.

WIND LOADS ON FAN BLADES & BLADE DYNAMICS

Cofimco S.r.l.

Summary

1. Purpose of the document

2. Introduction

3. Loads

4. Dynamics

5. Load mitigation

6. Conclusions

2

Purpose of the document

3

Purpose of the document

The main scope of this document is to study the loads acting on the blades with and without windand the related blade response. Three different conditions have been studied :

a) Wind Speed (WS) = 0b) WS = 25 m/s (≈ 60 mph), constantc) WS = 25 m/s, constant with wind screen

Nomenclature:

WS: Wind speedBNF: Blade Natural FrequencyBOF: Blade Operative FrequencyBPF: Blade Passing FrequencyLF: Load FrequencyLP: Load PeriodCPM: Cycles per MinuteAoA: Angle of Attack

CPS: Cycles per secondRPM: Revolution per minuteRBM, BM: Root Bending MomentSH: Shear (Vertical load)RT: Revolution periodVy: Axial component of the flow speedFy: Axial component of the load on bladesNL: Nominal Load (Fy)

4

Introduction

5

Introduction (1): Fan & blades features

The studied fan has the following features:

Low blade count,wide chord (blade A)

-Single cell-Diameter 34 ft (10363 mm)-Hub diameter ≈ 7 ft (2000 mm)-RPM 120 (2 cps )-Nominal power: ≈ 160 KW (217 HP)

6

Introduction (2): Blade Features

The fan blades A have the following properties: Blade A

Chord at root, mm 890 (2.9 ft)Chord at tip, mm 770 (2.5 ft)Blade Mass, Kg 112.6 (248 lbs)Blade count 6BOF mode1 & mode 2, Hz 6.7 – 39.5Main features Heavy, stiff, high BOF

7

Introduction (3): Study Approach

The study has been conducted by means of CFD (X_FLOW) simulation with thefollowing approach:

Phase 2 - Mitigation of the loads acting on the bridge

Step 2a → Blade A + Wind Screen

Load definition and response calculation @ WS = 25 m/s

Step 2b → Blade’s Chord Reduction and Count Increase: Blade B1

Load definition and response calculation @ WS = 25 m/s, no Wind Screen

Step 2c → Blade’s Chord Reduction, Count Increase, BOF Reduction and friction-damper: Blade B2

Load definition and response calculation @ WS = 25 m/s, no Wind Screen

Phase 1 – Blade A: load and blade response

Step 1a → Load definition and response calculation @ WS = 0 & WS = 25 m/s

Step 1b → Evaluation of the loads transmitted to the bridge for both WS = 0 & WS = 25 m/s

8

Loads

9

Loads (1): Blade A case studies

Second Case Study:WS = 25 m/s – no wind screensWind direction perpendicular tothe bridge(25 m/s ≈ 60 mph)

First Case Study:WS=0

Two cases have beenconsidered:

10

Loads (2): Flow Perturbations

WS= 0

WS= 25 m/s

The two case studies showed someinteresting differences. Among them, themost relevant are:

-Load Dynamic component amplitude-Number of pulses per round-Width (duration) of the pulses-Load spectra-Load pattern regularity

First case study: WS = 0

Second case study: WS = 25 m/s

11

Loads (3): Angle of Attack (AoA)

The ratio between the flow axial speed Vy and the blade speed defines the actualAngle of Attack (AoA).

The higher the WS, the higher theAoA fluctuation with the bladeposition around the fan and the loadvariation

NOMINAL AoA @ WS = 0

ALTERATED AoA @ WS >> 0(Upwind)-4

-2

0

2

4

6

8

10

12

14

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

Wind direction

High AoA (upwind) Low AoA (downwind)

Vy

12

Loads (4): Blade A @ 120 RPM

Vertical load, Ntime history @ WS = 0, WS = 7.5 m/s ≈ 17 mphWS = 15 m/s ≈ 33 mphWS = 25 m/s ≈ 60mph

Design Load: -2550 N

-7000

-6500

-6000

-5500

-5000

-4500

-4000

-3500

-3000

-2500

-2000

-1500

-1000

-500

0

500

1000

3.3 3.5 3.7 3.9 4.1 4.3 4.5 4.7 4.9 5.1 5.3

Loads are shifted in time for a better view

Revolution time @ 120 RPM = 0.5 s

13

Loads (5): Blade A @ WS = 0

Vertical load, time history, WS = 0

-4000

-3800

-3600

-3400

-3200

-3000

-2800

-2600

-2400

-2200

-2000

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7

Fy (N) – Blade A - WS=0

-4000

-3800

-3600

-3400

-3200

-3000

-2800

-2600

-2400

-2200

-2000

2 2.02 2.04 2.06 2.08 2.1 2.12 2.14 2.16 2.18 2.2 2.22 2.24

Fy (N) - W.S. = 0

Vertical load - Pulse detail:Pulse duration: 0.09 sPulse intensity: 1250 N

LP: 0.25 sLF: 4 Hz – 240 CPM - (2X)

