ASME 2002, Reno, 14-17 January

VIBRATIONS OF A THREE-BLADED WIND TURBINE ROTOR

DUE TO CLASSICAL FLUTTER

Morten Hartvig HansenWind Energy DepartmentRisø National Laboratorymorten.hansen@risoe.dk

ASME 2002, Reno, 14-17 January

Outline

• Motivation• Stall-induced vibrations versus classical flutter

• Turbine Model• Description and structural analysis

• Blade Flutter• Stability limits

• Effect of flapwise discretization

• Turbine Flutter• Stability limits

• Comparison with blade flutter

• Visualization of flutter mode

ASME 2002, Reno, 14-17 January

MotivationPost-design solution to stall-induced vibrations

• Softening of stall

Primary design solution to classical flutter

• Center of mass towards the leading

edge

• High torsional stiffness

• Improved turbine design??

ASME 2002, Reno, 14-17 January

Turbine model• 3N blade DOFs and 7 tower/nacelle DOFs• Center of mass and elastic axis at mid-chord• Blade Element Momentum theory• Quasi-steady aerodynamics, and no turbulence or shear

ASME 2002, Reno, 14-17 January

Campbell diagramsBasic model configuration: 1 flap (1.14 Hz), 1 lag (1.46 Hz), and 1 torsion (16.0 Hz) mode.

low range high range

ASME 2002, Reno, 14-17 January

Operation conditionsVariable speed and pitch turbine

ASME 2002, Reno, 14-17 January

Aerodynamic conditions in steady state

= Attached flow conditions

ASME 2002, Reno, 14-17 January

Single blade flutterAeroelastic damping of torsion mode in basic model configuration: 1 flap, 1 lag, and 1 torsion mode.

ASME 2002, Reno, 14-17 January

Single blade flutter

Pitching and flapping motion at 75 % radius, wind speed 20 m/s, andtorsion frequency 8.5 Hz.

ASME 2002, Reno, 14-17 January

Stability limits for blade flutter

Effect of the discretization of flapwise blade motion

ASME 2002, Reno, 14-17 January

Damping of blade torsion on turbine

Basic model configuration with original torsion frequency of 16 Hz

ASME 2002, Reno, 14-17 January

Comparison of flutter limitsThe critical torsion frequency is higher for turbine flutter!

ASME 2002, Reno, 14-17 January

Flutter motion

Pitching and flapping motion at 75 % radius, wind speed 20 m/s, andtorsion frequency 8.5 Hz.

ASME 2002, Reno, 14-17 January

Flutter whirling amplitudes

ASME 2002, Reno, 14-17 January

Conclusion

• Structural dynamics of turbines is important• Affects the risk of flutter (and stall-induced vibrations)

• Flutter analysis must include these effects

• Blade-only analysis is not conservative

• Flutter may become a problem for large turbines

Future

• Inclusion of unsteady aerodynamics

• Optimization of turbine dynamics

• Complete stability and optimization tool