other 25% thick, were (i) having suction side transition from laminar to turbulent
designed by Korea Aerospace Research Institute (KARI) flow in the L• region for angles of attack at Cl,max; and (ii)
[22]. The lift : drag ratio of these KARI airfoils is higher by obtaining a high Cl,max with simulated L• roughness [7].
20 30% compared with NACA643-618 and DU-91-W2- For the 18% thick airfoil, the drop in Cl,max was only 3.7%
250 airfoils. This was achieved by modifying these profiles at Re = 1.6 106, whereas for the 24% thick airfoil, it was
to increase the Cl and to decrease the Cd values. 7.4% with the standard zigzag tape. However, more severe
roughness caused reductions in Cl,max of 12 27%.
9. ROUGHNESS SENSlTlVlTY OF
THlCK AlRFOlLS 10. THlCK FLATBACK AlRFOlLS
Van Rooij and Timmer [21] reviewed the performance of a For the inboard region of large WT blades, thick blunt T• or
number of airfoils: DU, FFA, S8xx, AH, RisØ and NACA flatback airfoils were designed and tested [23]. These airfoils
series for inboard region (thick airfoils). They used the were designed to provide structural and aerodynamic perfor-
reference of zigzag tape with height of 0.35 mm at 0.05c to mance advantages structurally, the sectional area for a given
simulate L• contamination. They also found that in the maximum airfoil thickness increases, and aerodynamically,
inboard region, difference between two-dimensional (2D) Cl,max and lift curve slope increase; also, the sensitivity to
roughness reduces [24]. The main advantage of T• thickness
is that it allows for a portion of the pressure recovery to take
place in the wake of the airfoil, reducing the severity of the
adverse pressure gradient on the suction side and, hence,
reduces the chance of flow separation. This results in improved
performance in both clean and soiled conditions and improves
lift characteristics. Figure 14 shows the profiles and pressure
distributions of a sharp and a blunt T• airfoil. It can be seen
that the pressure gradient is mild for the thick T• case, and
the pressure recovery is continuing. This will alleviate the
Figure 13. Profiles of three 30% thick airfoils [21]. tendency of early flow separation.
TabIe I. Effect of roughness on the performance of (about) 25% thick airfoils [21].
Configuration Clean  Rough
Airfoil (L/D)max Cl,max (L/D)max Cl,max
Re = 3 106
DU-91-W2-250 127.6 1.37 61.8 1.16
NACA-63421-425 120 1.277 39 0.803
AH-93-W-257 120.7 1.41 55 1.04
S814 114.1 1.408 61.4 1.357*
Re = 1.6 106
FFA-W3-241 81 1.37 48.5 1.16**
RisØ-A1-24 89 1.36 57 1.17**
*Grit roughness at upper surface  x/c = 0.02 and lower surface  x/c = 0.1.
**ZZ-tape at x/c = 0.05 on upper surface and at x/c = 0.1 on lower surface.
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Blade sections for wind and tidal current turbines status and future M. R. Ahmed
Figure 14. Pressure distributions on the TR-35 and TR-35-10 airfoils
at Re = 4.5 106 and a =8o [24].
Three thick T• airfoils were designed by adding different
thicknesses to both the sides of the T• of FB3500 airfoil
(35% thick), and their performance characteristics were
studied under different conditions. The improved perfor-
mance for the fixed transition case for the thick T• airfoils,
Figure 15. Profiles of six low Re airfoils [25].
compared with the sharp T• airfoil demonstrated the reduc-
tion of L• soiling sensitivity [23]. However, a higher Cd is
the obvious cost of a thick T•. Site [26]. Design of low Re airfoils is still ongoing [27].
