Download Coast Steel Fabricators

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 - XLT
Size 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 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
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”
XLT Enclosure
Other Enclosure Styles
• Carousel Style
• Conventional Dome
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 Enclosure
Interior Layout
XLT Enclosure
Telescope Configuration
XLT Telescope
Configuration 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 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
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
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
Wind Engineering Tools
Computational Fluid Dynamics (CFD)
• Scale
• site topography
• enclosure internal
• enclosure external
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
XLT Enclosure
Preliminary CFD Analysis
• contours of turbulence intensity
XLT Enclosure
Preliminary CFD Analysis
• contours of velocity magnitude
XLT Enclosure
Preliminary CFD Analysis
• velocity vectors
XLT Enclosure