Effects of torsional degree of freedom, geometric nonlinearity, and gravity onaeroelastic behavior of large-scale horizontal axis wind turbine blades under varyingwind speed conditionsMin-Soo Jeong, Myung-Chan Cha, Sang-Woo Kim, In Lee, and Taeseong Kim

Citation: Journal of Renewable and Sustainable Energy 6, 023126 (2014); doi: 10.1063/1.4873130 View online: http://dx.doi.org/10.1063/1.4873130 View Table of Contents: http://scitation.aip.org/content/aip/journal/jrse/6/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Unsteady vortex lattice method coupled with a linear aeroelastic model for horizontal axis wind turbine J. Renewable Sustainable Energy 6, 042006 (2014); 10.1063/1.4890830 A numerical investigation of the stall-delay phenomenon for horizontal axis wind turbine AIP Conf. Proc. 1493, 389 (2012); 10.1063/1.4765518 Dynamic stall analysis of horizontal-axis-wind-turbine blades using computational fluid dynamics AIP Conf. Proc. 1440, 953 (2012); 10.1063/1.4704309 Power performance of canted blades for a vertical axis wind turbine J. Renewable Sustainable Energy 3, 013106 (2011); 10.1063/1.3549153 Aeroelastic Problems of Wind Turbine Blades AIP Conf. Proc. 1281, 1867 (2010); 10.1063/1.3498270

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Effects of torsional degree of freedom, geometricnonlinearity, and gravity on aeroelastic behavior oflarge-scale horizontal axis wind turbine blades undervarying wind speed conditions

Min-Soo Jeong,1 Myung-Chan Cha,2 Sang-Woo Kim,3 In Lee,4,a) andTaeseong Kim5

1Body Durability CAE Team, Research & Development Division, Hyundai Motor Co.,Hwaseong 445-706, South Korea2Ship Performance Research Department II, Hyundai Maritime Research Institute,Hyundai Heavy Industries Co. Ltd., Ulsan 682-792, South Korea3Launch Complex Team, KSLV-II R&D Program Executive Office, Korea AerospaceResearch Institute, Daejeon 305-806, South Korea4Division of Aerospace Engineering, Korea Advanced Institute of Science and Technology,Daejeon 305-701, South Korea5Technical University of Denmark, Department of Wind Energy, Risø Campus,Rokilde 4000, Denmark

(Received 13 September 2013; accepted 11 April 2014; published online 25 April 2014)

Modern horizontal axis wind turbine blades are long, slender, and flexible

structures that can undergo considerable deformation, leading to blade failures

(e.g., blade-tower collision). For this reason, it is important to estimate blade

behaviors accurately when designing large-scale wind turbines. In this study, a

numerical analysis considering blade torsional degree of freedom, geometric

nonlinearity, and gravity was utilized to examine the effects of these factors on the

aeroelastic blade behavior of a large-scale horizontal axis wind turbine. The results

predicted that flapwise deflection is mainly affected by the torsional degree of

freedom, which causes the blade bending deflections to couple to torsional

deformation, thereby varying the aerodynamic loads through changes in the

effective angle of attack. Edgewise deflection and torsional deformation are

mostly influenced by the periodic gravitational force on the wind turbine blade.VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4873130]

I. INTRODUCTION

Wind farms now contain a great number of large-scale horizontal axis wind turbines. One

of the significant developments in the design of individual wind turbines is an increase in their

power capacity to minimize the cost of energy.1 Wind fluctuations strongly affect power output,

control systems, and maintenance of wind turbines, and especially their safety and stability.

Large wind turbine may give rise to loads that vary along the blade and change quickly in

response to varying wind conditions, such as turbulent and sheared flow conditions. For design

purposes, it is important to understand the distribution of turbulence energy among the poten-

tially significant periods or frequencies of fluctuation. Wind turbulence has a strong impact on

blade deformation. Heavy turbulence may generate large variations of aerodynamic loads acting

on the blade and thus may result in turbine failures and reduced life of the turbine.1,2

Moreover, wind velocity is proportional to the height from the ground due to the surface rough-

ness. With the continuously increasing blade length of wind turbines, wind loads according to

a)Author to whom correspondence should be addressed. Electronic mail: inlee@kaist.ac.kr. Tel.: þ82-42-350-3717. Fax:

þ82-42-350-3710.

1941-7012/2014/6(2)/023126/19/$30.00 VC 2014 AIP Publishing LLC6, 023126-1

JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 6, 023126 (2014)

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blade position vary significantly, resulting in large unbalanced aerodynamic loads on the blades.

Thus, wind shear can also result in excessive blade deformations.

As mentioned above, the interaction between blade motions and wind speed variations

becomes more pronounced as wind turbines become larger.3 Fluid-solid coupling dynamics also

become more complex, and more serious coupling in wind turbines may cause blade damage

more easily. For instance, during the operation of wind turbines, a minimum clearance must be

maintained between the blade tips and the tower. For this reason, high blade stiffness is

required to avoid collisions between the blades and the tower; yet, in practice, the maximum

blade length is constrained by the required stiffness and stresses on blades. Therefore, to pre-

vent blade failures, simulations for accurately estimating the blade behavior of large-scale wind

turbines should be utilized in the design of flexible and slender wind turbine blades. To do this,

some important factors, such as torsional degree of freedom (DOF), geometric nonlinearity, and

gravity, must be considered in a numerical analysis. Many researchers in the wind industry and

research institutes use simulation codes to include the torsional DOF in estimates of the blade

deformations of their wind turbines4 (e.g., GH Bladed,5 HAWC2,6 BHawC,7 ADAMS,8

FLEX5,9 PHATAS,10 TURBU,11 etc.). Although these codes are relatively inexpensive compu-

