Wind Loading of Large TelescopesWind loading characteristics zIs it possible to characterize wind...

23 August, 2002 CHO SPIE Kona Conference (4837-40) #1

Wind Loading of Large Telescopes

Myung Cho, Larry Stepp, George AngeliAURA New Initiatives Office

David SmithMERLAB


Wind is a key factor in large telescope design

Driving factors:• Larger telescope size, larger wind-induced deflections• Telescope frequencies closer to peaks of wind spectra

Seeing and buffeting• wind helps to mitigate thermally-induced local seeing • wind buffeting affects pointing and tracking and causes localized deformations of mirrors

Wind Test (pressure and velocity)• Wind test on Gemini South Telescope, May 2000


Wind Test and Procedures

• Why: to identify source of pressure variations– Wind attack angles – Telescope pointing angle (Zenith angle) – Wind vent gate positions

• How: to setup various test configurations – 116 different combinations of wind test

• What: to measure wind pressure/velocities– 24/32 pressure; 6 anemometers– Pressure/velocities at 10 samples/second (no delay)


Gemini South Wind Tests(May 2000) Sensor locations

3-axis anemometers

Pressure sensors, 32 places

Pressure sensors

Ultrasonic anemometer

Ultrasonic anemometer


Overview of the L16, L9 Analysesstatistical approach - standard design of experiments (DOE)

• Largest effect is from vent gate position.• Elevation angle is NOT statistically significant.


Simultaneous Animations(c00030oo)

Wind Pressure (N/m2) Mirror Deformation (microns)

Wind Speed at 5 Locations (m/sec)


Structure functionCorrelation length, Lp

Are the pressures on the mirror correlated?If so, what is the limiting distance?

Solution: Structure functionD(ρ) = < | P(r+ ρ) - P(r) |2 >L p data set L p data set Lp data set

1.95 meter c00030oo 1.33 meter c04530oo 0.83 meter c09030oo1.80 meter t00030oo 1.71 meter t04530oo1.78 meter c00060oo 1.38 meter c04560oo 0.90 meter c09060oo1.88 meter d00060oo 0.90 meter d09060oo

wind direction90º

zenith angle 30o

zenith angle 60o

wind direction0º

wind direction45º

Correlation length is less than 2 meters for most wind cases

Strong dependence on wind direction


Wind loading characteristics

Is it possible to characterize wind pressure as a time varying average pressure pattern?

If so, average temporal pressure can be representable to wind pressure.Average pressure can be predictable from Computational Fluid Dynamics (CFD) modelsThen, characteristics of wind pressure and velocity from CFD models can be an input wind loading as PSD in FE analyses


Best Fit to Average Pressure

Wind pressure at sensor locations

Temporal Average pressure at sensor locations

Pi(t) = C(t) Pai + Pri(t)Pi(t) raw data at time t at ith sensor C(t) coefficient of fit at time tPai temporal average pressure at ith sensorPri(t) fit residual at time t at ith sensor

<Pi(t)2> = <C(t) Pai2 > + <Pri(t)2 > + 2 <Pri(t) * C(t) Pai>


Coefficient of Fit C(t)

C(t) time variation

<Pi(t)2> = <C(t) Pai2 > + <Pri(t)2 > + 2 <Pri(t) * C(t) Pai>

Fit ResidualAverage pressure is representable to wind pressure


Does C(t) contain velocity effects?

C(t) with wind velocity at M1

f –1.384Kolmogorov:

( ) 35−

= fCfΦ vv

Prms vs C(t) PSD fit (open-open)

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.00.0 0.5 1.0 1.5 2.0 2.5 3.0

Prms

C(t)

PSD

slop

e

c13560oo

t00000oo

c13560oo

(f n)

( n )

f –1.70

C(t) shows Kolmogrov velocity PSD f-5/3 rule


Comparison of Predicted and Measured Pressure Patterns

c00030oo

t00030ooMeasured pressure patterns

Facing into wind, Zenith angle = 30°

Pressure pattern using a simple disk predicted by CFD

(DeYoung)


Alternate CFD Model

• For further investigation, Oleg Likhatchev (University of Arizona) set up simple CFD models with a disk of D x H (8m x 2m)

– CFD code FLUENT; Quasi-steady assumption; Zenith angle = 30

Stepped CFD model shows similar effect to measured data(Low pressure in “leading edge”; High in “trailing edge”)Accurate CFD modeling is essential; NIO/TSU collaboration

“CFD modeling of turbulent shedding effects on 30m GSMT”

Poster paper by Xu (4840-43)


Segmented Mirror wind modeling

Goals in segment wind modeling To specify segment support stiffnessTo identify segment control bandwidthSubscale model to GSMT integrate modelEnables to build analytical tools for GSMT

Segment configurationDimension = 1.33 m point to pointSupport stiffness = 10 N/microns36 segments for a 8 m primary mirror


Gemini Imaginary Segmented Mirror

Gemini Segmented

Mirror Testbed (GSMT)

Gemini Mirror


Sample wind at t=0.1 sec. (t00000oo)

Wind pressure distribution Segment displacement pattern (microns)


Spot size for Diffraction limit (single segment)

0.5 arcseconds

0.1 arcseconds

nominal seeing


GISM Sample animation (t00000oo)

Spot diagram (arc-seconds)Segment displacement pattern (microns)


Simultaneous Animation (t00000oo)

Spot diagram (arc-seconds)Segment displacement pattern (microns)“Active optics and control architecture for GSMT”presented by Angeli (4840-22)


GISM wind effects (t00000oo)

90% energy encircled at 0.173 arcsec radius (for 5 minute integration)


Effect of vent gate positions

Pressure Deformation

Surface RMS = 0.31 microns

Surface RMS = 0.03 microns

Seg-Deformation

Surface P-V = 0.80 um

Surface P-V = 0.07 um

C00030oo

open

open

C00030cc

closed

closed


Summary and Conclusions

• Correlation length is less than 2 meters in most cases

• Average wind pressure: Low in “leading”; High in “trailing”

• CFD should reflect physical geometries, turbulent flow conditions• Average pressure pattern contains most energy (small fit

residuals)• Wind pressure is favorably expressed by time varying Pavg

• M1 wind deformations: astigmatisms; Segment in high

frequencies

• PSD of C(t) fits with frequency in fn, n ~ ( -1.4 to -2.4)

• On average C(t) fits show Kolmogrov velocity PSD f-5/3 rule

• Developed analytical/design tools for GSMT


Future Plan

Wind loading simulationWind loading characteristics (Lo, Lp, PSD,…)Gemini Segmented Mirror Testbed simulation

CFD analysesEstablish CFD models for GSMTIdentify and quantify key design parameters

More wind testsLonger time at higher rate in denser samplesShare expertise and collaborations


The Gemini South wind test results are availableon the AURA New Initiatives Office Web site at:

www.aura-nio.noao.edu

Wind Loading of Large TelescopesWind loading characteristics zIs it possible to characterize wind...

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