Low-Frequency Lighting Pole Vibrations
Muhammad. R. Hajj
ESM Department
Virginia Tech
Blacksburg, VA 24061
Supported by
Hapco Aluminum Pole Products
Abingdon, VA 24210
Technical Committee T-12 Meeting
Structural Supports for Highway Signs, Luminaires and Traffic Signals
July 7, 2009 New Orleans, LA
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 1
Motivation
– Observed field oscillations of lighting poles have a frequency near 1.5 Hz
(near first mode) in about 25 to 40 mph.
– No clear guidance regarding dynamic loads
– Discrepancies in static load coefficients (wind tunnel tests)
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 2
Objectives
Determine root causes of observed large amplitude oscillations of square lighting
poles
(1) Wind loads:
Aerodynamic load contains a component
that directly excites the low frequency
mode! Numerical simulations
(2) Structural nonlinearities
The shedding frequency might excite a higher
mode which passes energy to the first mode
through nonlinear mechanisms.
Address discrepancies in measured static load coefficients
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
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Structural Analysis
Observed field oscillations of lighting
poles have a frequency
p
q
y near 1.5 Hz ((first
mode) in about 25 to 40 mph.
1st mode
d
Round Corners Pole
No Luminaire
Sharp Corners Pole
O Luminaire
One
L i i
Round Corners Pole
One Luminaire
Sharp Corners Pole
Two Luminaires
Round Corners Pole
Two Luminaires
Freq [Hz]
U [mph]
Freq [Hz]
U [mph]
[ h]
Freq [Hz]
U [mph]
Freq [Hz]
p ]
U [[mph]
Freq [Hz]
U [mph]
2.08
5.48
1.54
4 043
4.043
1.41
3.69
1.13
2.97
1.27
3.34
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
2nd
mode
13.04
34.18
10.74
28 15
28.15
10.33
27.09
9.75
25.58
10.09
26.46
3rd mode
d
36.25
95.06
31.10
81 54
81.54
30.25
79.32
29.02
76.10
29.84
78.26
Lock-in resonance
between vortex
shedding and lighting
pole response is not
the cause of these
oscillations.
7 July 09 | 4
Structural Analysis
y
– Full Scale Tests
Out of Plane Motion
In Plane Motion
0.15
0.2
Horizontal Free Response
Vertical Free Response
0.15
0.1
01
0.1
Amplitude [Volts]
Amplitude [Volts]
0.05
0.05
0
-0.05
0
-0.05
-0.1
-0.1
-0.15
-0.2
0
5
10
15
Time [sec]
20
25
0
30
5
10
15
Time [sec]
20
25
30
-2
10
Horizontal Motion
Vertical Motion
-3
10
10
1.5
Hz
-4
10
8.5 Hz
-4
10
10
Pow
wer
10.1 Hz
-5
Pow
wer
1.5
Hz
-3
10.1
Hz
-55
10
-6
10
-6
10
-7
10
-7
-8
10
10
0
5
10
15
20
Frequency (Hz)
25
30
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
0
5
10
15
20
Frequency (Hz)
25
30
7 July 09 | 5
Wind Tunnel Tests
Size of test section
Blockage ratio
Geometric similarity
Aeroelastic scaling: matching
aerodynamic,
y
, structure and
coupling parameters
Instrumentation
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Numerical Simulations
Flow over finite length cylinders
Advances in computing power and methodologies
Infinite aspect ratio:
ratio the cylinder
c linder
reaches both sides of the
computational domain
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
60:1 aspect ratio: the cylinder
has a free end, so that end
effects can be accounted for
7 July 09 | 7
Sharp Edges Cylinder:
D
Drag
and
d Lift coefficient
ffi i t ti
time hi
histories
t i
Presence of a
relatively low
frequency component
in the 60:1 aspect
ratio case
Mean drag is lower in
the 60:1 aspect ratio
case
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
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Sharp Edges Cylinder:
Drag and
D
d Lift coefficient
ffi i t P
Power S
Spectra
t
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Nondimensional shedding
frequency St=0.137
St 0.137 is
obtained from the peaks in
lift coefficient spectra.
ag coefficient
coe c e spec
spectra
a
Drag
have peaks at twice the
shedding frequency.
A low frequency
q
yp
peak for
St=0.01 is present in drag
coefficient spectra of the
60:1 aspect ratio case.
Amplitude of lowfrequency peak is much
larger than that of the
vortex shedding
component.
t
7 July 09 | 9
Pathlines (60:1 aspect ratio)
Tip
End effects
Loss of coherence
in vortex shedding
g
Symmetry boundary condition
is imposed at the root.
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Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 11
Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 12
Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 13
Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 14
Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 15
Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 16
Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 17
Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 18
Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 19
Lift time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 20
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 21
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 22
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 23
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 24
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 25
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 26
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 27
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 28
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 29
Drag time histories as a function
off pole
l elevation
l
ti
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 30
Mean drag coefficient as a
f
function
ti
off pole
l elevation
l
ti
(square cylinder with sharp corners; r/B=0)
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
Drag coefficient mean
values in the root region
are close to those
obtained with the infinite
aspect ratio cylinder
Drag
g coefficient mean
value decreases with
height
p
discrepancies
p
Explains
between measured drag
coefficients
7 July 09 | 31
Mean drag coefficient as a
f
function
ti
off pole
l elevation
l
ti
(square cylinder with rounded corners; r/B=1/12)
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
Drag coefficient mean
value decreases with
height
The decrease is less
consistent than that of the
sharp corners cylinders
7 July 09 | 32
Effects of Luminaires
-4
10
-4
10
cd-historySquareRe100kAR60
cd-history-poleLamp0deg
cd-history-poleTwoLamps0deg
-6
10
cd-historySquareRe100kAR60
cd-history-poleLamp90deg
cd-history-poleTwoLamps90deg
-6
10
-8
10
-8
Power
Power
10
-10
-10
10
10
-12
10
-14
10
10
-12
10
-14
10
0
0.1
0.2
0.3
0.4
0.5
0.6
Freq * D/U
0.7
0.8
0.9
1
0
0.1
0.2
0.3
0.4
0.5
0.6
Freq * D/U
0.7
0.8
0.9
Zero and 90 degrees
g
angle
g of attack
Adding the luminaires reduces the amplitude of low-frequency variations in
the drag coefficient.
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
1
7 July 09 | 33
Conclusions
Presence of low-frequency component in the aerodynamic flow which could directly
excite the first mode.
– Associated with finite-length cylinders
– Could not be determined from wind tunnel tests
Mean drag coefficient varies along the finite-length pole.
Discrepancies in measured static coefficients is explained
explained.
– CD= 2.0 is on the high end
Luminaires reduce amplitude of low-frequency component in drag coefficient.
Numerical simulations may be better suited to characterize the aerodynamics of the
flow around finite length poles and aeroelastic aspects of these poles.
Recommendations:
– Need to address low-frequency variations in aerodynamic loads in code
–
Need for development of control procedures of low-frequency
low frequency oscillations
–
Full-scale measurements for validation of numerical simulations and control
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 34
Acknowledgemnts
Hapco: Joe Bowman, Ray Minor and Greg
Mercier
Students: Andrea Mola, Giancarlo Bordonaro
and Chris Mesrobian
2009 AASHTO Subcommittee on Bridges and Structures Annual Meeting
7 July 09 | 35