What Do We Need to Know about Wind for GSMT?

1. What Do We Need to Know about Wind for GSMT? George Angeli 26 November, 2001

2. Introduction • GSMT modeling environment • Wind information needed • Known (perceived) inconsistencies between models and experiments • What to expect from wind simulations?

3. Design process • Concurrent engineering (structural, optical and control) • Design verification through simulation • Feedback to reiterate and improve the design

4. Our approach

5. 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

6. Our approach • Potential drawbacks • Difficult to handle nonlinear effects in structure or optics • Limits of the linear optical approach should be explored and established

7. Structural model Modal description: State-Space description:

8. Optical model Modal description: Small deformations! P - optical sensitivity Zernike expansion: Ray tracing

9. Integrated model

10. 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

11. 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

12. 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

13. 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

14. 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)

15. 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

16. Experimental wind data

17. 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)

18. 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

19. 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

20. 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

21. Atmospheric model • Taylor’s frozen flow hypothesis • Atmospheric “dispersion” • Temporal PSD

22. 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

23. 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

24. Basic questions • How to scale our existing measurements to the GSMT? • What kind of additional measurements we need (if any)?

25. Pressure/Force PSD on primary mirror