One of the low Re airfoils, SG6043, was tested experimen-
tally in the low-speed wind tunnel at the University of the
11. LOW RE AlRFOlLS South Ä„acific and numerically with ANSYS-CFX for appli-
cations to WTs in the Ä„acific Island countries. The Re was
A number of sites in many countries have regions of low varied from 38 000 to 300 000, and Tu was varied from 1%
wind (4 5 m/s). WTs for such winds require low Re airfoils. to 10%. The a versus Cl and a versus Cd graphs are shown
Advances in the development of small WTs will provide in Figures 16 and 17, respectively [28]. It can be seen that
solutions to the energy requirements of many countries. Cl increases continuously up to 14o for the lowest Re of
Researchers are making performance data sets of low Re 38 000, after which it starts to decrease because of the sepa-
airfoils available, which will help in validating codes used ration of the flow from the upper surface. For the Re of
for design purposes. Aeroacoustic characteristics are also 100 000 and above, Cl increases up to 16o and then starts to
studied by researchers [25] because many small WTs need decrease beyond this angle. It is interesting to note that the
to be installed in populated areas where noise can be a major drop in the Cl values is gradual, as the point of separation
issue. Smaller WTs normally rotate at high rpm, which moves upstream from the T•. The highest values of Cl were
results in high tip speeds and hence higher noise. Figure 15 recorded for the Re of 200 000. For this Re, the value of Cl
shows the profiles of six of the airfoils that were studied in dropped only a little from 16o to 18o, indicating that the stall
detail for both aerodynamic and aeroacoustic performance, for this airfoil is not sudden. The drag coefficient increases
and the reports are made available by NR•L for designers. slowly with a for lower angles; from the angle of 14o for
All these are relatively thin airfoils with maximum thickness Re = 38 000 and of 16o for Re = 150 000, there is a significant
varying from 8.5% to 16.5%. The suffixes along with the increase in Cd, indicating the onset of stalling process.
names of the airfoils refer to multiple versions of those
airfoils. The airfoils were tested over a range of low Re
(generally 100 000 500 000). Some of these airfoils are 12. ACTlVE AND PASSlVE FLOW
employed by WT manufacturers such as Aeromag, World- CONTROL
Ä„ower Technologies and Southwest Wind Ä„ower. Apart
from these, a large number of low and high Re airfoils coor- Devices for flow control are commonly employed to improve
dinates, and data files are available at UIUC s Airfoil Data the aerodynamics performance of the blade section(s). The
Int. J. Energy Res. 2012; 36:829 844 © 2012 John Wiley & Sons, Ltd. 837
DOI: 10.1002/er
M. R. Ahmed Blade sections for wind and tidal current turbines status and future
chord. Liebeck s [33] results showed a significant
increment in lift compared with the baseline airfoil.
The increased lift was also measured and reported by
reference [17,34]. Liebeck suggested an optimal
Gurney flap height to be of the order of 1 2% of the
chord. The flap increases the pressure on the pressure
side, reduces pressure on the suction side and helps
the BL to remain attached till the T•. Gurney flaps
are preferable for the inner part of the blade to increase
Cl, allowing for a smaller chord for the same lift. In
another work, Tongchitpakdee et al. [35] tested the
effect of Gurney flap at 7 and 15 m/s and yaw angles
of 0o, 10o and 30o for S809 airfoil. They found that
both Cl and Ct increase at the lower wind speed. The
increase in the tangential force because of the deploy-
Figure 16. Variation of Cl with a for SG6043 airfoil at different
Re [28]. ment of the flap led to an increase in torque and power.
Some of the active control devices are as follows:
a) circulation control devices, which involve tangen-
tially blowing a small high-velocity jet over a curved
surface such as a rounded T• causing the BL and the
jet sheet to remain attached to the surface because of
the Coanda effect and turn without separation. This
causes the rear separation point to move to the lower
surface resulting in enhanced circulation around the
airfoil and hence enhanced lift [35]. The enhanced
circulation resulting from the blowing resulted in
an increased loading over the entire blade section.
The pressure distributions showed enhanced suction
both near L• and T•. This resulted in an increase in
Ct at the wind speed of 7 m/s and yaw angles of up to
Figure 17. Variation of Cd with a for SG6043 airfoil at different
30o [35].
Re [28].
b) traditional and nontraditional T• flaps, which change
the sectional camber by deflecting the T• portion of
objective also may be to reduce extreme loads and to mitigate the airfoil; the traditional flaps tend to be heavy and
fatigue loads. In general, the intent of these devices is to slow and take up a large portion of the chord; nontra-
delay or advance transition, to reduce or increase turbulence ditional flaps, on the other hand, have a quick activa-
or to prevent or promote separation. The ensuing effects tion, are lightweight and occupy less chord. With the
include lift enhancement, tangential force enhancement, drag use of smart materials, the nontraditional flaps are
reduction, mixing improvement, heat transfer enhancement found to be aerodynamically superior giving improve-
and flow-induced noise reduction [29]. The overall benefit ment in L/D [29].
