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SCIENCE CHINA Technological Sciences
Science China Press and Springer-Verlag Berlin Heidelberg 2014
tech.scichina.com link.springer.com
*Corresponding author (email: [email protected])
Special Topic: Engineering Thermophysics January 2015 Vol.58
No.1: 18 Article doi: 10.1007/s11431-014-5741-8
Numerical analysis and experimental investigation of wind
turbine blades with innovative features: Structural response
and
characteristics
CHEN Xiao*, QIN ZhiWen, YANG Ke, ZHAO XiaoLu & XU
JianZhong
National Laboratory of Wind Turbine Blade Research &
Development Center, Institute of Engineering Thermophysics, Chinese
Academy of Sciences, Beijing 100190, China
Received September 26, 2014; accepted December 3, 2014;
published online December 19, 2014
Innovative features of wind turbine blades with flatback at
inboard region, thick airfoils at inboard as well as mid-span
region and transversely stepped thickness in spar caps have been
proposed by Institute of Engineering Thermophysics, Chinese Academy
of Sciences (IET-Wind) in order to improve both aerodynamic and
structural efficiency of rotor blades. To verify the proposed
design concepts, this study first presented numerical analysis
using finite element method to clarify the effect of flat-back on
local buckling strength of the inboard region. Blade models with
various loading cases, inboard configurations, and core materials
were comparatively studied. Furthermore, a prototype blade
incorporated with innovative features was manu-factured and tested
under static bending loads to investigate its structural response
and characteristics. It was found that rotor blades with flatback
exhibited favorable local buckling strength at the inboard region
compared with those with conventional sharp trailing edge when
low-density PVC foam was used. The prototype blade showed linear
behavior under extreme loads in spar caps, aft panels, shear web
and flatback near the maximum chord which is usually susceptible to
buckling in the blades according to traditional designs. The
inboard region of the blade showed exceptional load-carrying
capacity as it survived 420% extreme loads in the experiment.
Through this study, potential structural advantages by applying
proposed structural features to large composite blades of
multi-megawatt wind turbines were addressed.
wind energy, rotor blade, flatback, local buckling, extreme
loads
Citation: Chen X, Qin Z W, Yang K, et al. Numerical analysis and
experimental investigation of wind turbine blades with innovative
features: Structural re-sponse and characteristics. Sci China Tech
Sci, 2015, 58: 18, doi: 10.1007/s11431-014-5741-8
1 Introduction
Wind energy is one of the lowest-priced renewable energy
technologies available today. Installed wind energy capacity both
worldwide and in China has grown exponentially over the past few
years and it is expected to increase significantly in the years to
come [1]. As one of the most critical compo-nents in wind turbine
system, rotor blades capture kinetic energy from wind and convert
it to mechanical energy,
which is eventually converted to electrical energy by
gener-ators. Rotor blades are thin-walled composite structures with
airfoil cross-sectional profiles. Typical construction of blade
cross sections is shown in Figure 1. Spar caps of rotor blades are
made of composite laminates and designed to carry primary bending
moments applied to the blades, while leading panel and aft panel
are made of sandwich construc-tions and designed to provide
aerodynamic profiles of blade cross sections. Shear webs are also
sandwich constructions and designed to support two spar caps and
transfer shear forces in the blades.
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2 Chen X, et al. Sci China Tech Sci January (2015) Vol.58
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Figure 1 (Color online) Typical construction of cross sections
of compo-site wind turbine blades.
Aerodynamic and structural performance of rotor blades
determines the overall performance of wind turbines. Re-searchers
[27] have been focusing on improving aerody-namic efficiency of
rotor blades as it directly affects the power output of wind
turbines and the associated cost of wind energy. Therefore,
aerodynamic performance of rotor blades is usually regarded to be
of primary concern in wind turbine design. Nevertheless, recent
failure accidents of rotor blades have attracted researchers
attention. Through a number of studies [811] it has been found that
local buck-ling of large aft panels and sharp trailing edge near
the maximum chord, failure of inboard region at the root
transi-tion, and spar cap delamination are among major failure
modes in composite rotor blades.
