NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Swept Blade Aero-Elastic Model for 

a Small Wind Turbine

Rick Damiani and Sang Lee

National Renewable Energy Laboratory

Scott Larwood

University of Pacifica

National Solar Conference

July 7, 2014

San Francisco, California

NREL/ PR-5000-62337
2

Acknowledgments

• U.S. Department of Energy (DOE) – For Competitiveness 

Improvement Project (CIP) funds and computer-aided 

engineering (CAE) tools funds

• Patrick Moriarty and Scott Schreck from NREL for helpful 

discussions
3

Outline

• Background on optimized/advanced rotors

• Numerical modeling tools

• Case study

• Summary of results
4

• Sweep introduces bend/twist coupling that can 

reduce loads at fixed rotor diameter size 

OR

• Increase rotor diameter at same loading level

• Reduce material

• Optimize utilization

• Reduce the levelized cost of energy (LCOE)

Need for Advanced Rotors
5

• MSC ADAMS – Multibody Dynamics:

o Commercial product

o Can be coupled to Aerodyn (FAST module) –

o FAST has a translator to convert FAST input to ADAMS input

o Highly customizable with user’s controller library (requires a bit of 

programming)

o Highly flexible – virtually any horizontal axis wind turbines

o High fidelity – as many degrees of freedom as the user requires

CAE Tools: FAST and ADAMS

Jonkman, J.; Buhl, M. (2005). “FAST User’s Guide,” 

NREL/EL-500-38230. Golden, CO: National 

Renewable Energy Laboratory.
6

• Original preprocessor would use the sheared approach:

o Leads to inaccurate representation of aerodynamic and inertial loads 

• New preprocessor follows curved properties of blade elements:

o Higher fidelity and physically consistent results

o Improved accuracy

FAST2ADAMS

Larwood, S. (2009). “Dynamic Analysis Tool Development 

for Advanced Geometry Wind Turbine Blades,” U.C. Davis 

Ph.D. Dissertation.
7

• FAST input model of a small wind turbine converted to ADAMS model using 

FAST2ADAMS

• NREL Small Wind Research Turbine (SWRT) baseline

• Design load cases (DLCs): 1.1 from IEC 61400-2

o Normal Turbulence Model

o Wind Speed = 8–16 m/s

• Five configurations: baseline and 0, 5, 10, 15, and 20 degree sweep angles

• Study blade and tower moments

o Maximum blade and tower loads (max in 10 minutes)

o Blade tip clearance

o Damage equivalent loads (fatigue) – blades and tower

Case Study

70% radius

ɸ Pitch axis
8

Blade configurations

baseline 5 deg 10 deg

15 deg 20 deg
9

• Time series 0 deg and 20 deg sweep at 14 m/s wind

Bending Moments

U: streamwise velocity Nac Yaw:Nacelle Yaw 

Bld OOP: blade root out-of-plane bending moment Twr Pitch Mom: tower pitch moment
10

• The mean power has not shown more than 5% 

variation across the numerous configurations

Note: This finding needs to be revised as the 

aerodynamic model does not include cross-flow 

effects 

• Furling impact on same order or greater than sweep 

impact

• Furling standard deviation slightly reduces with sweep

Power and Furling
11

• Blade-tip twist 

• 20 deg sweep doubles the twist at the blade tip

Maximum Blade Twist Angle from DLC 1.1

0

1

2

3

4

5

6

7

8

Load Cases

Blade Tip Twist Magnitudes [deg]

DLC20degDLC15deg

DLC10deg

DLC05deg

DLC00deg

TipRDzc1

TipRDzc2

TipRDzc3
12

0

0.05

0.1

0.15

0.2

0.25

Load Cases

Minimum Tower Clearance [m]

DLC20deg

DLC15deg

DLC10deg

DLC05deg

DLC00deg

TipClrnc1

TipClrnc2

TipClrnc3

• Blade root bending moment

• 20 deg sweep mitigates maximum 

blade moment by ~15%

• 10 deg sweep increases maximum 

blade moment by ~15%

Maximum blade load and Tower Clearance Statistics from DLC 1.1

0

1

2

3

4

5

6

7

Load Cases

Blade Moment Magnitudes [kNm]

DLC20degDLC15deg

DLC10deg

DLC05deg

DLC00deg

RootM1

RootM2

RootM3

Tower clearance almost unaffected
13

• Tower-base bending moments

• Twenty degree sweep mitigates max tower moment by 

~15%

• Five degree sweep increases maximum tower moment by 

~10%

Maximum tower Load from DLC 1.1

0

50

100

150

200

250

300

350

400

450

500

Load Cases

Tower Moment Magnitudes [kNm]

DLC20degDLC15degDLC10degDLC05degDLC00deg

TwrBsM

TwrBsMzt
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• Blade root bending moments

• Twenty degree sweep mitigates damage equivalent loads 

(DELs) by ~15% (flap) to 20% (edge)

• Ten degree sweep increases DELs by ~5% (edge)

Fatigue Load Statistics from DLC 1.1

0

0.2

0.4

0.6

0.8

1

1.2

05 deg 10 deg 15 deg 20 deg

DEL /DEL@0deg

Root-bending DEL ratios with respect to 

the baseline (m=10)

RootMxb1

RootMxb2

RootMxb3

RootMyb1

RootMyb2

RootMyb3
15

• Yaw-bearing bending moments DELs reduced (shaft life)

• Tower-base moments DELs reduced by ~20%

• Twenty degree sweep mitigates DELs by ~20%

Case Study – Fatigue Load Statistics from DLC 1.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

05 deg 10 deg 15 deg 20 deg

DEL/DEL@0deg

Yaw-bearing DEL ratios with respect 

to the baseline

YawBrMxp m=3

YawBrMxp m=4

YawBrMxp m=5

YawBrMyp m=3

YawBrMyp m=4

YawBrMyp m=5

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

05 deg 10 deg 15 deg 20 deg

DEL/DEL@0deg

Tower-base DEL ratios with respect 

to the baseline

TwrBsMxt m=3

TwrBsMxt m=4

TwrBsMxt m=5

TwrBsMyt m=3

TwrBsMyt m=4

TwrBsMyt m=5
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Conclusions

• Preliminary study – further investigation needed

• A new FAST2ADAMS preprocessor is now available for FAST 7.02 that allows for 

higher fidelity representation of presweep and bend of blades

• A case study on a small wind turbine revealed the capabilities of simulating the 

effect of blade sweep using ADAMS

• Results show promising effects in terms of load mitigation: 

• Max. loads decrease by 15% at 20 deg sweep (likely 10%–15%)

• DELs decrease by 20% at 20 deg sweep (likely 15%–20%)

• Little effect on power production (same rotor area) to be revised with 

improved aerodynamic modeling and possibly eliminating furling

• Use sweep to reduce costs by

• growing the rotor size and/or

• thinning the blade structure

• Use caution in selecting amount of sweep, as some angles may create 

nonlinear effects on the dynamics of the machine and increase loads
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

For More Information

Rick.Damiani@nrel.gov

Sang.Lee@nrel.gov