to the turbine performance with minimum tradeoff is nor- c) active translational microtabs deployed normal to the
mally the goal [30]. A review of the aerodynamic models surface close to the T.•. a maximum deployment
used to estimate the aerodynamic loads on WTs is presented height on the order of the BL thickness (1 2% chord).
by Hansen and Madsen [31]. The commonly used passive Deployment of the tabs effectively changes the
control devices are as follows: sectional camber of the rotor blade and the T• flow
conditions, thereby changing the aerodynamic charac-
a) fixed VGs, which are small solid plates mounted on teristics of the blade. The advantages of the microtab
the blade surface that promote mixing and mitigate are its low actuation power requirements, short actua-
boundary layer separation. VGs that are appropriately tion time, simplicity and the fact that it requires
sized and correctly oriented produce coherent helical minimal changes in the way blades are manufactured.
vortex structures that cause mixing between the air in Inclusion of gaps or serrations in the tabs was found to
the freestream and BL [32]. They are commonly used yield higher gains in terms of L/D for a given Cl
to reduce flow separation and increase Cl,max. compared with the solid tabs [36]. The overall drag
b) Gurney flaps, which are small tabs attached to the can be reduced with this concept. Similar concepts
lower surface of the airfoil in the vicinity of the T• are employed in miniature T• effectors (MiT•s) and
with a height that can vary from 1% to 5% of the microflaps [30].
838 Int. J. Energy Res. 2012; 36:829 844 © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Blade sections for wind and tidal current turbines status and future M. R. Ahmed
In addition to the above, various other passive and active peaks on the upper surface [43]. Figure 18 shows the pres-
flow control methods and devices are proposed and tested sure distribution on the RisØ-A1-24 airfoil at a =8o and Re =
[30,31]. MacĄhee and Beyene [37] simulated the aeroelastic 1.6 106. It can be seen from the figure that the suction peak
response of a morphing airfoil subjected to variable loading for this airfoil is not large at this angle; thus, it will be free
and found that superior lift to drag ratios are obtained over from cavitation. The suction on the upper surface is higher
a specified range of a. They claim that their passive pitch for the smooth airfoil compared with the case when L•
control method would achieve the same end result as an roughness is simulated. As a result, the value of Cl,max drops
active pitch-control device without expensive control and from about 1.4 to about 1.2 [18,19].
sensor equipment. Another method being investigated is In a work by Coiro et al. [44], a hydrofoil for a marine
the modification of the blade tip to enhance the performance turbine was designed by modifying S805 section and tested
[38,39]. Thus, it is clear that numerous methods and devices at Re = 500 000, which was the estimated Re at the tip. The
are developed and tested with the main objective of extract- modified section was named GT1. Cavitation number was
ing maximum power from the flowing wind. A rotor dynam- estimated to be sv = 4.1 at a velocity of 2.5 m/s at 10 C.
ics model that predicts the rotor speed for various turbine Figure 19 shows the pressure distribution on the surface of
configurations, operating over a wide range of wind condi- the hydrofoil at a = 6o and Re = 500 000. The modification
tions, is recently proposed by Ä„ope et al. [40]. significantly improved the pressure distribution and reduced
Of late, the concept of smart rotor control is catching up. the suction peak. The maximum L/D of the GT1 airfoil was
With increasing blade diameters, the need for more sophisti- 122 compared with about 88 for the S805 airfoil at this Re.
cated load control techniques has generated interest in locally Another popular airfoil that is used in TCTs is S814. Up to
distributed aerodynamic control systems with build-in intelli-
gence on the blades. Such concepts are often named in
popular terms  smart structures or  smart rotor control . A
comprehensive review of the smart rotor control research is
presented by Barlas and van Kuik [41].