At the design speed (120 RPM -2 CPS), we have:Revolution Period (RT) = 0.5 sLoad Period (LP) = 0.25 s

Static Load: 2550 N

Blade under the Bridge

14

Loads (6): Blade A @ WS=25

Vertical load, time history, WS = 25

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6

Fy (N) – Blade A - W.S. = 25 m/s

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3.5 3.55 3.6 3.65 3.7 3.75 3.8 3.85 3.9 3.95 4

Fy (N) - W.S. = 25 m/s

Vertical load – Pulse detail:Pulse duration: 0.38 sPulse intensity: 3450 N

LP: 0.5 sLF: 2 Hz – 120 cpm - (1X)

At the design speed (120 RPM -2 CPS), we have:Revolution Period (RT) = 0.5 sLoad Period (LP) = 0.5 s

Blade under the BridgeBlade UpwindBlade Downwind

15

Loads (7): Blade A - Loads comparison

Wind Speed = 0Wind Speed = 25 m/s

-8000

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6

Vertical load, comparison: WS=0 m/s vs WS=25 m/s

Fan speed:120 RPM, 2 CPS, RT=0.5 s

Lap Duration (RT) 0.5 s

Two distinct load peaks per revolution, nearly equivalentOne prevailing load peakper revolution

16

0

500

1000

1500

2000

2500

1 2 3 4 5 6 7 8 9 10 11 12

WS=0

WS=25

Loads (8): Blade A - Loads spectra

WS = 25 m/sVertical load spectrumThe presence of the 1X is anevidence of the single prevailing peak/revolution

WS = 0 m/sVertical load spectrumThe 2 peaks/revolution are evidenced by the 2X load

2X

4XNX

As a consequence, the load spectra have different shapes and significantcomponents

(only the dynamic components are shown)

17

1X

5X

3X

-7000

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3.6 3.85 4.1 4.35 4.6 4.85 5.1 5.35 5.6

Loads (9): Loads patterns

In order to avoid “random perturbations”, a load pattern hasbeen selected from each load time history, then replicatedperiodically with the same period as the original one. This wasnecessary to investigate the behavior of the blade clearing itfrom the influence of random load pulses

-6000

-5000

-4000

-3000

-2000

-1000

0

2 2.25 2.5 2.75 3 3.25 3.5 3.75 4 4.25 4.5 4.75 5

Nominal load (NL), N -2550

Min @ WS=0, N -3822

max @WS=0, N -2265

Dynamic +, N 285

Dynamic -, N -1272

min @ WS=25, N -5536

max @ WS=25, N -78

Dynamic +, N 2472

Dynamic -, N -2986WS=25: Total Dynamic 5458 N(214% of NL)

WS=0: Total Dynamic 1557 N(61% of NL)

Fan speed 120 RPM,Lap duration, 0.5 s

Loads sample

18

Loads (10): RBM origin

DISPLACEMENTS

As an elastic structure, a blade responds to a time-depending load accordingto several parameters:a) Load characteristics (Pulses’ shape, Pulses’ duration, Pulses’ frequency)b) BOF, Blade Mass, Blade Stiffness, Damping Factor.The blade’s response is characterized by a time-depending displacementshape that generates a time-depending vertical load Fy and a Root BendingMoment (RBM). This gets transmitted to the bridge causing vibration,particularly of the “Rocking” type

RBM

-250

-200

-150

-100

-50

0

50

-6000

-5000

-4000

-3000

-2000

-1000

0

1 1.25 1.5 1.75 2 2.25 2.5 2.75 3

Load

RBM

Bending Momentand Vertical Loadtransmitted to thebridge

19

SH

Dynamics

20

Dynamics (1): Modal analysis

Each blade’s response has been determined by modal analysis. The first and the second modes onlyhave been considered for the calculation.

