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Extra Large Telescope Wind Engineering

Extra Large Telescope Wind Engineering. Wind and Large Optical Telescopes. Wind is a key factor in the design of large telescopes: larger wind-induced deflections lower natural frequencies frequencies closer to peaks of wind velocity spectra Seeing

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Extra Large Telescope Wind Engineering

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  1. Extra Large Telescope Wind Engineering

  2. Wind and Large Optical Telescopes • Wind is a key factor in the design of large telescopes: • larger wind-induced deflections • lower natural frequencies • frequencies closer to peaks of wind velocity spectra • Seeing • large mirrors more difficult to maintain thermal equilibrium • wind helps to mitigate thermally-induced local seeing problems • wind buffeting affects pointing and tracking and causes localized deformations of mirrors

  3. Extra Large Telescope - XLT Size Comparison

  4. XLT Enclosure • “Calotte” Configuration • structurally-efficient spherical shell • stiff structure - less vibration • minimum air volume - efficient thermal control • round aperture - less turbulence • wind screens not required

  5. XLT Enclosure External Service & Maintenance Tower • no enclosure cranes - minimal handling equipment inside dome: • lighter enclosure - less power consumption, less heat generated • less obstructions to airflow • tower impacts airflow around and inside enclosure

  6. XLT Enclosure • Wind Control • wind fences • on aperture perimeter • impact of fence porosity • surface roughness • airflow around rounded bodies sensitive to roughness • ribs projecting 2% of diameter considered “very rough”

  7. XLT Enclosure Other Enclosure Styles • Carousel Style • Conventional Dome

  8. Site Conditions • Atmospheric Boundary Layer thickness depends of surface roughness and time of day • Turbulence caused by ridges, hollows and other topographical features • Wind speed-up over hills • Prevailing wind speed and direction • Air temperature and density

  9. XLT Enclosure Interior Layout

  10. XLT Enclosure Telescope Configuration

  11. XLT Telescope • Configuration Options • 3-Mirror Option • 2-Mirror Option (shown) • Primary Mirror Cell

  12. XLT Telescope Wind Interaction • Tripod or Quadrapod Configuration • Cylindrical Truss and Spider Configuration

  13. Wind Engineering Tools • Finite Element Analysis (FEA) • Static Analysis • Simplified: apply constant pressure q = 0.5•d•V2, where d = air density (1.29kg/m3 at 0C, 1atm), V = wind velocity (m/s), q (kPa); use dynamic factors for gusts, vortex shedding forces, and exposure conditions • Detailed: account for intensity of wind turbulence at site as function of structure height and terrain roughness; dynamic factors use empirical wind speed spectra and aerodynamic admittance functions

  14. Wind Engineering Tools • Finite Element Analysis (FEA) • Dynamic Analysis • Modal Analysis • vibration modes and frequencies • Transient Dynamic Analysis • time history - simplified input (ie. rectangular pulse function), input from CFD or sensor data • Response Spectrum Analysis • requires wind speed spectrum • Random Vibration Analysis

  15. Wind Engineering Tools • Water Tunnel • Accuracy • How did past experiments predict actual observatory conditions? • Natural Conditions • How can realistic velocity profile and turbulence be simulated? • Dimensional Scaling Problem • High Reynolds numbers require large and expensive test setup • Computational tools reduce need for scale testing

  16. Wind Engineering Tools • Computational Fluid Dynamics (CFD) • Scale • site topography • enclosure internal • enclosure external

  17. Wind Engineering Tools • Computational Fluid Dynamics (CFD) • Temperature • isothermal: applicable to higher wind speeds and larger scales • thermal variations: more important for enclosure interior environment • Turbulence Model • important factor in CFD environmental applications • standard k-e model: adequate for larger scale • increased computational complexity required for flows around “bluff-bodies” • RNG k-e model: more accurate and reliable for a wider class of flows than standard k-e model

  18. XLT Enclosure Preliminary CFD Analysis • contours of turbulence intensity

  19. XLT Enclosure Preliminary CFD Analysis • contours of velocity magnitude

  20. XLT Enclosure Preliminary CFD Analysis • velocity vectors

  21. XLT Enclosure

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