13. BLADES FOR TCTS
One of the major advantages of TCTs is that, although the
source fluctuates, it is highly predictable, unlike wind which
is highly random and unpredictable both in magnitude and
direction. As the density of water is 800 times that of air,
the forces acting on the TCT rotor are greater compared with
WTs. Thus, the structural requirements for TCTs tend to lead
to thicker sections, particularly near the root, which will
Figure 18. Pressure distribution on the RisØ-A1-24 airfoil at a =8o
increase the Cd. A bigger challenge in the design of blades and Re = 1.6 106 [19].
for TCTs is to avoid cavitation and still obtain an acceptably
higher Cl [42]. Cavitation occurs when the absolute value of
the pressure coefficient is greater than the cavitation number,
sv. Unlike WTs, which are getting bigger and bigger in size,
TCTs are limited in size, mainly by the size of channels in
which they are placed. Fixed speed, passively stall-controlled
TCTs are likely to have a lower annual energy production
compared with variable speed, pitch-controlled ones.
However, the flow conditions for TCTs are much more
steady and predictable; hence, it would not be justified to
use complex control systems. Unlike the WT blades that
have undergone extensive research and development and
many airfoil families have been developed, the blades for
TCTs are either designed by the manufacturers with specific
objectives, or in many cases, relatively thick WT blades that
do not have strong suction peaks are chosen and modified or
used directly.
In a work to optimize the hydrofoil for a marine turbine,
an optimization method was employed by NR•L with a
number of NACA and RisØ sections. The results again
converged only on thick hydrofoils. Thinner hydrofoils are
more susceptible to cavitation as they have large suction Figure 19. Pressure distribution on the GT1 hydrofoil [44].
Int. J. Energy Res. 2012; 36:829 844 © 2012 John Wiley & Sons, Ltd. 839
DOI: 10.1002/er
M. R. Ahmed Blade sections for wind and tidal current turbines status and future
a =8o, the suction peak is not very high and Cl is about 1.3 at Local inflow angles (LFA) were measured using five-hole
Re = 1 106, making it appropriate for these turbines. Goun- probes. LFA was defined as the angle between the local
dar et al. [45] designed and tested a hydrofoil that gives a inflow vector and the local blade chord line, measured at the
maximum Cl of greater than 2 without causing cavitation. probe tip. The measured LFAs at three locations for values
Cavitation causes structural damage to turbine blades, the of U1 ranging from 5 to 25 m/s for zero yaw operation are
pressures associated with bubble collapse are high enough shown in Figure 22. At 5 m/s, LFA began at approximately
to cause failure of metals [46]. Cavitation deteriorates the 5o for all three radial locations. At higher U1 values, the
performance of the blades; it causes lift to decrease and drag LFA plots started diverging, with the maximum divergence
to increase. Batten et al. [42] developed models to show for the 0.34R location. Thus, higher values of U1 prompted
when the cavitation inception occurs, how it will affect higher LFA. The variations of Cn with LFA for rotating and
the performance of marine current turbines and how cavita- non-rotating blades are shown in Figure 23. At lower LFAs,
tion can be avoided by using a section with a higher camber
or by changing the pitch angle of the blades. They also
suggested the use of suitable blade materials such as fibre-
reinforced plastics. Mueller and Wallace [47] proposed the
development of new blade coatings to offer increased cavita-
tion resistance. However, it is better to avoid cavitation with
appropriate design [45 47].
14. EFFECTS OF ROTATlON
The effects of rotation on the performance of a turbine blade
have been a matter of great interest and investigation by
researchers for many years [48 50]. However, a detailed
Figure 20. Comparison of pressure distributions on a rotating
understanding of the effects of rotating is still lacking, as
and non-rotating blade for a =5o and Re = 4 105 [50].
the effects depend on a number of parameters such as blade
profile, orientation, length, rotational speed, wind character-
istics; the effects also vary along the length of a rotating
blade. Ronsten [48] from his wind tunnel measurements
found that at lower a, there is a good agreement between
the pressure distribution and Cl for rotating and non-rotating
blades up to moderate angles of attack at all radial locations.
The hub region experienced higher loadings at low tip speed
ratios than what was predicted using 2D data. Tangler [49]
evaluated the measured NASA Ames Unsteady Aerodynamic
•xperiment post-stall blade element data and provided
guidelines for developing an empirical approach that predicts
post-stall airfoil characteristics. The three-dimensional effects
were found to cause delayed stall with higher Cl values
Figure 21. Comparison of pressure distributions on a rotating
compared with the 2D data, especially near the root region.
and non-rotating blade for a =20o and Re = 4 105 [50].