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Mode shape 1

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Mode shape 2

21

Dynamics (2): Blade A @ WS=0

-60

-56

-52

-48

-44

-40

-36

-32

-28

-24

-20

-16

-4000

-3800

-3600

-3400

-3200

-3000

-2800

-2600

-2400

-2200

-2000

-1800

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

Fy (N) - Tip displacement (mm)

Mass RBM SH Y des

-8437.2 -3224.6 -36.4BLADE A

FN1 = 6.68

112.6FN2 = 39.5

RBM Δ RBM Δ RBM % -59% SH Δ SH Y tip

-11572.5 Δ SH % -55% -4333.1 -51.2

-6606.7 Δ Y % 64% -2554.4 -27.94965.8 1778.8

22

Dynamics (3): Blade A @ WS=0

-7,000

-6,500

-6,000

-5,500

-5,000

-4,500

-4,000

-3,500

-3,000

-2,500

-2,000

-1,500

-1,000

-14,000

-13,000

-12,000

-11,000

-10,000

-9,000

-8,000

-7,000

-6,000

-5,000

-4,000

-3,000

-2,000

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

Mass RBM SH Y des

-8437.2 -3224.6 -36.4BLADE A

FN1 = 6.68

112.6FN2 = 39.5

RBM Δ RBM Δ RBM % -59% SH Δ SH Y tip

-11572.5 Δ SH % -55% -4333.1 -51.2

-6606.7 Δ Y % 64% -2554.4 -27.94965.8 1778.8

23

Dynamics (4): Blade A @ WS=25 m/s

-90

-82

-74

-66

-58

-50

-42

-34

-26

-18

-10

-2

6

14

-6000

-5500

-5000

-4500

-4000

-3500

-3000

-2500

-2000

-1500

-1000

-500

0

500

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

Fy (N) - Tip displacement (mm)

Mass RBM des SH des Y des

BLADE A

FN2 = 39.5

FN1 = 6.68

112.6 -8437.2 -3224.6 -36.4

RBM Δ RBM Δ RBM% -193% SH Δ SH Y tip

-17274.5 Δ SH % -188% -6417.2 -77.9

-980.0 Δ Y % 202% -340.5 -4.516294.5 6076.7

24

Dynamics (5): Blade A @ WS=25 m/s

Mass RBM des SH des Y des

BLADE A

FN2 = 39.5

FN1 = 6.68

112.6 -8437.2 -3224.6 -36.4

-7,200

-6,600

-6,000

-5,400

-4,800

-4,200

-3,600

-3,000

-2,400

-1,800

-1,200

-600

0

-20,000

-18,000

-16,000

-14,000

-12,000

-10,000

-8,000

-6,000

-4,000

-2,000

0

2,000

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

RBM Δ RBM Δ RBM% -193% SH Δ SH Y tip

-17274.5 Δ SH % -188% -6417.2 -77.9

-980.0 Δ Y % 202% -340.5 -4.516294.5 6076.7

25

Dynamics (6): Load /Response summary

Design conditions: Load = -2550 N TD = -36.4 mm VL0 = -3224 N RBM0 = -8437 Nm

26

WS = 0 WS = 25

LOA

D

Minimum, N -3822 -5536

Maximum, N -2265 -78

Dynamic +, N 285 2472

Dynamic -, N -1272 -2986

Pulse duration, s ≈ 0.09 ≈ 0.38

Spectrum 2X; 4X; NX 1X; 2X; 4X

RESP

ON

SE

Min Max Δ Min Max Δ

Tip Displacement, mm -51.2 -27.9 23.3 -77.9 -4.5 73.4

RBM, N*mm -11572.5 -6606.7 4965.8 -17274.5 -980.0 16294.5

Δ RBM / RBM0, % 59% 193%

SH, N -4333.1 -2554.4 1778.8 -6417.2 -340.5 6076.7

Δ SH / SH0 % 55% 188%

Load Mitigation

27

Load Mitigation (1): Blade A -Wind Screen

Screen Tunnel Test CFD simulation

The screen is defined by the so called

PRESSURE DROP COEFFICIENT:

Downwind pressure ≈ - 1000 Pa

K = 2 * ΔP / ( δ * V2)

For this simulation, a value of

K = 6.2

was selected from the several sizesavailable on the market.