tationally, some simulation codes (e.g., FAST12) do not include the torsional DOF of the blade;

thus, the predictions cannot consider the torsional deformations due to the aerodynamic pitching

moment. However, because the torsional deformation consequently affects the aeroelastic char-

acteristics as well as the aerodynamic characteristics through variations in the angle of attack,

an analysis including the torsional DOF should be utilized.13 In addition, the larger and more

flexible blade shape of the wind turbines introduces nonlinear blade behaviors.14 Numerous

approaches have been developed to cope with large deflection problems, such as elliptic integral

formulation, numerical integration with iterative shooting techniques, the incremental finite ele-

ment (FE) method, the incremental finite differences method, the method of weighted residual,

and the perturbation method.15,16 The European Commission-funded project UPWIND17–19

deals with nonlinear modeling of blades and the effects of including such geometric nonlinear-

ities. Also, the TURBU11 includes the effect of geometric nonlinearities.20 However, a fair

number of the available commercial programs for wind turbine design still use simplified linear

structural models, which cannot be applied to structures with considerable deformations.3,21–23

Furthermore, the blade behaviors are largely impacted by gravity, which induces excitation in

the rotating blade and becomes a more crucial vibration source as dimensions of the wind tur-

bine increases.24 For this reason, it is necessary to understand the various nonlinear interactions

including the effect of gravity, as well as the torsional DOF, on large-scale (larger than 5 MW)

wind turbine blades.

For fluid-structure interaction simulations, several different approaches for wind turbines

have been used. Until recently, the coupled computational fluid dynamics (CFD)-computational

structural dynamics (CSD) techniques have rarely been utilized for aeroelastic analysis of the

wind turbines.25,26 Baziles et al.25 and Yu and Kwon26 have developed the CFD-CSD methods

for estimating the static blade deflections and the aerodynamic results by the deformed blades.

These CFD-CSD computations have some limitations for use in an aeroelastic model, although

CFD approaches have ability to improve the prediction accuracy. A notable disadvantage of the

coupled CFD-CSD model is that it is computationally intensive and complex. For this reason,

blade element momentum (BEM)-CSD technique is the most widely used because of its sim-

plicity and low computational cost. Therefore, this study deals with the effects of torsional

DOF, geometric nonlinearity, and gravity on the aeroelastic behavior of a horizontal axis wind

turbine blade using the ABAQUS-BEM coupled method. The finite element software ABAQUS

is not a specific code for wind turbines, and has no built-in rotor-aerodynamics algorithms, but

allows users to link their own routines with aerodynamic modules.23 Therefore, in this study,

the ABAQUS/standard program is coupled with its aerodynamic solver based on a BEM

method, which takes in account the interaction of the factors of the torsional DOF, geometric

nonlinearity, and gravity. The results predict that flapwise deflections of a wind turbine blade

are predominantly influenced by the factor of the torsional DOF, while edgewise and torsional

deformations are mostly affected by the gravity.

023126-2 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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II. MODELING

A. Wind profiles under turbulent and sheared flow conditions

Wind condition is one of the most critical characteristics in wind turbine aerodynamics and

aeroelasticity. In fact, wind velocity varies in both time and space, and is determined by many

factors such as terrain and meteorological conditions.27,28 Because the wind condition, such as

turbulent flow, is a random parameter, measured wind data are usually considered using statisti-

cal methods.1 A turbulent flow environment includes critical features known to adversely affect

the aerodynamic and aeroelastic blade response and should be considered in the design phase

of wind turbine blades; however, the characteristics of the turbulent flow cannot be simulated

accurately by the normal turbulence models from the International Electro-Technical

Commission (IEC) 61400-1 standard.27 The National Renewable Energy Laboratory (NREL)

developed the TurbSim stochastic flow turbulence code to provide a numerical analysis of full-

field flow, including bursts of coherent turbulence associated with organized turbulent structures

in the flow.28 As mentioned earlier, the analysis code employs a statistical model to predict the

time series of three velocity components in a two-dimensional grid. The power spectra of the

three velocity components, and the spatial coherence, can be expressed in the frequency do-

main. Also, the time series is generated by an inverse Fourier transform (IFT).28 This study

utilizes the IEC Kaimal turbulence model for the numerical simulations, as it is capable of pro-

viding the most realistic flow conditions.29 Detailed descriptions of the Kaimal turbulence

model are reported in the NREL TurbSim User’s Guide technical report.28

Wind shear is a meteorological phenomenon in which wind velocity increases with the

increased height above the ground. The impact of height on the magnitude of the wind velocity

is related to surface roughness, and the wind profiles can be predicted using the following

Heckmann power equation:1,27

uðzÞ¼ uðz0Þz

z0

� �a

; (1)

where z is the height above the ground, z0 is the reference height for which wind speed u(z0) is

known, and a is the wind shear coefficient. In practice, a depends on a number of factors,

including the roughness of the surrounding landscape, height above the ground, time of day,

season, and locations.28 In the present research, a wind shear exponent of 0.2, which is the av-

erage value discussed in wind turbine design guideline, was used in all simulation cases.

