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Wind Tunnel Testing of a
Generic Telescope Enclosure
Tait S. Pottebaum
Douglas G. MacMynowski*
California Institute of Technology
June 2004
*formerly D. MacMartin
macmardg@cds.caltech.edu
Experiment
• Model:
– Empty telescope enclosure
– Square opening
• size appropriate for roughly f/1.3
– 30° Zenith angle (fixed)
– Diameter = 0.83m, ~1% scale
• Turbulent flow at M2 location
• Probably not turbulent at M1 location
• Data:
– Flow visualization
– Digital particle image velocimetry (DPIV) data in a vertical plane
containing the telescope axis near the dome opening
– Hot-wire anemometer data along the axis of the telescope
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Scaling
Dimensionless parameters
Re 
UL

Re exp  106
Re fs
(12 ms 1 )( 32m)

 34
1
Re exp (35ms )( 0.32m)
where L is the side length of the opening
Convective frequency scaling
U
f0 
L
f 0,exp, 35ms 1  109s1
f 0,fs ,12ms 1  0.37s1
Helmholtz frequency scaling
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c L
fH 
2 V
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fH,exp  62s 1
fH,f s  0.57s 1
V is the enclosed volume and c is the speed of sound
Experimental Setup
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•
•
•
•
Clear Lucite dome with opening
Camera and mirror for visualization & DPIV
Hotwire mounted on traverse
Mirror and optics for laser sheet
Lucas adaptive wall wind tunnel
– 5’ by 6’ un-adapted
• Mounted on turn-table
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Large scale
flow, 0° and
180 °
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Smoke Visualization
• 0° azimuth
• Smoke injected
from outside
the dome
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U∞
Velocity (hot-wire) spectrum
inside enclosure data at 0°, r/R = 0.934
Dominant 1st peak
Large 2nd peak
-5/3 slope
35m/s
20m/s
fH
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Shear layer modes:
• Frequency: f ~ 0.65nU/L
• Present for AZ ≤ 90°
• Mode n depends on speed;
influenced by Helmholtz mode
fH
DPIV data
• Focus on area near the opening
• Principle of measurements
–
–
–
–
–
Seed flow with tracer particles (water droplets)
Illuminate a thin sheet with a laser (vertical plane on centerline of dome)
Synchronize laser with the camera
Record images in pairs with small time separation
Correlate small regions of image to determine displacement
• Weaknesses

x 
u
t
– In regions of steep gradients, velocity is typically underestimated
– Scales smaller than the interrogation regions cannot be resolved
– Only the in-plane components of velocity are measured
• Obtain mean and statistics from large number of image pairs
– 2400 pairs for 0°
– 4495 pairs for 180°
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Sample data image: 1st snapshot
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Sample data image: 2nd snapshot
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Mean in-plane velocity, 0°
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
u Uinf
In-plane rms fluctuation, 0°
u
2
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 v2 
1/2
Uinf
Mean in-plane velocity, 180°
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
u Uinf
In-plane rms fluctuation, 180°
u
2
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 v2 
1/2
Uinf
Profiles on telescope axis
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Conclusions
• Upwind viewing
– Shear layer across enclosure opening periodically rolls up into
large vortices
• Frequencies are well described by convection velocity of shear
layers and a mode number
• Mode selection may be influenced by coupling of the shear layer
instability with Helmholtz oscillations
– Large fluctuation velocities are likely to exert significant unsteady
forces on the secondary mirror and support structure
• Downwind viewing
– Opening is inside the wake recirculation
– Mean velocity local maximum exists inside the dome
– Fluctuation levels are low, so most forces are likely to be steady
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Further analysis
• Data being used for comparison with CFD (Konstantinos
Vogiatzis, AURA NIO)
• Additional testing done with venting; data analysis in
progress.
– Significant attenuation of shear layer modes
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