Therefore, at 25 m/s, the expectedPressure Drop is

ΔP = 2363 PaTotal ΔP from CFD simulation ≈ 2300 Pa

Upwind pressure ≈ 1300 Pa

28

Load Mitigation (2): Blade A -Wind Screen

Wind Screen: 2.5 m below the fan inlet

More regular distribution of the flow vertical component (Vy)

Reduced perturbated areas

29

Load Mitigation (3): Blade A - Wind Screen

Mass RBM SH Y des

-8437.2 -3224.6 -36.4BLADE A

FN1 = 6.68

112.6FN2 = 39.5

RBM Δ RBM Δ RBM% -121% SH Δ SH Y tip

-14760.7 Δ SH % -118% -5638.7 -63.7

-4541.3 Δ Y % 123% -1820.8 -18.910219.4 3817.8

-84

-77

-70

-63

-56

-49

-42

-35

-28

-21

-14

-7

0

-6000

-5500

-5000

-4500

-4000

-3500

-3000

-2500

-2000

-1500

-1000

-500

0

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

Fy (N) - Tip displacement (mm)

30

Load Mitigation (4): Blade A - Wind Screen

-9,000

-8,000

-7,000

-6,000

-5,000

-4,000

-3,000

-2,000

-1,000

0

-18,000

-16,000

-14,000

-12,000

-10,000

-8,000

-6,000

-4,000

-2,000

0

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

Mass RBM SH Y des

-8437.2 -3224.6 -36.4BLADE A

FN1 = 6.68

112.6FN2 = 39.5

RBM Δ RBM Δ RBM% -121% SH Δ SH Y tip

-14760.7 Δ SH % -118% -5638.7 -63.7

-4541.3 Δ Y % 123% -1820.8 -18.910219.4 3817.8

31

Load Mitigation (5): Blades B @ WS=25

Higher blade count with reduced chord.With 6 blades, 50% of the blades can besimultaneously in the high AoA region,while with 10 blades only 40% of theblades, at the most, can be in thatcondition

6 blades10 blades

32

Load Mitigation (6): Blades B Features

Two blades from blade family B with the following properties:

Blade B1 Blade B2 (35F)

Chord at root, mm 500 (1.64 ft)Chord at tip, mm 448 (1.47 ft)Blade Mass, Kg 89 (196 lbs) 41.5 (92 lbs)Blade count 10BOF mode1 & mode 2, Hz 4.1 – 15.7 2.2 – 14.2

Main features Lighter, stiff, medium BOFVery Light, very low BOF, high

damping effect (*)

(*) Double FRP shaft withinner friction-based damper

Shape and size comparison:- Blade A- Blades B1 & B2

33

Load Mitigation (7): Blade B2 Features

Blade B2: double FRP shaft with innerfriction-based damper

Outer FRP shaft (glass + Carbon fibers)

Inner FRP shaft (glass fibers)

Stainless Steel frictiondamping sleeve

34

Blade B235F type

Load Mitigation (8): Blade B1 @ WS=25

Mass RBM SH Y des

-4031.2 -1567.4 -65.3FN2 = 15.7

BLADE B1

FN1 = 4.13

89.0

-120

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

-2600

-2400

-2200

-2000

-1800

-1600

-1400

-1200

-1000

-800

-600

-400

-200

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

Fy (N) - Tip displacement (mm)