B. Fluid-structure interaction analysis model

Blade element theory (BET) is an analytical method for predicting aerodynamic forces at

each blade section. Momentum theory denotes a control volume analysis of the blade loads

based on the conservation of the linear and angular momentum. The results of these two meth-

ods can be coupled with strip theory. The fundamental concept of the BEM method is to equal-

ize the linear and angular momentum changes of the masses flowing through the rotor plane

with the axial load and torque generated on the rotor blades. This equilibrium is accomplished

by considering the flow through annular strips of width and the aerodynamic loads on blade ele-

ments of the same width. Then, it is possible to compute the aerodynamic forces and pitching

moment for different conditions of wind speed, rotor speed, and collective pitch angle. The fac-

tors for calculating the induced velocity are defined as follows:13

a ¼ 4Ftiploss sin2/rCn

!þ 1

( )�1

; a0 ¼ 4Ftiploss sin / cos /rCt

� �� 1

� ��1

; (2)

where a and a0 denote the axial and tangential induction factors, respectively, and Cn and Ct

are the force coefficients in normal and tangential directions, respectively. The term r is the

blade solidity, which is defined as the ratio of blade area to rotor disk area (r¼Nb � c(r)/2pr,

023126-3 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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where Nb is the number of blades), and the term / is the angle between the rotor plane and the

resultant velocity, as shown in Fig. 1. The tip-loss factor developed by Prandtl was employed

to consider the impact of the discrete number of the blades, and the formulation can be

expressed as follows:

Ftiploss ¼2

pcos�1 exp �Nb

2

R� r

r sin /

� �� �; (3)

where the term R is the blade radius and r is an arbitrary position of the blade. Then, the effec-

tive angle of attack at the each blade section can be calculated as follows:

aeffective ¼ hflow � htotal ¼ tan�11� að ÞVwind cos /yaw

1þ a0ð ÞXr� Vwind sin /yaw

� �cos w

( )� htotal; (4)

where

htotal ¼ htwist þ hpitch þ hdeformed:

The term Vwind denotes the mean wind velocity; /yaw, the yaw angle; X, the rotor speed; W,

the azimuthal angle of the blade; htwist, the structural twist angle; hpitch, the collective pitch

angle; and hdeformed, the deformed angle caused by the torsional elastic motion.

By using a look-up table method of experimentally determined lift coefficient (Cl), drag

coefficient (Cd), and moment coefficient (Cm) data (as a function of the Reynolds number and

angle of attack), the aerodynamic forces in axial direction (Fu), tangential direction (Fv), normal

direction (Fw), and pitching moment (Mh) for each blade station can be estimated as follows:

Fu ¼ � Fgrav sin htotal cos w; (5)

Fv ¼1

2qCnV2

relcþ Fgrav sin htotal sin w ¼ 1

2qV2

relc

� �2

Cl cos /þ Cd sin /ð Þ þ Fgrav sin htotal sin w;

(6)

Fw ¼1

2qCtV

2relcþ Fgrav cos htotal sin w ¼ 1

2qV2

relc

� �2

Cl sin /� Cd cos /ð Þ � Fgrav cos htotal sin w;

(7)

Mh ¼1

2qCmV2

relc2; (8)

where the term q denotes the air density, Vrel is the resultant velocity, c is the chord length of

the blade, and Fgrav stands for the gravitational force (Fgrav¼m � g, where m is the mass of unit

FIG. 1. The local normal and tangential loads on a wind turbine blade.

023126-4 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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length, and g denotes the acceleration due to gravity). In order to apply the BEM method to the

horizontal axis wind turbine blades, some correction factors should be introduced into the simu-

lation process. The tip vortices generate multiple helical structures in the wake, which have a

large impact on the induced velocity distribution;30 but the BEM method cannot account for the

impact of vortices being shed from the blade tips into the wake.30 Thus, a tip-loss correction

model developed by Prandtl was applied to compensate for the deficiency of the induced veloc-

ity field. Yawed conditions can produce a skewed wake behind the rotor plane; therefore, the

BEM method requires correction to account for this skewed wake effect.20,31 Furthermore, the

aerodynamic forces for the BEM method demand correction to account for viscous effects. This

is accomplished by the Du-Selig dynamic stall model. Finally, the rotational augmentation cor-

rections for three-dimensional delayed stall were applied using AirfoilPrep,32 which uses the

Selig and Eggers methods to modify the lift and drag coefficients of the rotating blade.

In the present study, the BEM method is coupled with the ABAQUS/standard program to

examine the aeroelastic blade behaviors of large-scale wind turbines. State-of-art wind turbine

blades are generally not simple to model due to the distribution of anisotropic material proper-

ties and the complexity of their cross-section. Although these complex wind turbine blades can

be modeled accurately using complete finite element methods, they are too detailed for fluid-

structure interaction analysis. Thus, simplified beam model based on a series of equivalent

beam elements (the beam elements in ABAQUS that use linear and quadratic interpolation, i.e.,

element type B31) is used for aeroelastic simulation. Strong coupling, which means that the

aerodynamic loads (Fu, Fv, Fw, Mf, Mb, Mh) and structural displacements (u, v, w, f, b, h) are

exchanged at each time step, is employed for a fluid-structure interaction analysis as presented

in Fig. 2. The coupling schemes can be classified by the type and order of the interaction

method used for the aerodynamic solver (i.e., fluid) and the structural solver (i.e., structure).33

In this study, a first order implicit–explicit coupling scheme, which fundamentally consists of

the two steps, was employed.

As seen in Fig. 3, the phenomena by feeding back changes in structural geometry due to

elastic deformation of the blade is coupled with the aerodynamic solver to re-predict the aero-

dynamic loads. The discrete equation of motion at time iþ 1 for the horizontal axis wind tur-

bine blade is defined as follows:34

M½ � €uf giþ1 þ C½ � _uf giþ1 þ fintf giþ1 � fextf giþ1 ¼ rf giþ1 ¼ 0; (9)

where [M] and [C] denote the mass and damping matrices, respectively, and vector {r}iþ1 is

the residual, or out-of-balance force, which is zero when equilibrium is satisfied. By subtracting

from Eq. (9), the equivalent equation of motion at time i, an incremental form is obtained as

follows:

FIG. 2. The process of fluid-structure interaction analysis of wind turbine blades.

