Giant Magellan Telescope site and enclosure computational fluid dynamics modeling and analysis

The Giant Magellan Telescope (GMT) is currently planned for construction at Las Campanas Peak in northern Chile. Part of the next generation of extremely large telescopes, GMT will be one of the most powerful ground-based telescopes in operation in the world. Due to the larger aperture envisioned for GMT, characterization and control of the air flow entering and circulating within the enclosure will be required to maintain the highest possible image quality. Aero-thermal interactions between the site topography, enclosure, internal systems, and optics are complex. A key parameter for image quality is the thermal gradient between the terrain and the air mass entering the enclosure, and how quickly that gradient can be dissipated to equilibrium. Because the thermal gradients are highest near the ground, an important function of the GMT enclosure is to minimize the flow of ground-layer air entering the enclosure. By doing so, a more uniform air density above the telescope will enable higher image quality. The design of the GMT lower enclosure is driven by equipment storage and access requirements but also directly impacts the origin and quality of the air entering the enclosure aperture. To ensure the highest quality GMT optical performance, Computational Fluid Dynamics (CFD) models and specialized analyses are utilized to evaluate several lower enclosure designs for their ability to limit the amount of ground-layer air entering the enclosure aperture. Lower enclosure designs with traditional solid outer walls promote the formation of “necklace” vortices, which tend to direct near-surface air, containing steep thermal gradients, into the enclosure aperture, potentially reducing image quality. Modifications to the lower enclosure, such as perforating the outer walls, are shown to suppress these necklace vortices at the expense of added structural complexity and/or reduced internal storage space. Initial isothermal CFD simulations defined the minimum height above terrain reached by the flow-path upwind of the observatory as a proxy to characterize the quality of air entering the enclosure, with lower heights associated with steeper thermal gradients. Based on these results, the most promising designs are further refined and subjected to additional higher fidelity CFD analyses, which includes a terrestrial thermal boundary layer. These simulations are also surveyed to quantify the aero-thermal environment along telescope optical paths, permitting evaluation and comparison of the predicted optical performance of the final candidate enclosure designs. Results from preliminary water tunnel testing of select lower-enclosure designs have increased our confidence in the CFD simulations.