35

RBM Δ RBM Δ RBM% -78% SH Δ SH Y tip

-5545.6 3140.3 Δ SH % -50% -2742.0 -90.9

-2405.3 Δ Y % 83% -1535.0 -36.91207.0

Load Mitigation (9): Blade B1 @ WS=25

Mass RBM SH Y des

-4031.2 -1567.4 -65.3FN2 = 15.7

BLADE B1

FN1 = 4.13

89.0

36

-3,000

-2,800

-2,600

-2,400

-2,200

-2,000

-1,800

-1,600

-1,400

-1,200

-1,000

-800

-600

-400

-7,500

-7,000

-6,500

-6,000

-5,500

-5,000

-4,500

-4,000

-3,500

-3,000

-2,500

-2,000

-1,500

-1,000

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

RBM Δ RBM Δ RBM% -78% SH Δ SH Y tip

-5545.6 3140.3 Δ SH % -50% -2742.0 -90.9

-2405.3 Δ Y % 83% -1535.0 -36.91207.0

Load Mitigation (10): Blade B2 (35F) @ WS=25

-260

-240

-220

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

-2600

-2400

-2200

-2000

-1800

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

Mass RBM SH Y des

BLADE B2

FN1 = 2.16

41.5FN2 = 14.2

-2548.4 -1957.3 -157.0

37

RBM Δ RBM Δ RBM% -53% SH Δ SH Y tip

-3068.7 Δ SH % -31% -2333.6 -186.0

-1718.1 Δ Y % 50% -1721.0 -107.11350.7 612.6

Load Mitigation (11): Blade B2 (35F) @ WS=25

Mass RBM SH Y des

BLADE B2

FN1 = 2.16

41.5FN2 = 14.2

-2548.4 -1957.3 -157.0

38

RBM Δ RBM Δ RBM% -53% SH Δ SH Y tip

-3068.7 Δ SH % -31% -2333.6 -186.0

-1718.1 Δ Y % 50% -1721.0 -107.11350.7 612.6

-2,500

-2,400

-2,300

-2,200

-2,100

-2,000

-1,900

-1,800

-1,700

-1,600

-1,500

-1,400

-1,300

-1,200

-1,100

-1,000

-900

-4,000

-3,750

-3,500

-3,250

-3,000

-2,750

-2,500

-2,250

-2,000

-1,750

-1,500

-1,250

-1,000

-750

-500

-250

0

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4 4.1 4.2 4.3 4.4 4.5

Load Mitigation (12): Load summary

39

Blade A - Design conditions: Load = -2550 N TD = -36.4 mm SH0 = -3224 N RBM0 = -8437 Nm

Blade B1 - Design conditions: Load = -1530 N TD = -65.3 mm SH0 = -2424 N RBM0 = -4031 Nm

Blade B2 - Design conditions: Load = -1530 N TD = -157 mm SH0 = -1957 N RBM0 = -2548 Nm

Blade A Blade A Blade A Blade B1 Blade B2 (35F)

WS = 0 (REFERENCE) WS = 25 WS = 25, Wind Scr. WS = 25 WS = 25

LOA

D

Minimum, N -3822 -5536 -4936 -2250 -2250

Maximum, N -2265 -78 -1160 -530 -530

Dynamic +, N 285 2472 1390 1030 1030

Dynamic -, N -1272 -2986 -2386 -670 -670

Pulse duration, s 0.09 0.38 0.36 0.25 0.25

Variation of tip displacement and RBM

RESP

ON

SE

Δ Tip Displacement, mm 23.3 73.4 44.9 54 79

Δ RBM, N*mm 4966 16295 10220 3140 1351

Δ RBM / RBM0, % 0.59 1.93 1.21 0.78 0.53

SH, N 1779 6077 3817 1206 613

Δ SH / SH0 % 0.55 1.88 1.04 0.5 0.31

RBM mitigation with ref.N.A. 328% 206% 63% 27%

to Blade A, WS = 0

Load Mitigation (13): Blade B2 (35F) on-site test

40

Vibration amplitudes were greatly reduced using the new FRP-Carbon shafts.On top of the motor, a huge vibration reduction was achieved compared to a standard FRP blade with rigid connection to the hub, evidencing a great reduction in the BM transmitted to the bridge.

FRP-Carbon shaftsHand Laminated Blades

Z

YMotor Top

X Y Z Y

3.1 4.2 7.8 12.2

2.8 2.1 2.7 6.1

Gearbox, Direction

32ft - 6 bladesmm/s RMS

Fan type:

Hand Laminated

FRP - Carbon Shaft 35F

Test carried out @ WS ≈ 0

Conclusions

41

Conclusions (1)

Wind can dramatically increase the RBM variation transmitted to the bridge and,consequently, the vibration level, up to more than three times the nominal value in theconditions simulated in this report (120 rpm, WS=25 m/s)

For the above reasons, the loads and stress on the drive chain components can reach up tothree times the nominal level (in the conditions simulated in this report)

While, at nominal conditions, the geometry (layout) of the cell defines the harmoniccomponents of the loads (2X and 4X mainly), with strong wind other harmonic components canappear. The simulation of blade A at 90 rpm showed a relevant value of the 3X harmoniccomponent.

42

Conclusions (2)

Wind screens can reduce the wind effect significantly, both in terms of vibration level andperformance (reduction of 37 % in the conditions simulated in this report)

Using a fan with reduced-chord blades and increased blade count can reduce the wind effect as wellin terms of vibration level.

Generally, “softer” blades (low BNF like the 35F) can significally mitigate the vibrations induced onthe structure

The introduction of an enbedded damping device can take under control the blade displacement andthe related loads even close to resonance conditions, increasing the blade safety margin and mitigatingfurtherly the vibrations induced to the structure

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Conclusions (3)

The reduction of the blade chord, together with the soft shaft and the embedded friction-based

damping system, will have a great beneficial effect on the duty life of the cell but cannot avoid thelack of performances of the cell due to the wind

The combination of all the above means (reduced blade chord, low-natural frequency blade anddamping system and Wind Screens) will likely achieve the optimum task of mitigating the wind effect

on the performance reduction and on the vibration level, increasing the drive system life andreducing the maintenance requirements

NOTE: the different BPF and BOF MUST be taken into consideration. The above conclusions are trueunder the condition that the BPF and BOF don’t match with one of the bridge NFs or resonancecomponents

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