023126-5 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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M½ � D€uf gi þ C½ � D _uf gi þ Dfintf gi � Dfextf gi ¼ Drf gi ¼ 0: (10)

By a first-order linearization of the internal force and external force, we can obtain

Dfintf gi ¼@fint

@u

� i

Duf gi ¼ Kint½ �i Duf g; (11)

Dfextf gi ¼@fext

@u

� i

Duf gi ¼ Kext½ �i Duf g; (12)

where [Kint] denotes the consistent tangent stiffness and [Kext] stands for the load stiffness.

Inserting these linearized increments into Eq. (10) gives an equation where only the accelera-

tions and velocities at iþ 1 are unknown remains as follows:

M½ � D€uf gi þ C½ � D _uf gi þ Kint½ � � Kext½ �ð Þ Duf gi ¼ Drf gi: (13)

In a nonlinear context, [Kint] and [Kext] are in general functions of the displacement.

Equivalently, the gradients of the internal force and external force need not be linear.

Therefore, due to the linearization in Eqs. (11) and (12), the equilibrium equation at time iþ 1

is no longer exactly satisfied, giving a non-zero residual {r}iþ1. Details on these formulations

are well documented in the literature.34,35 In the present study, the discrete equation of motion,

as presented in Eq. (9), can be solved using the FE software ABAQUS/standard. The aeroelas-

tic model based on the ABAQUS-BEM coupled method is employed to predict the blade defor-

mations of the wind turbines at different operation conditions. The ABAQUS/standard program

is a powerful tool, especially for a complicated and flexible blade structure, because this tool

can offer several relevant modeling options.36 First, the slender and flexible wind turbine blade

is expected to experience the large deformations, contains structural couplings, and could have

a changing tangential stiffness matrix for nonlinear analysis;36 and second, an integrated Python

application programming interface (API) allows the user to completely manipulate ABAQUS

models and initiate analysis from the ABAQUS Python script command line. Thus, a Python

script can act as the bonding element between ABAQUS/standard program and any other code

or program, such as MATLAB.36,37

III. ANALYSIS MODEL: NREL 5 MW REFERENCE WIND TURBINE (RWT)

This study uses the NREL 5 MW RWT to investigate the aeroelastic behaviors through

fluid-structure interaction analysis. The geometric parameters and operational conditions are

presented in Table I.

The operational ranges can be divided into two sections: (1) the variable speed operational

range (from cut-in wind speed of 3 m/s to rated wind speed of 11.4 m/s) and (2) the pitch-

controlled operational range (from a rated wind speed of 11.4 m/s to a cut-out wind speed of

FIG. 3. Fluid-structure coupled computational process (implicit–explicit coupling scheme).

023126-6 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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25 m/s). Details on the structural and aerodynamic properties are well documented in the previ-

ous literature.38

A. Validations

1. Dynamic structural responses of the wind turbine blade

Table II presents the six lowest natural frequencies of the NREL 5 MW RWT blade at

zero rotational speed. Comparing the natural frequencies against the BModes39 and FAST40

codes, and showing good agreement for all modes, can be seen as a validation of the dynamic

structural behavior modeling capabilities of the beam approach using the ABAQUS/standard

program. BModes, which was developed by NREL, is a finite element code, and can provide

dynamically coupled modes for a beam model.39 Also, FAST code is an assumed-modes

approach code that uses only uncoupled modes for the flapwise and edgewise degrees of free-

dom. The natural frequencies are computed by conducting an eigenvalue analysis on the first-

order state matrix created from a linearization analysis in FAST code.40

B. Steady-state blade deflections under uniform flow conditions

The fluid-structure interaction analysis was performed for the NREL 5 MW RWT blade at

a rated wind speed under uniform flow condition. At the rated wind speed of 11.4 m/s, the cor-

responding rotor speed is 12.1 rpm, and the pitch control angle is set to zero degrees. For the

structural calculations, 50 finite elements were used along the span to model the structure of

the blade. The elastic axis was assumed to be located at the quarter-chord as presented by

NREL.38 Fig. 4 shows the blade tip deformation during the coupling iterations. The results pre-

dicted that the convergence was obtained in less than coupling iterations of five.

Fig. 5 shows the blade tip deflections under normal operating conditions for wind ranges

from cut-in to cut-out wind speed. These steady-state blade deformations are nondimensional-

ized by radius of the NREL 5 MW RWT blade. The rotor speed and collective pitch angle con-

trols are based on the data given by NREL.38 The present results are compared with those

obtained by FAST-AeroDyn for the blade bending deflections. FAST code is a modal-based

code which includes neither a torsion DOF of the blade nor non-linear geometric couplings, so

this prediction ignores torsional deformations due to the aerodynamic pitching moments that

TABLE I. Main characteristics of the NREL 5 MW reference wind turbine.36

Rated power 5 MW

Number of blades 3

Rotor/hub diameter 126 m/3 m

Cut-in/rated/cut-out wind speed 3/11.4/25 m/s

Cut-in/rated rotor speed 6.9 rpm/12.1 rpm

Airfoil section DU and NACA airfoils

Basic control Variable speed, collective pitch

TABLE II. Natural frequencies for the first 6 modes at zero rotation speed (unloaded case).