In order to increase structural performance of blades, sandia
national laboratories (SNL) [1214] conducted a so-called blade
system design study (BSDS) in which structural innovations such as
flatback airfoils, thick root diameter, carbon spar cap, etc. were
proposed and structural advantages of these innovations were
demonstrated by ex-periments using subscale prototype blades.
Meanwhile, a series of research programs have been carried out at
Na-tional Laboratory of Wind Turbine Blade Research &
De-velopment Center, Institute of Engineering Thermophysics,
Chinese Academy of Sciences (IET-Wind) aiming to im-prove both
aerodynamic and structural performance of
composite wind turbine blades. From material and structural
point of view, the proposed blades were featured with (i)
glass/polyester composites for lower material and manufac-turing
cost compared with more commonly used glass/ epoxy composites, (ii)
flatback at the inboard region and thick airfoils at both inboard
and mid-span region for larger bending stiffness and strength
compared with sharp trailing edge and thin airfoils used in the
conventional blades, and (iii) transversely stepped thickness in
spar caps for more efficient use of materials against external
bending loads due to the increase of the area moment of inertia of
the blade cross-section. The structural features of the proposed
blades are illustrated in Figure 2. Major difference between the
BSDS blade and the one proposed by IET-Wind is shown in Table
1.
Although structural advantages of the BSDS blade have been
studied by SNL, the comparisons of structural perfor-mance were
made between blades with different geometries, material layups, and
applied loads. It should be noted that comparative study on blades
with a single variable would give more conclusive information to
blade designers than the one with multiple variables especially
when the effect of one particular variable is of interest.
Considering the flat-back at the inboard region is one of the most
important in-novations proposed by both SNL and IET, this
structural feature is treated as a key variable in the current
study and comparative study is conducted numerically to clarify its
effect on local buckling strength of the blades. Furthermore, a
prototype blade with the joint use of flatback, thick air-foils and
transversely stepped spar cap thickness has been manufactured and
tested in order to verify the proposed design concepts with
emphasis on structural response and characteristics of the blades.
It is expected that more in-sights into structural performance
could be gained through this study and eventually more reliable and
cost-effective blade designs for wind energy utilization could be
achieved.
2 Numerical modeling and analysis
In numerical modeling, a blade with structural features of
flatback at the inboard region, thick airfoils at both inboard
Figure 2 (Color online) Structural features of blades proposed
by IET-Wind.
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Chen X, et al. Sci China Tech Sci January (2015) Vol.58 No.1
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Table 1 Comparison of structural features between blades
proposed by SNL and IET
SNL IET
Thin, large diameter root Regular root size
Thick airfoil at inboard region Thick airfoil at inboard and
mid-span region Spar cap with full length and
constant-thickness Spar cap with transversely
stepped thickness Carbon spar cap Glass spar cap
Epoxy resin Polyester resin
and mid-span regions, and transversely stepped thickness in spar
caps was analyzed. Although structural advantages of many features
are readily assessed according to basic struc-tural mechanics,
those of flatback at the inboard region have not been thoroughly
investigated by the existing studies [1214], in which different
blades of multiply variables were compared with. In this study, the
blade geometric con-figurations at the inboard region with flatback
was deter-mined to be a key variable in the numerical modeling and
other variables such as outboard geometry, material layups, and
applied loads were kept the same among the blade models to be
analyzed. In this study, particular focus was paid to local
buckling strength of blades with different in-board
configurations.
2.1 Blade geometry and material properties
Four types of blades with a total length of 10.3 m were modeled
using the general finite element program Abaqus [15], and they were
labeled as BS1: A baseline blade model with a conventional sharp
trailing edge at the inboard region; BS2-1: A blade model with a
conventional sharp trailing edge and a longer chord length at the
inboard region; BS2-2: A blade model same as BS2-1 but with an
additional shear web; BF: A blade model with a flatback and a same
chord length as BS1, see Figure 3. All blade models had same
configurations at outboard regions which dominate aerody-namic
performance of the blades. Physical dimensions of aerodynamic
profiles of four blade models along their spans are shown in Figure
4. Except inboard configurations, all blades had the proposed
features as thick airfoils at inboard and mid-span regions and
transversely stepped thickness in spar caps.
