Wind Measurements on the Gemini
8m Primary Mirrors
Myung K. Cho
Larry M. Stepp
26-27 November, 2001
NIO Wind Loading Workshop
1
Goals of Wind Test on Gemini
• To understand wind effects on Gemini M1
• To evaluate the validity of the math/numerical
models developed in the design stage
• To understand environmental parameters during
observation
• To develop a vent gate control algorithm for
operation of Gemini telescopes
• To extrapolate wind effects measured on an 8m
telescope to larger telescopes
26-27 November, 2001
NIO Wind Loading Workshop
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26-27 November, 2001
NIO Wind Loading Workshop
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Wind Test Setup
• Wind pressure measurements on the primary
– Dummy mirror surface covered with plywood
– 24 pressure sensors installed for the first test series
• In conjunction with structural accelerometer measurements
– 32 pressure sensors installed for all other test sets
• Wind velocity measurements (6 anemometers)
–
–
–
–
Two locations on elevation axis (+X and –X axes)
Top and bottom sides of M1 cell (+Y and –Y axes)
Behind the secondary mirror assembly (M2)
At the top of the dome (Dome location)
26-27 November, 2001
NIO Wind Loading Workshop
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24 INITIAL PRESSURE SESOR LOCATIONS
8 ADDITIONAL PRESSURE SENSOR LOCATIONS
1
2
25
27
26
4
3
8
13
14
29
19
28
15
20
11
12
16
17
18
21
22
31
30
32
23
26-27 November, 2001
6
10
9
7
5
24
NIO Wind Loading Workshop
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Instrumentation
3-axis anemometers
Pressure sensors, 32 places
26-27 November, 2001
NIO Wind Loading Workshop
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Wind test hardware/software
• Wind data acquisition (National Instruments)
– Acquisition PC board: NI PCI-6033E I/O
– Interface: SCB-100, 100 pin connector block box
– Data logger: NI LabView software on a pentium PC
• Wind pressure sensor: Setra model 264
– Bi-directional pressure transducer
– Pressure range: ± 50 Pascals (N/m2)
• Wind velocity sensor: Young model 8100
– Ultrasonic 3 directional anemometer
– Velocity range: 0 – 20 meters per second (m/s)
26-27 November, 2001
NIO Wind Loading Workshop
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26-27 November, 2001
NIO Wind Loading Workshop
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26-27 November, 2001
NIO Wind Loading Workshop
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26-27 November, 2001
NIO Wind Loading Workshop
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Sensor Locations
Ultrasonic anemometer
Ultrasonic anemometer
26-27 November, 2001
Pressure sensors
NIO Wind Loading Workshop
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Anemometer at +X axis
26-27 November, 2001
NIO Wind Loading Workshop
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Anemometer on top of the dome
26-27 November, 2001
NIO Wind Loading Workshop
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NIO Wind Loading Workshop
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Wind Test Procedures
• Total of 116 wind test sets with parameters of:
– Wind attack angles (azimuth angle)
– Telescope zenith angles
– Vent gate positions
• Wind pressure measurements (24/32 sensors)
– Pressure sensor readings at a sampling rate of 10 samples
per second (10Hz) for 5 minutes
• Wind velocity measurements (5-6 anemometers)
– 3 readings at each anemometer at 10Hz for 5 minutes
• Simultaneous data acquisition – no time delay
26-27 November, 2001
NIO Wind Loading Workshop
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Wind Test Data Reduction
• Results from wind pressure measurements
–
–
–
–
–
–
Time history and frequency response of each sensor
Temporal and spatial average of wind pressure
Temporal and spatial RMS of wind pressure
Resulting surface distortions
Zernike coefficients of surface distortions
Structural Function for pressure difference variations
• Results from wind velocity measurements
– Average resultant wind velocity
– RMS resultant wind velocity
• Animations
26-27 November, 2001
NIO Wind Loading Workshop
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Sample test case (c00030oo)
•
•
•
•
•
Typical case among a total of 116 test cases
Wind attack direction to SLIT = 0 degree
Telescope Zenith angle = 30 degrees
Upwind gate: open
Down wind gate: open
(The open-open condition is most severe.)
