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George Angeli 26 November, 2001 What Do We Need to Know about Wind for GSMT?
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Introduction l Wind information needed l Known (perceived) inconsistencies between models and experiments l GSMT modeling environment l What to expect from wind simulations?
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Design process l Concurrent engineering (structural, optical and control) l Design verification through simulation l Feedback to reiterate and improve the design
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Our approach
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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)
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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
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Structural model Modal description: State-Space description:
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Optical model Modal description: P - optical sensitivity Zernike expansion: Small deformations! Ray tracing
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Integrated model
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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
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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
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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
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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
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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
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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
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Experimental wind data
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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.)
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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
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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
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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
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Atmospheric model l Taylor’s frozen flow hypothesis Atmospheric “dispersion” Temporal PSD
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Atmospheric model l Problem Infinite energy @ 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
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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
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Basic questions l How to scale our existing measurements to the GSMT? l What kind of additional measurements we need (if any)?
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Pressure/Force PSD on primary mirror
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