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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 - PowerPoint PPT Presentation
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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
• 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
Extra Large Telescope - XLTSize Comparison
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
XLT EnclosureExternal 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
XLT EnclosureWind 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”
XLT EnclosureOther Enclosure Styles
• Conventional Dome• Carousel Style
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
XLT EnclosureInterior Layout
XLT EnclosureTelescope Configuration
XLT TelescopeConfiguration Options
• 3-Mirror Option
• 2-Mirror Option (shown)
• Primary Mirror Cell
XLT Telescope Wind Interaction
• Tripod or Quadrapod Configuration
• Cylindrical Truss and Spider Configuration
Wind Engineering ToolsFinite Element Analysis (FEA)
• Static Analysis
• Simplified: apply constant pressure q = 0.5••V2, where = 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
Wind Engineering ToolsFinite 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
Wind Engineering ToolsWater 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
Wind Engineering ToolsComputational Fluid Dynamics (CFD)
• Scale
• site topography • enclosure internal
• enclosure external
Wind Engineering ToolsComputational 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 - model: adequate for larger scale
• increased computational complexity required for flows around “bluff-bodies”
• RNG - model: more accurate and reliable for a wider class of flows than standard - model
XLT EnclosurePreliminary CFD Analysis
• contours of turbulence intensity
XLT EnclosurePreliminary CFD Analysis
• contours of velocity magnitude
XLT EnclosurePreliminary CFD Analysis
• velocity vectors
XLT Enclosure