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Extra Large Telescope Wind Engineering

This document discusses the critical role of wind engineering in the design and operation of extra large telescopes. It highlights how wind affects mirror deformations, tracking accuracy, and overall performance. The report covers innovative enclosure designs such as the "Calotte" configuration that minimize vibration and improve airflow. It also examines wind control mechanisms, including fences and computational fluid dynamics (CFD) strategies for predicting turbulence and environmental interactions. Key methodologies for static and dynamic analysis are explored to ensure optimal telescope functionality.

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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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