Sicot et al. [50] studied the effects of rotation and turbulence
on the turbine blade performance in a wind tunnel. The Re on
the blade was 1.5 105 to 4 105. They found that at lower
angles of attack, the pressure distributions on the rotating and
non-rotating blades are similar, as can be seen from Figure 20
for a = 4.8o. At stall and post-stall angles, they recorded a
stronger suction on the suction surface for a rotating blade
compared with the non-rotating case. Figure 21 shows the
pressure distributions for a = 20o and Re = 4 105. It can
be seen that the pressure on the lower surface is slightly higher
for the rotating case; on the other hand, the suction on the
upper surface is much stronger for the rotating blade com-
pared with the parked one, indicating an augmentation of
forces. Similar rotational augmentation of forces including
stall delay and lift enhancement was reported after extensive
wind tunnel experimentation by Schreck and Robinson [51]. Figure 22. Measured LFA as a function of freestream velocity
They employed the S809 airfoil between 0.25R and the tip. at three locations [51].
840 Int. J. Energy Res. 2012; 36:829 844 © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Blade sections for wind and tidal current turbines status and future M. R. Ahmed
magnitude was 1.42. Olorunsola [53] presented results of
yaw force at different wind speeds and found that at non-zero
yaw angles, the yaw force increases significantly at high
wind speeds.
It is known that changes in wind magnitude and direction
are frequent; these can significantly affect the performance of
the turbine, as discussed above and may cause rotational
augmentation or dynamic stall. Similarly, the turbulence
level of the wind changes, which is likely to compound the
adverse effects discussed above. Although the modern tur-
bines are equipped with control systems, the changes in wind
speed and direction may outpace the control actuation rates,
leading to the problems discussed. This particular detailed
Figure 23. Measured Cn values at different LFAs at 0.3R for rotating
study was performed on a particular blade under specificcon-
and non-rotating blade [51].
ditions; the extent of force augmentations may be different
for different blade profiles, sizes and operating conditions.
the Cn values rise linearly and the slopes for rotating and non-
rotating blades are similar. However, at higher LFAs, the Cn
values for the rotating blade rise steeply and attain a maxi-
14.1. Tip speed ratio and wake rotation losses
mum at 45o. They observed considerable stall delay and stall
Cn amplification at other blade locations of 0.47R, 0.63R and A higher tip speed ratio (higher rotational speed) means the
0.8R [52]. However, stall delay and Cn amplification were desired power can be generated by a lower torque (reduced
consistently greater for radial locations farther inboard on weight of rotor shaft and gearbox). A three-bladed WT rotor
the blade. performs optimally at a tip speed ratio of 7 to 8. However, a
When the inflow is yawed (the rotor plane yawed with higher tip speed ratio means a higher noise level. At tip
respect to U1), the vectors ©r andU1 are no longer perpen- velocities greater than 70 m/s for WTs, noise becomes a
dicular. Thus, local inflow magnitude and LFA vary during major concern. For offshore WTs, it is possible to exceed
the rotation cycle, depending upon whether the blade section tip velocities beyond 70 m/s, as noise is not a major concern.
is advancing upstream into the wind or retreating down- For TCTs, optimum performance is achieved at a tip speed
stream from it. As a consequence, LFA no longer remains ratio of 3 to 4 [44]. However, for both WTs and TCTs, the
constant as the blade rotates in azimuth. For a U1 of strength of the structure is a concern at high tip speed ratios
13 m/s and yaw angle of 40o, the pressure distributions on as the force on the hub increases. A challenge for the
the blade at 0.47R are shown in Figure 24. As can be seen designers is thus to design blades that are thin, light-weight
from the figure, the dynamic stall vortex (indicated by the and still capable of taking the load.
suction peak) was first detected just aft of the L• at 0.04c Ideally, a turbine should have no wake rotation losses.
and corresponding to c = 72.8o. Thirty-five msec later, Slow-rotating, high torque turbines experience more wake
when c = 57.8o, the suction peak had moved to 0.36c, rotation losses compared with high-speed turbines with
where it had broadened and magnitude decreased to 2.42. low torque [54], which again requires that blades should
After another 66 msec, when c = 29.4o, the suction peak rotate at high speeds.
had moved further downstream to 0.68c and was no longer
distinct (because of the growth of the vortex), and the  Cp
15. FUTURE WORK
" As discussed above, different blade profiles/blades and
flow conditions will encounter different levels of force
augmentation and dynamic stall while in operation. It
would be good to incorporate some, if not all, of these
aspects into initial design. Design of blade should
continue till it is tested in the field for an extended
period and then modified by addressing the problems
encountered. A challenge is to reduce uncertainty levels
in field measurements.
