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