Wind Tunnel Testing of a Generic Telescope Enclosure M 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 macmardg@cds.caltech.edu

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

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

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

Large scale flow, 0° and 180 °

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

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

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°

Sample data image: 1st snapshot

Sample data image: 2nd snapshot

Mean in-plane velocity, 0°

In-plane rms fluctuation, 0°

Mean in-plane velocity, 180°

In-plane rms fluctuation, 180°

Profiles on telescope axis

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

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