European Wind Energy Conference and Exhibition

2011Brussels, Belgium

EWEC 2011 Brussels 14-17 March 2011

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

EWEC 2011 Brussels 14-17 March 2011 2

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)

EWEC 2011 Brussels 14-17 March 2011 3

x

z

y

η0, s

ξ0

ξ

ζ0

ζη, s

ze(s)

v(s)

u(s)

w(s)+ze(s)

undeformed

deformed

0 u(s)

y(s) v(s) u ,w , 0

0 w(s)

r = E( )

ez s non-linear

Modelling issues: structural part

Ze: the pre-sweep

s

e

0

ˆ(s) (s) u (s) (w (s) z (s)) ds a swept blade twists when it flaps

Non-linear beam model

EWEC 2011 Brussels 14-17 March 2011 4

Modelling issues: structural part

Bending-torsion coupling on pre-swept blades

2 2t

t

1 ˆEI EI sin(2 ( )) u w2

ˆEI EI cos(2 ( )) u w

2t e

2t t t t e

t e

t t t t e

t

1 ˆEI EI sin(2 ( )) z2

ˆ ˆEI cos sin( ) EI sin cos( ) z

ˆEI EI cos(2 ( )) u z

ˆ ˆEI cos cos( ) EI sin sin( ) u z

ˆEI EI sin(2 ( )) w

e

t t t t e

z

ˆ ˆEI cos sin( ) EI sin cos( ) w z

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:

EWEC 2011 Brussels 14-17 March 2011 5

a flap-torsion coupling appears in all flapwise modes

aft sweep (4.5 m tip deflection)

1st flapwise 2nd flapwise

Modelling issues: structural part

EWEC 2011 Brussels 14-17 March 2011 6

Modelling issues: aerodynamic part

aerodynamic analysis of the deformed blade geometry –

non linear aeroelastic coupling

inboard vortices are shed ahead of those at tip inducing an up-wash

GENUVP free wake code

EWEC 2011 Brussels 14-17 March 2011 7

Results

sweep geometry defined in UPWIND project

different tip offsets ranging from 1m-6m are analyzed

b

0e

tip 0

r rz a

r r

a 3mb 2

a 3mb 4

EWEC 2011 Brussels 14-17 March 2011 8

Results

comparison of BEM against free wake for straight blade

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 10 20 30 40 50 60 70

bla

de

no

rma

l fo

rce

[N

/m]

radius [m]

GASTGENUVP

GENUVP - free wake codeGAST - BEM

0

50

100

150

200

250

300

350

400

450

0 10 20 30 40 50 60 70

bla

de

ta

ng

en

tia

l fo

rce

[N

/m]

radius [m]

GASTGENUVP

U=8 m/s

0

0.5

1

1.5

2

2.5

3

3.5

4

0 10 20 30 40 50 60 70

axia

l in

du

ce

d v

elo

cit

y [

m/s

]

radius [m]

GASTGENUVP

norm

al f

orce

dis

t. (

Nt/

m)

tang

entia

l for

ce d

ist.

(N

t/m

)ax

ial i

ndu

ced

vel

ocity

(m

/s)

EWEC 2011 Brussels 14-17 March 2011 9

Results

comparison of BEM against free wake for swept bladeU=8 m/s, b=2

GAST GENUVP

Increase of loading towards the tip

Lower loads in more inboard sections

Increasing tip sweep

norm

al f

orce

dis

t. (

Nt/

m)

tang

entia

l for

ce d

ist.

(N

t/m

)

EWEC 2011 Brussels 14-17 March 2011 10

Results

comparison of BEM against free wake for swept bladeU=8 m/s, b=4

GAST GENUVP

Similar behavior but larger effect for higher curvature

BEM computations are expected to over predict power

Increasing tip sweep

EWEC 2011 Brussels 14-17 March 2011 11

Results

U=8m/s:

Free-wake simulations

Angle of attack distributions for swept blade

• moderate tip offset (a=3) affects little the a.o.a except at the tip region

• increasing “a” and “b” the complete blade is affected

EWEC 2011 Brussels 14-17 March 2011 12

Results

comparison of BEM against free wake for swept blade – aerodynamic analysis results

U=8 m/s

b=2 b=4 b=2 b=4 b=2 b=4

GAST -0.07 -0.12 -0.53 -0.86 -2.01 -3.23

GENUVP 1.08 1.68 1.40 1.06 -3.30 -9.23

a=1 a=3 a=6

Aerodynamic Power – % variation wrt straight blade

power scaled for the same blade length

BEM

Free W

EWEC 2011 Brussels 14-17 March 2011 13

Results

comparison of BEM against free wake for swept blade – aeroelastic analysis results

U=8 m/s, b=2GAST GENUVP

U=8 m/s, b=4

GAST GENUVP

EWEC 2011 Brussels 14-17 March 2011 14

Results

comparison of BEM against free wake for swept blade – aeroelastic analysis results

U=8 m/s

Aerodynamic Power – % variation wrt straight blade

power scaled for the same blade length

b=2 b=4 b=2 b=4 b=2 b=4

GAST -0.28 -0.47 -1.60 -2.76 -4.90 -8.17

GENUVP -1.85 -3.05 -7.27 -11.60 -17.61 -25.77

a=1 a=3 a=6

BEM

Free W

EWEC 2011 Brussels 14-17 March 2011 15

• 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)

Conclusions

EWEC 2011 Brussels 14-17 March 2011 16

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

Acknowledgements

EWEC 2011 Brussels 14-17 March 2011 17

Thanks for your attention

END