Mode number BModes (Hz) FAST (Hz) Present (Hz)

1 0.69 0.68 0.673

2 1.12 1.10 1.106

3 2.00 1.94 1.926

4 4.12 4.00 3.955

5 4.64 4.43 4.427

6 5.61 5.77 5.511

023126-7 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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occur with device actuations. The blade tip deflections at the rated wind speed are about

0.090�R and 0.0097�R in the flap and edge directions, which are very similar to the existing

results from FAST-AeroDyn.38 The overall tendency is well-predicted by the present method,

demonstrating its capability.

C. Results and Discussions

1. Effects of torsional degree of freedom, geometric nonlinearity, and gravity on

aeroelastic behavior under uniform flow conditions

a. Effect of torsional degree of freedom on blade deflections. To further investigate the effect

of blade torsional behavior on blade deformation, an ABAQUS-BEM coupled analysis includ-

ing torsional DOF was performed for the wind speed ranges from 3 m/s to 25 m/s. Fig. 6 shows

the present results of the tip deflections compared with the numerical results by CFD-CSD

coupled methods41 for the flapwise, edgewise bending deflections and torsional deformation.

The blade bending deflections are nondimensionalized by the radius of the blade. Some differ-

ences of flapwise and edgewise deflections at the rated wind speed were observed between the

present analytical method which includes the factor of the torsional DOF and the present ana-

lytical method that ignores such factors, with offsets of 18.37% and 15.38%, respectively. Also,

an elastic torsional deformation of �3� (toward the feather direction) at the rated wind speed of

11.4 m/s is shown during the normal operating conditions. For this reason, the present result,

which considers the torsional DOF, can lead to a noticeable under-prediction of blade tip dis-

placement as compared to the present results that ignores the torsional DOF. In other words, it

is clear that the torsional deformation considerably affects the aeroelastic behaviors of the

blade. Comparisons were also made with the present method and the CFD-CSD coupled

FIG. 4. Blade tip displacements during coupling iteration of the fluid-structure interaction at a rated wind speed of

11.4 m/s.

FIG. 5. Steady-state blade tip displacements under uniform flow condition.

023126-8 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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method,41 because both methods take into account the torsional DOF, and good agreement was

shown between the two methods of blade bending deflections. A similarity was observed in the

result of the torsional deformation, in which a negative peak at the rated wind speed was

shown.

The radial distributions of blade-bending deflections and torsional deformation at the rated

wind speed of 11.4 m/s are presented in Fig. 7. The present results also show good agreement

with the numerical results obtained using the CFD-CSD coupled method.41

To investigate the effect of blade deformations on the aerodynamic forces, the distributions

of the normal and tangential force obtained using the ABAQUS-BEM coupled method, called

deformed blade, and the BEM only computations, called undeformed blade, were compared

with those of the CFD-CSD coupled method41 in Fig. 8. These results demonstrate that blade

deformation leads to a significant reduction in both normal and tangential aerodynamic forces.

This variation of the aerodynamic loads is directly related to the reduced effective angle of

FIG. 6. Blade tip displacements under uniform flow conditions, considering torsional DOF.

FIG. 7. Spanwise distribution of blade deflections at the rated wind speed considering torsional DOF.

023126-9 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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attack resulting from the reduction of the torsional deformation, as shown in Fig. 7.

Comparisons were also made between the results from the present method and the CFD-CSD

coupled method,41 and good agreement was shown for the aerodynamic force in the normal and

tangential directions.

b. Effect of geometric nonlinearity on the blade deflections. To examine the effect of geometric

nonlinearity on the blade deformations, a geometrical nonlinear analysis using the ABAQUS-

BEM coupled method was made under normal operating conditions. The computed results of

the nondimensional blade tip deflections, compared with other results by FAST-AeroDyn38 for

the flapwise and edgewise deflections, are presented in Fig. 9. Some discrepancies of flapwise

and edgewise deflections at the rated wind speed of 11.4 m/s between the present nonlinear

analysis and linear analysis are observed, with offsets of 8.31% (6.08% in the case of simula-

tion including the torsional DOF) and 1.41% (2.61% in the case of simulation including the tor-

sional DOF), respectively. In the edgewise deflection, because the stiffness is relatively high

and resultant deflections are small, it is clear that the difference between the linear model and

the nonlinear model is negligible.

c. Gravity effect on the blade deflections. Fig. 10 shows the nondimensional tip deflections at

a rated wind speed of 11.4 m/s, which are driven only by the periodic gravitational loading.

Results excluding the gravity effect are also presented for comparison. The mean values of the

tip displacements caused by the gravity are nearly similar to the results obtained using the sim-

ulation that ignores the gravity effect. The edgewise and flapwise motions are dominated by

gravity, which is seen as the oscillations on the scale of 4.957 s (corresponding to the rotor

FIG. 8. Spanwise distribution of aerodynamic forces at the rated wind speed considering torsional DOF.

FIG. 9. Blade tip displacements under uniform flow conditions considering geometric nonlinearity.

023126-10 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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speed of 12.1 rpm). In addition, some differences of flapwise and edgewise deflections at the

rated wind speed were observed between the present analysis with or without the consideration

of gravity effect, with offsets of 6.58% (4.75% in the case of linear analysis model) and 4.32%

(4.25% in the case of linear analysis model), respectively. These magnitudes of unsteady varia-

tions are fairly large, indicating that the unsteadiness due to the gravitational loading is signifi-

cant. The variation of edgewise deflection is nearly identical to that induced by gravity only, as

shown in Fig. 10(b), indicating that the blade behavior in the edgewise direction is mostly

affected by gravitational loading.