Material layups of four blades were assigned to be iden-tical
when blade regions are the same. The material layups of the
additional shear web in BS2-2 were the same as those of the primary
shear web in other three blades. The material layups as well as the
geometry of the flatback region in BF were identical to those of
the additional shear web in BS2-2. Because mechanical properties of
core materials signifi-cantly affect buckling strength of sandwich
constructions in the blades, two different core materials, i.e.,
PVC foam and balsa wood, which are commonly used in wind turbine
blades were selected to perform parametric study in the numerical
analysis. Typical material properties of cores used in this study
are shown in Table 2.
2.2 Element type and mesh density
Shell elements S4R with an offset-node formulation were used for
outer blade skins and those with a conventional mid-node
formulation were used for shear webs. S4R is a 4-node,
quadrilateral, stress/displacement shell element with reduced
integration and a large-strain formulation. A typical size of 25 mm
25 mm was used to mesh the models, before this mesh size was
determined, a mesh convergence study has been conducted. It was
found that when the blades were meshed with typical sizes of 35
mm35 mm, 25 mm25 mm, and 15 mm15 mm, the results of the first
natural fre-quencies of the blades between 25 mm25 mm mesh and 15
mm15 mm mesh were below 2%, and the results of the first bucking
eigenvalues between two meshes were below 4%, therefore, a mesh
size of 25 mm25 mm was deemed sufficient. This mesh size resulted
in a total number of shell elements ranging approximately from
24000 to 26000 for four blade models.
2.3 Boundary and loading conditions
Fixed boundary was applied to the blade roots. Applied loads of
four blades were assumed to be identical consider-ing that outboard
regions controlling aerodynamic perfor-mance of the four blades
were the same. Static bending loads were applied to the blades to
simulate extreme wind loads that the blades were expected to
sustain in 20 years design lives according to IEC standard 61400-1
[16] and
Figure 3 (Color online) Blade geometry and cross-sections of
blades considered in numerical modeling.
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4 Chen X, et al. Sci China Tech Sci January (2015) Vol.58
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Table 2 Properties of core materials used in sandwich panels
Core material Density (kg/m3)
Compressive modulus (MPa)
Shear modulus (MPa)
PVC foam 60 70 20 Balsa wood 155 4100 166
Figure 4 (Color online) Physical dimensions of the blades.
GL Guideline for the Certification of Wind Turbines [17]. There
were four loading cases in terms of bending direc-tions for each
blade and they are schematically shown in Figure 5 taking the blade
model BF for example. The load-ing cases of Flap_max and Flap_min
primarily result in compression at suction side and pressure side
of the blades, respectively; while the loading cases of Edge_max
and Edge_min primarily result in compression at leading edge and
trailing edge of the blades.
Desired distributions of bending moments in each load-ing case
were calculated by IEC and GL and they were ap-proximated by
piece-wise linear fits achieved by point loads introduced at 4 and
8 m blade spans, see Figure 6. Point loads with resultants equal to
the forces necessary to gener-ate bending moments were equally
distributed on the spar caps responsible for loading-carrying. The
use of this kind of loading introduction was also intended to
simulate the actual test setups in the blade load tests which will
be fur-
Figure 5 Loading directions of the blades.
ther discussed in section 3. Representatively, boundary and
loading conditions of the blade model BF in the Flap_ max loading
case are shown in Figure 7.
2.4 Simulation results and discussion
There were two categories of FE analysis conducted on the blade
models, the one was stiffness analysis with incremen-tal loading in
order to obtain the deflections and spar cap strains of the blades,
and the other one was linear buckling analysis with specified
loads, i.e., 100% applied loads, in order to evaluate local
buckling strength of the blades under extreme wind loads. Because
local buckling strength of four blades with different inboard
configurations was of major interest in numerical analysis, FE
results from linear buck-ling analysis is presented and discussed
in this section, while FE results from the stiffness analysis were
compared with and validated against those from the blade test
pre-sented in section 3.
