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VORTEX-INDUCED VIBRATIONS
PHYSICAL ASPECTS AND PREDICTION TOOLS
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Critical wind velocity
Stnb
v yiicrit
,,
⋅=
b: cross-wind dimension ni,y: natural frequency St: Strouhal number
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ee
aa nm
cπ
ζ4
=eee
ee
ee
aa m
bdmn
bndbnmn
yF ρρζ ==∝
)/(/ 22
−−=
2
1bam
bdKL
y
eaGa
σρζ
KONSTRUKSJONSSEMINAR 29. AUGUST 2012
Influence of turbulence
0 200 400 600Measuring time. [sec.]
ymax / d
-0.75
0
0.75
-0.75
0
0.75
-0.75
0
0.75vm ≈ 0.96 vr
vm ≈ vr
vm ≈ 1.05 vr
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Eurocode EN 1991-1-4:2005
• Approach 1: Vortex-resonance model • Approach 2: Spectral model
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Approach 1 versus approach 2
• Approach 2 allows for the consideration of different turbulence intensities, which may differ due to meteorological conditions.
• For regions where it is likely that it may become very cold and stratified flow condition may occur (e.g. in coastal areas in Northern Europe), approach 2 may be used.
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Vortex-shedding action – inertia forces – approach 1 and 2
max,2
, )()2()()( ysnsmsF yiyiw ⋅Φ⋅⋅⋅⋅= π
py ky ⋅= σmax
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Scruton number – approach 1 and 2
2,2
bm
Sc eis
⋅⋅⋅
=ρδ
δs: structural damping – logarithmic decrement mie/(ρ·b2): mass ratio
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Approach 1 – deflection from vortex shedding
latw cKKScStb
y⋅⋅⋅⋅=
112
max
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Approach 2 – deflection from vortex shedding
hb
mb
abKSc
CStb e
L
ya
cy2
22
14
1 ρ
σπ
σ
⋅
−−⋅
=
aL: determines the amplitude of large vibrations Cc: determines the amplitude of small vibrations Ka: determines the critical Scruton number between large and small vibrations
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Approach 2 Aerodynamic damping as function of turbulence intensity
)()Re(),Re( max, vvava IKKIK ⋅=
vv IK 31−=
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Approach 2 – circular cylinders
0 5 10 15 20 25Scruton number, Sc.
0.00
0.15
0.30
0.45
σy / d
Re = 105
Re = 106
Re = 5 ∙ 105
0 5 10 15 20 25Scruton number, Sc.
0.00
0.15
0.30
0.45
σy / dSmooth flow, Iv = 0%
Iv = 0%
Re = 105
Iv = 10%
Iv = 20%
0 5 10 15 20 25Scruton number, Sc.
0.00
0.15
0.30
0.45Re = 106
0 5 10 15 20 25Scruton number, Sc.
0.00
0.15
0.30
0.45
σy / d
σy / dRe = 5 ∙ 105
Iv = 0%
Iv = 10%
Iv = 20%
Iv = 0%
Iv = 10%
Iv = 20%
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Approach 2 – full scale measurements
0.0
0.5
1.0
( ymax / d )observed
( ymax / d )predicted
0.1 1 10 100Reynold number, Re∙10-5.
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Analysis for various structural elements Approach 2, Eurocode:
hd
md
daKSc
CStd e
L
ya
cy2
2max,
2
14
1 ρ
σπ
σ
−−
=
aL: determines the amplitude of large vibrations Cc: determines the amplitude of small vibrations Ka: determines the critical Scruton number between large and small vibrations
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Figure:The aerodynamic parameters Cc,max and Ka,max for circular cylinders in smooth flow. Due to uncertainty of Strouhals number minimum values at 0.0075 and 0.75 for Cc,max and Ka,max, respectively, are used.
Analysis for various structural elements Approach 2, Eurocode:
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Figure: The response for elements on which damages have been observed is shown in red, and the response for elements on which no damages have been observed is shown in green. The special cases of Statfjord and Gullfaks are shown in black. The dashed lines show upper and lower limits of the responses as function of the Scruton number Sc divided by 4πKa, where Ka is the aerodynamic damping parameter.
Analysis for various structural elements Approach 2, Eurocode:
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Wind-tunnel tests
Vortex-induced vibrations of spanbreakers
• 1:1 scale of spanbreakers in a bay
• Influence of structural damping
• Determination of aerodynamic damping (Ka)
• Influence of Reynolds number
• Influence of incoming turbulence
Full-scale tests to determine the structural damping
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Parametric excitation
1.02.03.0
6.05.04.0
7.08.0
10.09.0
Vibration amplitude [-]
Force frequency n [-]n1 12n 2n F
Parametric excitation
n : First natural frequencyn : Second natural frequency
1
2
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What is the problem?
Main engine and/or wind • Main engine - induced vibrations: Not the only source • Wind – induced vibrations: Not only source
Conbined effect of main engine and wind
KONSTRUKSJONSSEMINAR 29. AUGUST 2012