2. Effects of torsional degree of freedom, geometric nonlinearity, and gravity on

aeroelastic behavior under combined flow conditions of turbulence and wind shear

To achieve maximum efficiency, modern blades are lightweight and flexible with large

diameters, and are therefore more susceptible to blade failure problems, such as blade–tower

collisions. In large-scale horizontal axis wind turbines, wind conditions of turbulence and wind

shear considerably alter the effective loads on the rotor blades and thus, the blade aeroelastic

behavior. Therefore, analysis for estimating the blade responses resulting from turbulent and

sheared flow conditions should be implemented.

In this study, the fluid-structure interaction analysis using the ABAQUS-BEM coupled

method was performed under the assumption of fixed values of rotor speed and no collective

pitch control under the combined turbulent and wind shear flow conditions. The aeroelastic

model is employed to analyze the blade response to wind speed oscillations. In order to gener-

ate the turbulent flow using TurbSim,28 the Kaimal turbulence model recommended by IEC

61400–3 (Ref. 42) was applied. The mean wind speed was set to 11.4 m/s (i.e., rated wind

speed), and the turbulence intensity of 10% (Kaimal turbulence model, Class A) was used with

41-by-41 vertical and lateral grid points. Total simulation time was 600 s, and a power law

exponent of 0.2 for the sheared flow was employed in all simulation cases. Thus, the combined

turbulent and sheared flow conditions (red solid line), as presented in Fig. 11, were used for the

simulations. In other words, Fig. 11 presents that the blade tip experiences the wind speed

when the blade rotates. Also, we assumed that the blade begins to rotate from azimuth angle of

zero, where the blade was positioned at vertical up.

To investigate the effects of torsional DOF, geometric nonlinearity, and gravity on the

blade aeroelastic behaviors, the fluid-structure interaction analysis including all such effects was

performed. Figs. 12–14 show the predicted flapwise tip displacements of the NREL 5 MW

RWT blade throughout the numerical simulations. The results demonstrate how the blade inter-

acts with wind speed variations. In other words, a change in the magnitude of wind velocity

has a considerable impact on the magnitude of blade deflections. These analysis results were

compared with the baseline results, which included the torsional DOF, geometric nonlinearity,

and gravity. The numerical analysis for investigating the effect of torsional DOF on the blade

deflection was conducted, and the blade responses are shown in Fig. 12. The results predict that

FIG. 10. Blade tip displacements at the rated wind speed, with consideration of gravitational loading.

023126-11 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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relatively low flapwise tip deflections are caused by the torsional deformations. This tendency

is also observed by the histogram plot. Analytical methods including the factor of the torsional

DOF can result in different predictions of blade behavior, since the variation of aerodynamic

loads resulted from the torsion deformations affects the blade-bending displacements. Similar to

the results shown in Fig. 6, analytical methods that consider the torsional DOF may cause a no-

ticeable under-prediction of flapwise tip deflections. A power spectral density (PSD) function is

the most common method of representing the responses in the frequency domain, and is

obtained by utilizing the fast Fourier transform (FFT). This PSD simply presents the frequency

contents of the time response and is an alternative way of specifying the time history of the

blade deflection. As seen in the figures, the dominant responses are observed in the vicinity of

FIG. 11. Pure turbulent flow condition (blue dashed line) and combined turbulent and wind shear flow conditions (red solid

line) at the blade tip for a total simulation time of 600 s.

FIG. 12. Blade flapwise tip deformations under combined turbulent with sheared flow conditions, considering the torsional

DOF.

023126-12 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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FIG. 13. Blade flapwise tip deformations under combined turbulent with sheared flow conditions, considering the geomet-

ric nonlinearity.

FIG. 14. Blade flapwise tip deformations under combined turbulent with sheared flow conditions, with consideration of

gravitational loading.

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the frequencies of 0.2 Hz (1P), 0.4 Hz (2P), and 0.6 Hz (3P). These oscillations result from var-

iations in both the gravitational force and the aerodynamic loads due to the sheared flow. The

results predict that the most dominant flapwise deflection is at a 1P frequency, since the rotation

frequency of the rotor is, at most, 0.2 Hz (1/rev). Also, notable differences in the response mag-

nitudes are observed due to the effect of torsional DOF.

Figs. 13 and 14 show the impacts of geometric nonlinearity and gravity on the blade aeroe-

lastic responses. Compared to the effect of torsional DOF, these factors have a relatively small

influence on flapwise deflections. As seen in the figures, a comparison with the results of a lin-

ear model indicates that suppression of certain high-order nonlinear models can lead to a no-

ticeable under-prediction of the blade deflections due to under-predicted aerodynamic loads. For

this case, it is seen that geometric nonlinearity is more pronounced when the blade is largely

deformed. The gravity effect arises from the rotation of the rotor blade, thus exciting blade at a

frequency corresponding to the rotational frequency of the wind turbine. Similar to the results

shown in Fig. 12, the dominant response in flapwise tip displacements is at the frequency of

1P. It is also shown that these oscillations are caused by variations of loads on the blade due to

the wind flow conditions and gravity. The root-mean-square deviation (RMSD) is a commonly

employed measure of the differences between values predicted by a numerical model and the

values in the baseline case. Thus, when measuring the average difference between two time se-

ries x1,i and x2,i, the formula is defined as follows:43

RMSD ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

n

Xn

i¼1

x1;i � x2;ið Þ2s

; (14)

where n is the number of predictions. In the present study, the RMSD values were calculated

for each simulation case for comparison with the baseline result. The RMSD value of the effect

of the torsional DOF is 0.9545, whereas the RMSD values of the effects of the geometric nonli-

nearity and gravity are 0.3969 and 0.2523, respectively. In other words, the torsional DOF has

a stronger effect on flapwise blade deflection than the other factors. However, because substan-

tial differences between the blade responses are still observed, the factors of the geometric

nonlinearity and gravity cannot be ignored when attempting to accurately predict the blade

behavior of large-scale horizontal axis wind turbines.

