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What Do We Need to Know about Wind for GSMT?. George Angeli 26 November, 2001. Introduction. GSMT modeling environment. Wind information needed. Known (perceived) inconsistencies between models and experiments. What to expect from wind simulations?. Design process.
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What Do We Need to Know about Wind for GSMT? George Angeli 26 November, 2001
Introduction • GSMT modeling environment • Wind information needed • Known (perceived) inconsistencies between models and experiments • What to expect from wind simulations?
Design process • Concurrent engineering (structural, optical and control) • Design verification through simulation • Feedback to reiterate and improve the design
Our approach • Advantages • Highly improved simulation speed • Significantly reduced computer requirements • Potential for more complex model • Each discipline (mechanical, optical, control) keeps its preferred “native” tools and environment (unlike IMOS but like IODA) • Affordable price
Our approach • Potential drawbacks • Difficult to handle nonlinear effects in structure or optics • Limits of the linear optical approach should be explored and established
Structural model Modal description: State-Space description:
Optical model Modal description: Small deformations! P - optical sensitivity Zernike expansion: Ray tracing
What to expect? • Improve the design of the structure to make it less sensitive to wind load by • Optimizing the shape and surface of structural elements to minimize the wind-to-force efficiency • Optimizing the geometry of structure to minimize the coupling of wind power into higher order modes • Recognize the need and location of additional damping and stiffening
What to expect? • Aid the enclosure design to optimize its effect on the wind by • Optimizing the shape and surface of the dome • Optimizing the vents and opening on the dome to achieve the required filtering effect
What to expect? • Verify the control architecture by • Estimating the amplitude and bandwidth for wind induced deformation of telescope structure and primary mirror • Recognizing the need and location of actuators and sensors • Determining the necessary range and speed of actuators and sensors
What to expect? • Aid the design of the various feedback loops by • Providing well defined disturbance signals to reject • Help to estimate the optical performance of the telescope
Need to know… • Time evolution of wind forces on structural nodes • Wind characteristics • Velocity distribution in the vicinity of the structure with spatial sampling rate of node distances • Pressure distribution on the primary mirror with at least 3 samples per segments (to resolve torque)
Need to know… • Drag and lift: • Wind-to-force conversion • Vortex shedding (buffeting with Strouhal frequency at low Reynolds number) • Aerodynamic attenuation of large structures • Effect of enclosure generated turbulence • Validity of first order approximation
Use of experimental wind data • Current approach • Using Gemini South wind measurements • Problems • Real amplitude and direction time functions, no “assumptions” • Limited relevance (different place, different size) • No simulation flexibility (given sampling rate, sample length, amplitude, etc.) • Limited environment control (vent gates, direction, elevation, etc.) • Limited feedback to design (no understanding of process)
Use of simulated wind data • CFD output • Amplitude and direction time functions • Flexible environmental and simulation parameters • Problems • Limited understanding of the process • Time and resource consuming • Off-line calculations and data transfer
Use of calculated wind data • Wind generated in Matlab • Calculation based on mathematical wind model (mean velocity and direction, velocity, pressure and direction PSDs, cross-correlations) – filtered random variable • Process understanding applicable to design optimization • Flexible environmental and simulation parameters • On-line data generation • Problems • Significant research effort • Probably: simplifying assumptions
Atmospheric model • Kolmogorov’s isotropic turbulence theory • Energy cascade: large eddies ⇒ small eddies • Outer scale L0: turbulence not isotropic • Inner scale l0: turbulence disappears, energy dissipated through viscosity • Inertial subrange • Spatial PSD
Atmospheric model • Taylor’s frozen flow hypothesis • Atmospheric “dispersion” • Temporal PSD
Atmospheric model • Infinite energy @ κ=0 (outside of outer scale) • Problem • Solution • Von Karman spectrum • Davenport spectrum • HOWEVER, inside the enclosure and around the structure the turbulence is NOT isotropic and homogenous
Basic questions • How are the statistics of the random process of wind changing due to: • the mountain top environment; • the enclosure; • the telescope itself • How is the interaction between the wind and telescope structure changing due to: • the enclosure; • the telescope itself
Basic questions • How to scale our existing measurements to the GSMT? • What kind of additional measurements we need (if any)?