26-27 November, 2001
NIO Wind Loading Workshop
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Wind pressure variation at sensor #1
time history and frequency response
0
8
10
6
10
4
10
6
5
2
4
3
10
2
0
10
-2
10
-4
Sensor # 1 (C00030oo)
7
10
magnitude
pressure (N/m2)
Sensor # 1 (C00030oo)
10
1
0
0
50
100
150
200
Time History: time (second)
26-27 November, 2001
250
300
SUM = 9118
10
-3
10
NIO Wind Loading Workshop
-2
-1
0
10
10
10
Frequency Response Function: frequency (Hz)
1
10
18
Average wind pressure
time history and frequency response
0
1
10
0.5
10
0
10
6
5
-0.5
4
3
10
2
-1
10
-1.5
10
-2
AVERAGE Pressure (C00030oo)
7
10
magnitude
pressure (N/m2)
AVERAGE Pressure (C00030oo)
1.5
1
0
0
50
100
150
200
Time History: time (second)
26-27 November, 2001
250
300
SUM = -226
10
-3
10
NIO Wind Loading Workshop
-2
-1
0
10
10
10
Frequency Response Function: frequency (Hz)
1
10
19
RMS wind pressure variation time history
and frequency response
RMS Pressure (C00030oo)
10
4
10
6
3.5
5
10
3
magnitude
pressure (N/m2)
RMS Pressure (C00030oo)
7
4.5
2.5
4
10
3
10
2
2
10
1.5
1
10
1
0.5
0
0
50
100
150
200
Time History: time (second)
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250
300
SUM = 6141
10
-3
10
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-2
-1
0
10
10
10
Frequency Response Function: frequency (Hz)
1
10
20
Response of Primary Mirror
to Wind Loading
• Response of mirror determined by stiffness of
mirror and load-spreading properties of support
system
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–
–
–
Primary mirror material: Corning ULETM fused silica
Primary mirror diameter: 8.1 m
Primary mirror thickness: 0.2 m
Mirror support: 120-point hydraulic whiffletree
26-27 November, 2001
NIO Wind Loading Workshop
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Contour Plots
(c00030oo)
Wind Pressure (N/m2)
26-27 November, 2001
Mirror Deformation (microns)
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Simultaneous Animations
(c00030oo)
Wind Pressure (N/m2)
26-27 November, 2001
Mirror Deformation (microns)
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Wind Speed at 5 Locations (m/sec)
23
Effect of vent gate positions
Pressure
Deformation
C00030oo (open, open)
Fz=26N, Mx=151N-m, My=-10N-m
Surface RMS = 0.31 microns
C00030cc (closed, closed)
Fz=-7N, Mx=8N-m, My=-16N-m
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NIO Wind Loading Workshop
Surface RMS = 0.03 microns
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Mirror Deformation vs Pressure
RMS Deformation (microns)
0.600
0.500
0.400
0.300
0.200
0.100
0.000
0.00
26-27 November, 2001
2.00
4.00
6.00
NIO
Wind Pressure
Loading Workshop
RMS
(pascals)
8.00
10.00
25
Mirror Deformation vs Wind Speed
0.700
RMS surface (microns)
0.600
0.500
0.400
0.300
0.200
0.100
0.000
0.0
26-27 November, 2001
2.0
4.0
6.0
NIO
Wind Speed
Loading Workshop
Wind
at M1
(m/sec)
8.0
10.0
26
Mirror Deformation vs Wind Speed
0.700
RMS surface (microns)
0.600
0.500
0.400
0.300
0.200
0.100
0.000
0.0
26-27 November, 2001
2.0
4.0
6.0
NIO
Wind Speed
Loading Workshop
Wind
at M1
(m/sec)
8.0
10.0
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Mirror Deformation vs Wind Speed
0.700
Vent gates controlled to limit wind
speed at mirror to < 3 m/sec
RMS surface (microns)
0.600
0.500
0.400
0.300
0.200
0.100
0.000
0.0
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2.0
4.0
6.0
NIO Wind Loading Workshop
Wind Speed at M1 (m/sec)
8.0
10.0
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Effect of Vent Gate Positions
Average Attenuation of Outside Wind Velocity
Vent Gate Positions
Relative Wind
Speed at M1
Relative Wind
Speed at M2
Closed-Closed
13%
64%
Closed-Open
37%
74%
Open-Closed
35%
57%
Open-Open
74%
61%
(upwind-downwind)
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NIO Wind Loading Workshop
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NIO Wind Loading Workshop
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Overview of the L16 Analysis
(David Smith)
L16 factorial experiment includes:
•
•
•
Azimuth Angle of Attack (AoA) at 0 and 45 degrees
Elevation at 30 and 60 degrees of Zenith angle
Upwind Vent Gate (UVG) Open and Closed
•
Downwind Vent Gate (DVG) Open and Closed
Results:
•
•
•
The largest effect is from UVG position.
Elevation angle was not a statistically significant.
For the outside wind sensors (dome), the only
significant effects: AoA and wind velocity itself.
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NIO Wind Loading Workshop
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average pressure PSD(P00060oo)
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average pressure PSD(Q04530oo)
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Average Pressure PSD Data
- Effect of Upwind Vent Gate
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Average Pressure PSD Data
- Effect of Elevation
 Note: No elevation dependence on average pressure on primary
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Definition of Structural Function
Structure tensor D employed to define random wind
pressure difference between two points.
For homogeneous and isotropic wind pressure,
structural function D(d):
D(d) = < [P(r+d) – P(r )]2 >
d = separation between two points
r = position on the mirror surface
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NIO Wind Loading Workshop
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Average Pressure Structure Function
(C00030oo)
Average Structural Function for C00030oo
4
RMS pressure, Prms (N/m2)
3.5
3
2.5
2
1.5
D(d) = 0.096 d 0.41
1
0.5
0
Prms = 0.076124 d ** 0.4389
0
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1000
2000
3000
4000
5000
sensor spacing, d (mm)
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6000
7000
8000
Amplitude strongly dependent on vent gate setting
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Extrapolation to 30 Meters
RMS Pressure Difference (pascals)
0.25
Pressure variation on 30-m mirror about twice 8-m
0.2
0.15
0.1
Prms = 0.04d^0.5
0.05
0
0 2001
26-27 November,
10
NIO
Wind Loading Workshop
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Sensor Separation (meters)
30
38
Summary and Conclusions
• Wind loading on M1 is strongly dependent on vent gate openings
• Wind loading on M2 is not strongly dependent on vent gate
openings
• Control algorithm will maintain wind speed at M1 < 3 m/sec
• With vent gates closed, M1 deformations remain within error
budget even in high winds
• Pressure variations on M1 are larger than average pressure
• M1 wind deformations are dominated by astigmatism
• M1 deformations are proportional to RMS pressure variations on
surface
• M1 deformations ~ proportional to (wind velocity at mirror)²
• Pressure structure functions fit 0.5 power law
• Structure functions allow extrapolation to larger telescopes –
pressure range for 30m twice that of 8m
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NIO Wind Loading Workshop
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The Gemini South wind test results are available
on the AURA New Initiatives Office Web site at:
www.aura-nio.noao.edu
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NIO Wind Loading Workshop
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