Figs. 15–17 present the edgewise tip displacements of the NREL 5 MW RWT blade. It is

shown that the combined flow conditions of turbulence and wind shear are the primary factors

that lead to the considerable blade tip deformations in all directions. Similar to the numerical

results of the flapwise tip displacements, as presented in Figs. 12–14, the edgewise tip dis-

placements show almost irregular blade behavior. As seen in Figs. 15 and 16, small edgewise

displacements caused by torsional DOF and geometric nonlinearity are observed. However,

Fig. 17 shows that gravity most largely affects the edgewise deflections. This is because gravi-

tational force tends to dominate edgewise loading on the blade. The results in the frequency

domain predicted that the dominant responses are in the vicinity of the frequencies of 0.2 Hz

(1P), 0.4 Hz (2P), and 0.6 Hz (3P). Also, the dominant response in edgewise tip displacements

is shown to be at a 1P frequency, since the oscillations result from the variations of the flow

conditions and the gravitational force. The RMSD obtained by gravity is relatively high com-

pared to the RMSD obtained by the other factors. Therefore, a conclusion can be made that

edgewise blade motion is primarily driven by gravity, yet nearly unaffected by geometric

nonlinearity.

Figs. 18 and 19 show the torsional tip deformations of the wind turbine blade under com-

bined turbulent and sheared flow conditions. The results predict that the torsional tip deforma-

tions have almost periodic behavior. Similar to the edgewise tip displacements, as shown in

Figs. 15–17, gravitational force most greatly affects torsional motion. The results also demon-

strate that geometric nonlinearity fairly influences the torsional tip deformations. The RMSD

value observed for gravity is relatively high compared to the RMSD values for the other fac-

tors. This is because gravity has a large effect on torsional blade motion as well as on edgewise

023126-14 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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FIG. 16. Blade edgewise tip deformations under combined turbulent with sheared flow conditions, considering the geomet-

ric nonlinearity.

FIG. 15. Blade edgewise tip deformations under combined turbulent with sheared flow conditions, considering the torsional

DOF.

023126-15 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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FIG. 17. Blade edgewise tip deformations under combined turbulent with sheared flow conditions, with consideration of

gravitational loading.

FIG. 18. Blade torsional tip deformations under combined turbulent with sheared flow conditions, considering the geomet-

ric nonlinearity.

023126-16 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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motion. As a result of the introduction of gravity, the amplitudes of torsional deformations

become significantly higher than the deformations caused by geometric nonlinearity.

The main contribution of this study is the implementation of a fluid-structure interaction

analysis of a large-scale wind turbine blade that considers the effects of the torsional DOF, geo-

metric nonlinearity, and gravity. In each simulation of the given conditions, a fluid-structure

interaction analysis is performed on the NREL 5 MW reference wind turbine blade. The values

of the RMSD, as presented in Table III, are calculated by comparing the present results with

the baseline result, and to examine the interaction of each factor with the blade tip displace-

ments. As discussed above, a conclusion is drawn that the effect of the torsional DOF is the

main contributor to blade motion in the flapwise direction, while gravity most greatly affects

the blade displacements in the edgewise and torsional directions.

FIG. 19. Blade torsional tip deformations under combined turbulent with sheared flow conditions, with consideration of

gravitational loading.

TABLE III. RMSD values of the numerical predictions under combined flow conditions of turbulence (turbulent intensity

of 10%) and wind shear (power law exponent of 0.2).

Direction Considered effect Root-mean-square deviation

Flapwise tip displacements Torsional DOF 0.9548

Geometric nonlinearity 0.3969

Gravity 0.2523

Edgewise tip displacements Torsional DOF 0.1271

Geometric nonlinearity 0.0239

Gravity 0.2612

Torsional tip displacements Torsional DOF n/a

Geometric nonlinearity 0.1696

Gravity 0.3738

023126-17 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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IV. CONCLUSIONS

State-of-the-art horizontal axis wind turbines use extremely long and flexible blades that

can undergo large blade deformations. For this reason, accurately predicting blade behavior is

important to avoid blade failures, e.g., blade–tower collisions. In this study, the influences of

torsional DOF, geometric nonlinearity, and gravity in a fluid-structure interaction analysis on

the aeroelastic behavior of a large-scale wind turbine blade have been investigated under uni-

form flow conditions and under combined flow conditions of turbulence and wind shear. The

results predicted that the flapwise tip deformations are mostly influenced by torsional DOF,

which is mainly driven by coupling to torsional elastic deformations. Also, it is shown that

gravity is the greatest influence on edgewise and torsional tip deformations, whereas the other

factors have less impact on the blade deformations in these directions. However, because no-

ticeable differences are still observed as a result of geometric nonlinearity, as well as the fac-

tors of the torsional DOF and gravity, none of these factors can be ignored when attempting to

accurately predict the aeroelastic behavior of large-scale horizontal axis wind turbine blades.

Therefore, the results from this study suggest that torsional DOF, geometric nonlinearity, and

gravity should be considered in the fluid-structure interaction analysis for the design of flexible

and slender wind turbine blades.

ACKNOWLEDGMENTS

This research was supported by WCU (World Class University) program through the National

Research Foundation of Korea, funded by the Ministry of Education, Science and Technology

(R31-2008-000-10045-0). The authors are grateful for this support.

1W. Tong, Wind power generation and wind turbine design (Kollmorgen Corporation, WIT Press, Boston, 2010).2D. G. Wilson, D. E. Berg, B. R. Resor, M. F. Barone, and J. C. Berg, in AWEA Windpower 2009 Conference &Exhibition, Chicago, Illinois, USA, 4–7 May 2009.

