Aeroelastic stability analysis and passive instability suppression

1
Aeroelastic stability analysis andpassive
instability suppression

• EWEC 2006 Athens
• Thomas Buhl, Helena Markou,
• Morten H. Hansen, Kenneth Thomsen
• and Flemming Rasmussen

Speaker
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Stability Mechanisms

• Effect of flap/edgewise frequency coincidence
• Effect of flap/edgewise whirling coupling on
  damping
• Effect of torsional stiffness on damping
• Can whirl-flutter happen on a wind turbine?

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Effect of flap/edgewise frequency coincidence
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Effect of flap/edgewise frequency coincidence

• ASR turbine
• Isolated blade analysis
• Edge frequency lowered towards the flap frequency
• Four intervals
• 25
• 50
• 75
• 100

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Effect of flap/edgewise frequency coincidence
Flapwise mode

• Intermediate stiffness reductions gives virtually
  no change
• Full coincidence
• Decreases damping below 21 m/s

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Effect of flap/edgewise frequency coincidence
Flapwise mode

• Intermediate stiffness reductions gives virtually
  no change
• Full coincidence
• Decreases damping below 21 m/s
• Increases damping above 21 m/s

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Effect of flap/edgewise frequency coincidence
Edgewise mode

• Intermediate stiffness reductions gives virtually
  no change
• Full coincidence
• Increases damping below 19 m/s

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Effect of flap/edgewise frequency coincidence
Edgewise mode

• Intermediate stiffness reductions gives virtually
  no change
• Full coincidence
• Increases damping below 19 m/s
• Decreases damping above 19 m/s

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Effect of flap/edgewise frequency coincidence

• Sectional work

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Effect of flap/edgewise frequency coincidence
Mode shape at 22 m/s
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Effect of flap/edgewise whirling coupling on
damping
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Effect of flap/edgewise whirling coupling on
damping
Edgewise mode

• ASR turbine
• Full turbine analysis
• 100 coincidence increases edgewise damping

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Effect of flap/edgewise whirling coupling on
damping
Flapwise mode

• 100 coincidence decreases flapwise damping
• Negative damped above 22 m/s

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Effect of flap/edgewise whirling coupling on
damping
Original ASR
Shaft reduced to 2 of ori.
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Effect of flap/edgewise whirling coupling on
damping
1st edgewise
1st flapwise
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Effect of flap/edgewise whirling coupling on
damping
1st edgewise
Bladetip trace
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Effect of torsional stiffness on damping
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Effect of torsional stiffness on damping
PRVS turbine Torsional frequency reduced from
10.3 Hz to 3.8 Hz
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Effect of torsional stiffness on damping
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Effect of torsional stiffness on damping
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Effect of torsional stiffness on damping
Tip speed 130 m/s (31 rpm)
Flap modes
Torsion
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Effect of torsional stiffness on damping
1st edge
Sectional work
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Can whirl-flutter happen on a wind turbine?
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Can whirl-flutter happen on a wind turbine?
PRVS - Tower top element reduced in stiffness
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Conclusions

• ASR An isolated blade analysis of flap-edgewise
  frequency coincidence showed that the damping
  could be decreased and result in instabilities.
• ASR The coupling of the flap/edgewise whirling
  modes can lead to increased damping of the
  edgewise mode when the modes couple at
  standstill.
• PRVS The critical relative wind speed for which
  flutter occurs is 130.75m/s when the torsional
  stiffness is reduced to 20 of the original.
• PRVS The reduction in torsional stiffness leads
  to negative damping of the first edgewise mode
• PRVS Whirl flutter was found when the tower top
  stiffness in the yaw and tilt directions was
  reduced to 1.5 of the original or less.