In total, 32 numerical analyses were performed to study buckling
strength of blades in terms of four inboard config-urations, four
loading cases and two core materials. The
Figure 6 (Color online) Bending moments applied to the
blades.
Figure 7 (Color online) Boundary and loading conditions of the
BF blade in the Flap_max loading case.
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Chen X, et al. Sci China Tech Sci January (2015) Vol.58 No.1
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lowest eigenvalues for linear buckling of all analyses were
normalized by those obtained from the baseline blade mod-els, i.e.,
BS1, with the same loading cases and core materi-als. The results
are summarized in Figure 8.
2.4.1 Blades with PVC foam core It can be seen that when PVC
foam core materials were used, buckling strengths of the baseline
blade were im-proved by new configurations introduced at the
inboard regions in BS1-1, 2 and BF. The blade BF exhibited most
significant improvement in buckling strength in the loading cases
Edge_min and Flap_max compared with other blades. Particularly, the
blade BF showed a 25% increase in buckling strength compared with
its baseline blade in the loading case Edge_min. The blades BS1-1
and 2 showed preferable buckling strength in the loading case Edge_
max, while in the loading case Flap_min, only a few percentages of
improvement in buckling strength were found in BS1-1, 2 and BF.
These trends are regarded to be reasonable considering the
loading directions in each case. In the loading cases Edge_min and
Flap_max, the trailing edge sides most susceptible to local
buckling were primarily subjected to compression, the introduction
of longer chord length in BS1-1, 2 and flatback in BF greatly
increased local buckling resistance of inboard regions. In the
loading case Edge_ max, the trailing edge sides were primarily
subjected to tension, larger bending stiffness due to longer chord
length in BS-1, 2 therefore exhibited more significant increase in
buckling resistance than the flatback in BF. In the loading case
Flap_min, the trailing edge sides were partially sub-jected to
tension due to airfoil twist, the improvement of buckling strength
was not significant by introducing either longer chord length or
flatback. Furthermore, by examining buckling modes of each blade,
it was noticed that buckling regions of the BF blade at the
trailing edge sides were smaller than those of other three blades,
suggesting that
Figure 8 (Color online) Normalized eigenvalue for linear
buckling of different blades under various loading cases.
buckling modes were suppressed by introducing flatback at the
trailing edge sides. Representatively, buckling modes of blades in
the loading case Flap_max are shown in Figure 9(a).
2.4.2 Blades with balsa wood core When cored with balsa wood,
the blades exhibited only slight difference, approximately within
3%, in buckling strength for different loading cases and inboard
configura-tions. This can be explained by the locations of buckling
modes found at the middle spans of the blades as shown in Figure
9(b). Because four blades were only different in the inboard
regions, it is not surprising that buckling strength of four blades
did not differ much. It should be emphasized that although balsa
wood could considerably improve buck-ling strength of sandwich
panels in the blades, it is much more expensive than low-density
PVC foam material, meanwhile, weight penalty due to large material
density is also of concern for wind turbine blades. For four blades
studied in this study, their weights were increased by 5% to 6%
when balsa wood was used.
3 Experimental investigation
Based on simulation results and discussion presented in section
2, it is evident that the blade with flatback showed stronger local
buckling strength at the inboard regions than the blade with
conventional sharp trailing edges when other variables such as
outboard geometry, material layups, and applied loads were the
same. Therefore, it is expected that local geometry modification
from the commonly used sharp trailing edge to the flatback could be
an efficient way to
Figure 9 (Color online) Buckling modes of blades in the loading
case Flap_max.
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6 Chen X, et al. Sci China Tech Sci January (2015) Vol.58
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increase local buckling strength of wind turbine blades. In this
section, a prototype blade BF cored with PVC foams was manufactured
and tested under static bending to verify the numerical results and
further examine structural charac-teristics of blades with the
proposed features.