" A problem with field measurements is the high level of
unsteadiness in the wind that the researchers have no
control over. This is compounded by variations in
temperature and pressure. This not only introduces
Figure 24. Measured pressure distribution at three c values for uncertainties of a higher order but also calls for a more
U1 = 13 m/s, yaw angle of 40o and at 0.47R [51]. accurate assessment of the available energy [55].
Int. J. Energy Res. 2012; 36:829 844 © 2012 John Wiley & Sons, Ltd. 841
DOI: 10.1002/er
M. R. Ahmed Blade sections for wind and tidal current turbines status and future
" Looking at the profiles of most of the thick airfoils with contamination. The effect of L• roughness
(e.g., Figures 10 and 13), it can be seen that at slightly increases with thickness. For sections less than 18%
higher angles, the pressures on both the surfaces do thick, effect of roughness on the lift-curve slope is small.
not contribute significantly to Ct. The added lower " There is still a concern about increasing Cl (as it
surface thickness does not help contribute much to Ct, increases the thrust more). Some of the designs started
although some contribution may come from the part with the intention of maximizing the tangential force
aft of the maximum thickness location. If the objective coefficient; however, it is still a challenge.
is to get a strong component in the direction of rotation, " To maximize the tangential force component, a good
then the section design needs to be modified. A good contribution to lift needs to come from the region
contribution to lift needs to come from the region close close to the L• such that there is a component in the
to the L• such that there is a component in the negative negative drag direction (this also will reduce drag).
drag direction (this will also reduce drag). Some active It is achieved partially with active and passive flow
and passive devices, for example, in reference [35] have control devices, but the difficulty of increasing Ct
shown positive results in terms of increasing Ct, but the without increasing Cl much still remains.
Cl increased as well.
" Although there are some attempts to get more contribu-
NOMENCLATURE
tion to the rotation from the root region (e.g., reference
[7]), a lot of work needs to be performed on the root
c = chord length, m
region airfoils and supplementary devices that enhance
Cd = coefficient of drag
the contribution from this region.
Cl = coefficient of lift
" With the help of a feedback control system, it is possible
Cl,max = maximum coefficient of lift
to reduce unsteady loads with dynamically deployed
Cn = normal force coefficient
Gurney flaps [56]. However, more work is needed on
Cp = coefficient of pressure
this type of flaps.
Ct = tangential force coefficient
" With a growing concern about the adverse effects of
D = drag force, N
large WTs and WT farms on weather, future research
L = lift force, N
works also should pay equal attention to the turbulence
r = radial distance from hub, m
and mixing caused in the wake. At the same time, the
R = blade length, m
generation of rotational kinetic energy in the wake
Re = Reynolds number
results in less energy extraction by the rotor than would
Tu = turbulence intensity, %
be expected without wake rotation.
U1 = freestream mean velocity, m/s
" With a growing interest in wind farms, detailed investi-
x = distance along chord from the leading
gation of wakes assumes more significance and impor-
edge, m
tance. This is another area, which needs more attention.
xtr = distance along chord where transition
" For offshore WTs and TCTs, maintenance and repairs
occurs, m
are normally expensive and time consuming. Taking
a = angle of attack, degrees
this into consideration will lead to lean design.
c = blade azimuth angle, degrees
" High-design lift coefficients allow design of slender
sv = cavitation number
blades while maintaining high aerodynamic efficiency.
© = blade rotation rate, rad/s
Also, the rated power will be reached at relatively low
wind speed.
" Another challenge is to design blades that give consis-
Acronyms
tent output at varying flow conditions with a simple
control system.
HAWT = horizontal axis wind turbine
" It is disappointing to see some large wind turbines
LFA = local inflow angle
employingverythinblades(almostlikeflatplates)near
L• = leading edge
the tip for structural reasons. The engineering commu-
TCT = tidal current turbine
nityneedstostrivetoovercomethisproblem.
T• = trailing edge
VG = vortex generator
WT = wind turbine
16. CONCLUSlONS
" After nearly two decades of extensive research, a num-
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DOI: 10.1002/er