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

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

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

Our approach

l Advantages  Highly improved simulation speed  Significantly reduced computer requirements  Potential for more complex model  Affordable price  Each discipline (mechanical, optical, control) keeps its preferred “native” tools and environment (unlike IMOS but like IODA)

Our approach l 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: P - optical sensitivity Zernike expansion: Small deformations! Ray tracing

Integrated model

What to expect?  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 l Improve the design of the structure to make it less sensitive to wind load by

What to expect?  Optimizing the shape and surface of the dome l Aid the enclosure design to optimize its effect on the wind by  Optimizing the vents and opening on the dome to achieve the required filtering effect

What to expect?  Estimating the amplitude and bandwidth for wind induced deformation of telescope structure and primary mirror  Determining the necessary range and speed of actuators and sensors  Recognizing the need and location of actuators and sensors l Verify the control architecture by

What to expect?  Providing well defined disturbance signals to reject l Aid the design of the various feedback loops by l Help to estimate the optical performance of the telescope

Need to know… l Time evolution of wind forces on structural nodes  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) l Wind characteristics

Need to know… l Wind-to-force conversion  Drag and lift:  Validity of first order approximation  Vortex shedding (buffeting with Strouhal frequency at low Reynolds number)  Aerodynamic attenuation of large structures  Effect of enclosure generated turbulence

Experimental wind data

Use of experimental wind data l Problems  Using Gemini South wind measurements  Limited feedback to design (no understanding of process) l Current approach  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.)

Use of simulated wind data l CFD output  Amplitude and direction time functions  Flexible environmental and simulation parameters l Problems  Limited understanding of the process  Time and resource consuming  Off-line calculations and data transfer

Use of calculated wind data l Wind generated in Matlab l Problems  Process understanding applicable to design optimization  Calculation based on mathematical wind model (mean velocity and direction, velocity, pressure and direction PSDs, cross-correlations) – filtered random variable  On-line data generation  Significant research effort  Flexible environmental and simulation parameters  Probably: simplifying assumptions

Atmospheric model l Kolmogorov’s isotropic turbulence theory  Energy cascade: large eddies ⇒ small eddies  Outer scale L 0 : turbulence not isotropic  Inner scale l 0 : turbulence disappears, energy dissipated through viscosity  Inertial subrange  Spatial PSD

Atmospheric model l Taylor’s frozen flow hypothesis  Atmospheric “dispersion”  Temporal PSD

Atmospheric model l Problem  Infinite κ=0 (outside of outer scale)  Von Karman spectrum  Davenport spectrum l Solution l HOWEVER, inside the enclosure and around the structure the turbulence is NOT isotropic and homogenous

Basic questions l How are the statistics of the random process of wind changing due to: l How is the interaction between the wind and telescope structure changing due to:  the mountain top environment;  the enclosure;  the telescope itself  the enclosure;  the telescope itself

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

Pressure/Force PSD on primary mirror