Characterization of wind loading of telescopes

Angeli, George Z.; Cho, Myung K.; Sheehan, Mike; Stepp, Larry M.

doi:10.1117/12.489825

Cited by 27 publications

(22 citation statements)

References 9 publications

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“…Some limited but valuable data is available from measurements taken at Gemini South observatory. 9,10 More extensive data under more controlled conditions has been collected in scaled wind-tunnel experiments; while this approach has been used for many years, 11 modern experimental techniques allow significantly more data to be collected. 12,13 Finally, computational fluid dynamics (CFD) [13][14][15] is an ideal design tool, and can take advantage of the experimental data for validation, so that CFD can be used with confidence in future design work.…”

Section: -8mentioning

confidence: 99%

“…The outer scale L s associated with the turbulence observed near the secondary mirror, and the turbulence near the primary mirror when the vents are closed is roughly equal to the diameter D s of the dome opening. 5,10 The outer scale L v associated with the turbulence over M1 that is caused by flow through open vents is related to the height D v of the vent gates. 10 The spatial correlation length of pressure on a surface resulting from turbulence is related to the outer scale by = L/ √ 70.8.…”

Section: Mechanismsmentioning

confidence: 99%

“…5,10 The outer scale L v associated with the turbulence over M1 that is caused by flow through open vents is related to the height D v of the vent gates. 10 The spatial correlation length of pressure on a surface resulting from turbulence is related to the outer scale by = L/ √ 70.8. 19 The spatial correlation must be accounted for in computing forces to apply to the structure; an exponential correlation function R(r) = e −|r|/ is used herein as an approximation.…”

Section: Mechanismsmentioning

confidence: 99%

“…This likely explains why the shear layer mode is relatively weak in data collected at Gemini, and only visible at all in the data collected with the vent gates closed. 10 To account for this effect in the model, we introduce a second scale factor α s < 1. An accurate assessment of this factor will require detailed modeling with particular dome and shutter designs; until this has been done we assume that α s = 0.25 will not be difficult to achieve.…”

Section: Mechanismsmentioning

confidence: 99%

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Parametric modeling and control of telescope wind-induced vibration

et al. 2004

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A parametric model of the dynamic performance of an optical telescope due to wind-buffeting is presented. The model is being developed to support the design of next generation segmented-mirror optical telescopes through enabling rapid design iterations and allowing a more thorough exploration of the design space. A realistic performance assessment requires parametric descriptions of the wind, the structural dynamics, active control of the structure, and the optical response. The current model and its assumptions are presented, with the primary emphasis being on the parameterization of the wind forces. Understanding the temporal spectrum and spatial distribution of wind disturbances inside the telescope enclosure is one of the most challenging aspects in developing the overall parametric model. This involves integrating information from wind tunnel tests, computational fluid dynamics, and measurements at existing observatories. The potential and limitations of control to mitigate the response are also discussed, with realistic constraints on the control bandwidth obtained from the detailed structural model of a particular point design. Finally, initial results are presented on performance trends with a few key parameter variations.

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Section: -8mentioning

confidence: 99%

Section: Mechanismsmentioning

confidence: 99%

Section: Mechanismsmentioning

confidence: 99%

Section: Mechanismsmentioning

confidence: 99%

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Parametric modeling and control of telescope wind-induced vibration

et al. 2004

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“…3,4 Previous models of wind loading on the telescope structure for TMT have been based on a parametric model [4][5][6] informed by a combination of Gemini data, 7,8 wind tunnel testing 9 and CFD. Advances in CFD now allow the unsteady forces on the structure and optics to be computed directly, as a function of external wind speed, orientation of the telescope with respect to the wind, and with enclosure vents either open or closed.…”

Section: Introductionmentioning

confidence: 99%

Unsteady wind loads for TMT: replacing parametric models with CFD

MacMartin

Vogiatzis

2014

SPIE Proceedings

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Unsteady wind loads due to turbulence inside the telescope enclosure result in image jitter and higher-order image degradation due to M1 segment motion. Advances in computational fluid dynamics (CFD) allow unsteady simulations of the flow around realistic telescope geometry, in order to compute the unsteady forces due to wind turbulence. These simulations can then be used to understand the characteristics of the wind loads. Previous estimates used a parametric model based on a number of assumptions about the wind characteristics, such as a von Karman spectrum and frozen-flow turbulence across M1, and relied on CFD only to estimate parameters such as mean wind speed and turbulent kinetic energy. Using the CFD-computed forces avoids the need for assumptions regarding the flow.We discuss here both the loads on the telescope that lead to image jitter, and the spatially-varying force distribution across the primary mirror, using simulations with the Thirty Meter Telescope (TMT) geometry. The amplitude, temporal spectrum, and spatial distribution of wind disturbances are all estimated; these are then used to compute the resulting image motion and degradation. There are several key differences relative to our earlier parametric model. First, the TMT enclosure provides sufficient wind reduction at the top end (near M2) to render the larger cross-sectional structural areas further inside the enclosure (including M1) significant in determining the overall image jitter. Second, the temporal spectrum is not von Karman as the turbulence is not fully developed; this applies both in predicting image jitter and M1 segment motion. And third, for loads on M1, the spatial characteristics are not consistent with propagating a frozen-flow turbulence screen across the mirror: Frozen flow would result in a relationship between temporal frequency content and spatial frequency content that does not hold in the CFD predictions.Incorporating the new estimates of wind load characteristics into TMT response predictions leads to revised estimates of the response of TMT to wind turbulence, and validates the aerodynamic design of the enclosure.

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Buffeting of large telescopes: Wind-tunnel measurements of the flow inside a generic enclosure

Pottebaum

MacMynowski

2006

Journal of Fluids and Structures

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Characterization of wind loading of telescopes

Cited by 27 publications

References 9 publications

Parametric modeling and control of telescope wind-induced vibration

Parametric modeling and control of telescope wind-induced vibration

Unsteady wind loads for TMT: replacing parametric models with CFD

Buffeting of large telescopes: Wind-tunnel measurements of the flow inside a generic enclosure

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