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 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 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 V 2, where = air density (1.29kg/m 3 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 - 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 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