3.1 Test program
Static bending was applied to the blade which was root- fixed at
a test stand. Two cranes were used to introduce loads at 4 and 8 m
blade spans as shown in Figure 10.
The loads were applied in a step-wise form following a test
sequence as shown in Table 3. Once a loading case was finished, the
blade was rotated along its longitudinal axis to a desired position
and tested in a subsequent loading case.
At each load level, applied loads were recoded by load cells
mounted on cranes, deflections of the blade were measured at two
loading saddle locations and at the blade tip using draw-wire
displacement transducers. Blade strains were recorded by strain
gauges located along the center line of spar caps, at the middle of
flatback, aft panels and shear webs. Only strains longitudinal to
the blade axis were measured on spar caps, while both longitudinal
and trans-verse strains were measured at other locations by the
corre-sponding strain gauges labeled as 0 and 90, respectively. The
use of these orthogonal gauges were intended to better indicate the
occurrence of local buckling at flatback, aft panels, and shear
webs during the tests through changes of load-strain path
directions.
The blade successfully passed 100% test loads simulating the
extreme loads in the loading cases of Edge_min, Flap_min, and
Edge_max. In the following loading case Flap_max, it was decided to
load the blade to failure if it could survive 100% test loads.
Applied loads continued to increase and when they were close to
220% test loads, the blade failed catastrophically at 6-m span.
Measurements of
Figure 10 (Color online) Experimental setups for the blade
test.
Table 3 Load test history of the blade
Sequence Cases Loading history
1 Edge_min 0-40%-60%-80%-100%-unloading 2 Flap_min
0-40%-60%-80%-100%-unloading 3 Edge_max
0-40%-60%-80%-100%-unloading
4 Flap_max 0-40%-60%-80%-100%-120%-
140%-160%-180%-200%- blade failure
deflections and strains at the failure load were not able to be
indentified clearly due to the rapid failure of the blade, the
average values of measurements taken from 200% test load step to
the final failure were used to approximate structural response at
the load step of 210%.
3.2 Experimental results and discussion
3.2.1 Deflection and spar cap strains Measurements of
deflections and spar cap strains in each loading case were compared
with the predictions from nu-merical analysis. Representatively,
the results for the load-ing case Flap_max are shown in Figure 11,
where results of some load steps were not shown for the sake of
clarity in presentation. It can be seen that experimental
measurements and numerical predictions are with good agreement. The
tip deflections of the blade at the extreme loads and near the
ultimate failure loads were about 1.37 and 2.56 m, respec-tively;
the corresponding longitudinal strains of spar caps at 6-m span
were around 3500 and 6700 at both pressure and suction side.
3.2.2 Local strains of the maximum chord section Local strain
response of the inboard region of the blade with flatback is of
particular interest in the experimental study. As buckling is
reported as one of dominating structural re-sponse near the maximum
chord of conventional blades with sharp trailing edge. The strain
records of the current blade measured near the maximum chord, i.e.,
2-m span, were carefully examined and they are shown in Figure
12.
It can be seen that strains measured at spar caps, aft panel,
and the flatback near the maximum chord exhibited linear relation
to applied loads in all loading cases up to the ex-treme loads,
although slight nonlinear response was found at shear web when
loads approached the extreme loads in the loading case Flap_max.
This observation suggests that in general the blade has sufficient
buckling resistance in these regions. Considering the loading
process to the final failure loads of the blade in the Flap_max
case, it is inter-esting to notice that spar cap and aft panel
remained in line-ar response as shown in Figures 12(a) and (b).
While shear web started to show obvious nonlinear behavior beyond
the extreme loads. Longitudinal strain and transverse strain
changed load paths simultaneously with the increase of ap-plied
loads suggesting the occurrence of buckling at this location as
shown in Figure 12(c). Similarly, the flatback near the maximum
chord showed linear response up to 140% test load and local
buckling started to occur after-wards as shown in Figure 12(d).
It is important to note that although local buckling was
detected at shear web and flatback near the maximum chord, the
corresponding buckling loads were beyond the extreme loads.