3B. S. Kallesøe, Wind Energy 14, 209 (2011).4O. Passon, M. Kuhn, S. Butterfield, J. Jonkman, T. Camp, and T. J. Larsen, in EAWE Special Topic Conference: TheScience of Making Torque from Wind (Lyngby, Denmark, 2007), pp. 28–31.

5Garrad Hassan & Partners, Bladed Theory Manual Version 4.1 Multibody Dynamics (St. Vincent’s works, Silverstonelane, Bristol, Garrad Hassan and Partners Ltd., 2011).

6T. J. Larsen and A. M. Hansen, Risø DTU, Document No. Risø-R-1597 (ver. 3-1), Roskilde, Denmark, 2007.7R. Rubak and J. T. Petersen, in Proceedings of Copenhagen Offshore Wind, Denmark, 26–28 October 2005.8M. L. Buhl, A. D. Wright, and K. G. Pierce, National Renewable Energy Laboratory, Document No. NREL/CP-500-28848, USA, 2001.

9T. Hald and M. Høgedal, Proceedings of Copenhagen Offshore Wind, (Denmark, Oct. 2005).10C. Lindenburg, Energy Research Centre of Netherlands, Document No. ECN-I-05-005, The Netherlands, 2005.11T. G. Engelen, in Proceedings of European Wind Conference, Milan, Italy, 2007.12J. M. Jonkman and M. L. Buhl, National Renewable Energy Laboratory, Document No. NREL/EL-500-38230, USA,

2005.13M. S. Jeong, I. Lee, S. J. Yoo, and K. C. Park, Energies 6, 2242 (2013).14T. Kim, A. M. Hansen, and K. Branner, Renewable Energy 59, 172 (2013).15J. W. Larsen and S. R. K. Nielsen, Int. J. Nonlinear Mech. 41, 629 (2006).16D. Mohammad and A. Samir, Mech. Res. Commun. 32, 692 (2005).17E. S. Politis and V. A. Riziotis, The importance of nonlinear effects identified by aerodynamic and aeroelastic simulations

on the 5MW reference wind turbine (Center for Renewable Energy Sources, Pikermi, Greece, 2007).18B. S. Kallesøe and M. H. Hansen, Risø DTU, Document No. Risø-R-1642 (EN), Roskilde, Denmark, 2008.19V. A. Riziotis, S. G. Voutsinas, E. S. Politis et al., in Proceedings of the EWEC 2008 Scientific Track, Brussels, Belgium,

2008.20J. G. Leishman, Principles of helicopter aerodynamics (Cambridge University Press, Cambridge, 2006).21J. W. Larsen, Nonlinear dynamics of wind turbine wings (Aalborg University, Denmark, 2005).22G. Yuan and Y. Chen, Comput. Struct. Eng. 2009, 521.23M. L. Buhl and A. Manjock, National Renewable Energy Laboratory, Document No. NREL/CP-500-39113, USA, 2006.24Y. Guo, J. Keller, and W. LaCava, National Renewable Energy Laboratory, Document No. NREL/CP-5000-55968, USA,

2012.25Y. Bazilevs, M. C. Hsu, J. Kiendl et al., Int. J. Numer. Methods Fluids 65, 236 (2011).26D. O. Yu, H. I. Kwon, and O. J. Kwon, “50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and

Aerospace Exposition,” AIAA Paper No. 2012-1292, 2012.27International Standard IEC 61400-1, International Electrotechnical Commission, 2005.28B. J. Jonkman and M. L. Buhl, National Renewable Energy Laboratory, Document No. NREL/TP-500-39797, USA,

2006.

023126-18 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

143.248.103.56 On: Thu, 11 Sep 2014 00:22:15

29T. Burton, D. Sharpe, N. Jenkins, and E. Bossanyi, Theoretical prediction of dynamic inflow derivatives (John Wiley &Sons, Chichester, 2001).

30O. L. H. Martin, Earthscan (Sterling, 2008).31D. M. Pitt and D. A. Peters, “Theoretical prediction of dynamic inflow derivatives,” Vertica 5, 21 (1981).32See http://wind.nrel.gov/designcodes/preprocessors/airfoilprep/ for NWTC Computer-Aided Engineering Tools

(AirfoilPrep by Dr. Craig Hansen).33M. A. Fernandez, SeMa J. 55, 59 (2011).34Y. L. Young, J. Fluids Struct. 24, 799 (2008).35Y. L. Young, J. Fluids Struct. 23, 269 (2007).36Abaqus software, version 6.7, Abaqus, Inc., Rhode Island, 2007.37D. Verelst, Graduation Report, Delft University of Technology, 2009.38J. Jonkman, S. Butterfield, W. Musial, and G. Scott, National Renewable Energy Laboratory, Document No. NREL/TP-

500-38060, USA, 2009.39See http://wind.nrel.gov/designcodes/preprocessors/bmodes/ for NWTC Design Codes (BModes by Gunjit Bir).40See http://wind.nrel.gov/designcodes/simulators/fast/ for NWTC Design Codes (FAST by Jason Jonkman, Ph.D.).41D. O. Yu and O. J. Kwon, “51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace

Exposition,” AIAA Paper No. 2013-0911, 2013.42International Standard IEC 61400-3, International Electrotechnical Commission, 2009.43R. J. Hyndman and A. B. Koehler, Int. J. Forecasting 22, 679 (2006).

023126-19 Jeong et al. J. Renewable Sustainable Energy 6, 023126 (2014)

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