1. Wind Tunnel Testing of a Generic Telescope EnclosureM
Thirty
Meter
Telescope
Wind Tunnel Testing of a Generic Telescope Enclosure
Tait S. Pottebaum
Douglas G. MacMynowski*
California Institute of Technology
June 2004
*formerly D. MacMartin

2. Experiment Model: Data: 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

3. Scaling Dimensionless parameters
where L is the side length of the opening
Convective frequency scaling
Helmholtz frequency scaling
where V is the enclosed volume and c is the speed of sound

4. Experimental Setup 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

5. Large scale flow, 0° and 180 °

6. Smoke Visualization U∞
0° azimuth
Smoke injected from outside the dome

7. 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 fH
Shear layer modes:
Frequency: f ~ 0.65nU/L
Present for AZ ≤ 90°
Mode n depends on speed; influenced by Helmholtz mode

8. 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
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°

9. Sample data image: 1st snapshot

10. Sample data image: 2nd snapshot

11. Mean in-plane velocity, 0°

12. In-plane rms fluctuation, 0°

13. Mean in-plane velocity, 180°

14. In-plane rms fluctuation, 180°

15. Profiles on telescope axis

16. Conclusions Upwind viewing Downwind 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

17. 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