Furthermore, by examining strain levels at buckling loads, it can
be found that all local strains were within 200 ,
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Chen X, et al. Sci China Tech Sci January (2015) Vol.58 No.1
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Figure 11 (Color online) Comparison of blade deflections and
spar cap strains. (a) Blade deflections; (b) spar cap strains.
Figure 12 (Color online) Local strain responses of the blade.
(a) Spar cap strains; (b) aft panel strain; (c) shear web strains;
(d) flatback strain.
which are much smaller than the failure strains of composite
materials used in the blade. Therefore, the local buckling response
of shear web and flatback in the loading process to the blade
failure was not able to cause material failure at these locations.
This observation is of significance consid-ering that when the
conventional sharp trailing edge is sub-jected to buckling, it
usually exhibits local buckling and the associated material failure
due to large strains [10]. The flatback used in the current blade
exhibited great potential to improve local buckling strength which
is usually one of the weakest links in structural systems of wind
turbine
blades.
3.2.3 Ultimate strength of inboard region Because the blade
failure at 6-m span prohibited the as-sessment of the inboard
region at the ultimate failure, it was decided to conduct an
additional static bending test with an intention to fail the blade
at the inboard region in the load-ing case Flap_max. This load test
was achieved by using the loading saddle previously mounted at 4-m
span. Pulling forces were continuously applied and monitored. When
the root moment reached approximately 294 kNm which was
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8 Chen X, et al. Sci China Tech Sci January (2015) Vol.58
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about 420% of the root moment caused by the 100% ex-treme loads,
the inboard region did not exhibit any sign of failure.
The load test was then aborted due to safety concern.
Subsequently, the inboard region of the blade was visually
inspected and no material failure was found. It was con-cluded that
the inboard region of the blade had exceptional ultimate
strength.
4 Conclusions and future work
To verify innovative structural features of wind turbine blades
proposed by IET-Wind, a rotor blade with flatback, thick airfoils,
and transversely stepped thickness in spar caps was numerically
analyzed and experimentally investi-gated focusing on its
structural response and characteristics. The following conclusions
were drawn.
By comparing linear buckling strength of blades with different
inboard configurations in numerical analysis, it was found that the
blade with flatback showed more favora-ble buckling resistance at
the inboard regions than those with the conventional sharp trailing
edge when low-density PVC foam was used. However, no significant
advantage of flatback was found when balsa wood was used due to
buck-ling mode occurred at the mid-span where blade geometry,
material layups, and applied loads were identical. In load tests,
the prototype blade with the proposed features exhib-ited linear
behavior under extreme loads in spar caps, aft panels, shear web
and flatback near the maximum chord which is usually regarded to be
susceptible to buckling. In the failure test under flapwise
bending, the shear web and the flatback near the maximum chord
experienced local buckling around 100% and 140% extreme loads,
respec-tively, and they continued to sustain applied loads up to
220% extreme loads without any material failure. The in-board
region of the blade showed exceptional ultimate strength as it
survived 420% extreme loads in the experi-ment.
Followed by this study, three blades identical to the pro-totype
have been manufactured and installed on a 100 kilo-watt wind
turbine. A series of field tests will be carried out after an
ongoing trial run period to study aerodynamic, aer-oacoustic,
aeroelastic, and structural performance of rotor blades
incorporated with innovative features proposed by IET-Wind. Other
studies are also planned, with the objec-
tive of applying the proposed features to large rotor blades for
multi-megawatt wind turbines.
This work was supported by the National Natural Science
Foundation of China (Grant No. 51405468).
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/IncludeProfiles false /MultimediaHandling /UseObjectSettings
/Namespace [ (Adobe) (CreativeSuite) (2.0) ]
/PDFXOutputIntentProfileSelector /DocumentCMYK /PreserveEditing
true /UntaggedCMYKHandling /LeaveUntagged /UntaggedRGBHandling
/UseDocumentProfile /UseDocumentBleed false >> ]>>
setdistillerparams> setpagedevice