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Wind Tunnel Testing of a Generic Telescope Enclosure Tait S. Pottebaum Douglas G. MacMynowski* California Institute of Technology June 2004 T T M Thirty.

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Presentation on theme: "Wind Tunnel Testing of a Generic Telescope Enclosure Tait S. Pottebaum Douglas G. MacMynowski* California Institute of Technology June 2004 T T M Thirty."— Presentation transcript:

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

2 T T M Thirty Meter Telescope 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

3 T T M Thirty Meter Telescope 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 T T M Thirty Meter Telescope 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 T T M Thirty Meter Telescope Large scale flow, 0° and 180 °

6 T T M Thirty Meter Telescope Smoke Visualization 0° azimuth Smoke injected from outside the dome U∞U∞

7 T T M Thirty Meter Telescope Velocity (hot-wire) spectrum inside enclosure 35m/s 20m/s data at 0°, r/R = Large 2 nd peak Dominant 1 st peak fHfH fHfH -5/3 slope Shear layer modes: Frequency: f ~ 0.65nU  /L Present for AZ ≤ 90° Mode n depends on speed; influenced by Helmholtz mode

8 T T M Thirty Meter Telescope 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 T T M Thirty Meter Telescope Sample data image: 1 st snapshot

10 T T M Thirty Meter Telescope Sample data image: 2 nd snapshot

11 T T M Thirty Meter Telescope Mean in-plane velocity, 0°

12 T T M Thirty Meter Telescope In-plane rms fluctuation, 0°

13 T T M Thirty Meter Telescope Mean in-plane velocity, 180°

14 T T M Thirty Meter Telescope In-plane rms fluctuation, 180°

15 T T M Thirty Meter Telescope Profiles on telescope axis

16 T T M Thirty Meter Telescope 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

17 T T M Thirty Meter Telescope 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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