Initial computational fluid dynamics modeling of the Giant Magellan Telescope site and enclosure Ryan Danks* a , William Smeaton a , Bruce Bigelow b , William Burgett b a Rowan, Williams, Davies & Irwin Inc., 600 Southgate Dr., Guelph, ON, Canada, N1G 3W6 b GMTO Corporation, 465 N. Halstead Ave., Suite 250, Pasadena, CA, USA, 91101 ABSTRACT In the era of extremely large telescopes (ELTs), with telescope apertures growing in size and tighter image quality requirements, maintaining a controlled observation environment is critical. Image quality is directly influenced by thermal gradients, the level of turbulence in the incoming air flow and the wind forces acting on the telescope. Thus any ELT enclosure must be able to modulate the speed and direction of the incoming air and limit the inflow of disturbed ground-layer air. However, gaining an a priori understanding of the wind environment’s impacts on a proposed telescope is complicated by the fact that telescopes are usually located in remote, mountainous areas, which often do not have high quality historic records of the wind conditions, and can be subjected to highly complex flow patterns that may not be well represented by the traditional analytic approaches used in typical building design. As part of the design process for the Giant Magellan Telescope at Cerro Las Campanas, Chile; the authors conducted a parametric design study using computational fluid dynamics which assessed how the telescope’s position on the mesa, its ventilation configuration and the design of the enclosure and windscreens could be optimized to minimize the infiltration of ground-layer air. These simulations yielded an understanding of how the enclosure and the natural wind flows at the site could best work together to provide a consistent, well controlled observation environment. Future work will seek to quantify the aero- thermal environment in terms of image quality. Keywords: Computational Fluid Dynamics, site microclimate, enclosure design, telescope siting 1. INTRODUCTION The Giant Magellan Telescope 1 (GMT) is a ground based ELT, currently under construction at Las Campanas Observatory, Chile, which when completed will consist of seven 8.4m primary mirror segments providing ten times the resolution of the Hubble Space Telescope. While modern active and adaptive optics systems help mitigate the impact of temperature gradients and wind induced movement, a well-designed enclosure can reduce the degree of those impacts. For an ELT this is particularly important, as the large size of the primary mirror leads to increased wind forces acting on them, which can potentially strain correction systems. Thus, ELT projects are increasingly employing modeling tools like CFD, to qualify and quantify airflow through and around the enclosure and telescope 2,3 . This paper presents the results of preliminary work conducted by the authors to better understand the interaction between the wind microclimate at Las Campanas and the design of the enclosure. The computational fluid dynamics (CFD) simulations in this work were preliminary in nature, with the goal being to use simpler, more nimble models to assess a number of possible scenarios to establish patterns and trends rather than absolute values. In addition to work independently conducted by The Boeing Company 4 , the authors conducted over 30 simulations to characterize the existing wind conditions, and also investigate the following questions: 1. How does the geometry of the enclosure influence the air flow through and around it? 2. How does the enclosure geometry interact with the natural wind speed variations at the site, and can those interactions be taken advantage of to improve the performance of the enclosure? 3. How does the windscreen and enclosure vent configuration impact the above results, and how can they be used most effectively? *ryan.danks@rwdi.com; phone 1 519 823-1311; fax 1 519 823-1316; rwdi.com Modeling, Systems Engineering, and Project Management for Astronomy VI, edited by George Z. Angeli, Philippe Dierickx, Proc. of SPIE Vol. 9911, 991113 © 2016 SPIE · CCC code: 0277-786X/16/$18 · doi: 10.1117/12.2233288 Proc. of SPIE Vol. 9911 991113-1 Downloaded From: http://spiedigitallibrary.org/ on 08/11/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx