Advanced Aeroelastic Modeling of Swept Rotor Blades

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Advanced Aeroelastic Modeling of Swept Rotor
Blades
Vasilis A. Riziotis , Dimitris I. Manolas, Spyros
G. Voutsinas National Technical University of
Athens School of Mechanical Engineering Fluids
Section
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Rationale of sweep

• Sweeping of blades aims at reducing loads
• Sweeping activates flap-torsion coupling which
  can be very beneficial in mitigating loads
• Flap-torsion coupling is also possible by
  structurally tailoring the blade (Sandia Lab)
• In aerodynamic terms, as the outer part of the
  blade bends it also twists giving lower angles of
  attack and therefore lower aerodynamic loads
• Load reduction is always important due to its
  direct impact on the cost of energy (e.g.
  lowering the loads allows the increase of rotor
  diameter for the same given strength)

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Modelling issues structural part
Non-linear beam model
Ze the pre-sweep
a swept blade twists when it flaps
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Modelling issues structural part
Bending-torsion coupling on pre-swept blades

• In case of large bending deflections additional
  non-linear terms will become significant in
  torsion moment equation

Including the effect of blade sweep more terms
will appear related to ze
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Modelling issues structural part

• aft sweep (4.5 m tip deflection)

1st flapwise
2nd flapwise
a flap-torsion coupling appears in all flapwise
modes
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Modelling issues aerodynamic part
inboard vortices are shed ahead of those at tip
inducing an up-wash
aerodynamic analysis of the deformed blade
geometry non linear aeroelastic coupling
GENUVP free wake code
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Results
sweep geometry defined in UPWIND
project different tip offsets ranging from 1m-6m
are analyzed
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Results
comparison of BEM against free wake for straight
blade
U8 m/s
tangential force dist. (Nt/m)
normal force dist. (Nt/m)
GAST - BEM
axial induced velocity (m/s)
GENUVP - free wake code
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Results
comparison of BEM against free wake for swept
blade
U8 m/s, b2
Increase of loading towards the tip Lower loads
in more inboard sections
normal force dist. (Nt/m)
Increasing tip sweep
tangential force dist. (Nt/m)
GAST
GENUVP
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Results
comparison of BEM against free wake for swept
blade
U8 m/s, b4
Similar behavior but larger effect for higher
curvature BEM computations are expected to over
predict power
Increasing tip sweep
GAST
GENUVP
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Results
U8m/s Free-wake simulations Angle of attack
distributions for swept blade

• moderate tip offset (a3) affects little the
  a.o.a except at the tip region
• increasing a and b the complete blade is
  affected

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Results
comparison of BEM against free wake for swept
blade aerodynamic analysis results
U8 m/s
Aerodynamic Power variation wrt straight blade
BEM
Free W
power scaled for the same blade length
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Results
comparison of BEM against free wake for swept
blade aeroelastic analysis results
GAST
U8 m/s, b2
GENUVP
GAST
GENUVP
U8 m/s, b4
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Results
comparison of BEM against free wake for swept
blade aeroelastic analysis results
U8 m/s
Aerodynamic Power variation wrt straight blade
BEM
Free W
power scaled for the same blade length
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Conclusions

• Blade sweep activates flapwise bending/torsion
  coupling
• Aft sweeping gives rise to nose down torsion
  deformation and potentially reduces flapwise
  loads
• Reduction in loads is accompanied by a reduction
  in power
• Comparing BEM based against free-wake aeroelastic
  simulations indicates that BEM models
  underestimate power loss.
• As expected BEM cannot properly account for the
  near wake induced effects driven by skewed shape
  of the tip of the blade
• Power loss increases with blade curvature (b
  parameter) and tip offset (a parameter)

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Acknowledgements

• This work was partly funded by the European
  Commission under contract SES6 019945 (UpWind
  Integrated Project).

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Thanks for your attention END