Unsteady Computational and Experimental Fluid Dynamics Investigations of Aerodynamic Loads of Large Optical Telescopes

Abstract

The present chapter includes a comprehensive literature review of flow simulations and measurements of aerodynamic loads on large optical telescopes, and a discussion on a recent computational fluid dynamics (CFD) investigation, supported by wind tunnel measurements. The investigation was performed at the National Research Council of Canada (NRC) and was focused mainly on predicting and measuring unsteady wind loads on a scaled wind tunnel model of a Very Large Optical Telescope (VLOT), and on a VLOT full-scale model housed within spherical enclosures. Optical telescopes are usually housed in confined enclosures with apertures to limit buffeting problems caused by environmental disturbances such as wind and onsite thermal plumes or by the combined effect. In other words, the enclosures enhance the telescope pointing and tracking performance and also preserve the primary mirror optical shape by minimizing or reducing the unsteady wind loads and air circulation fluctuations inside the enclosure. Most of the present generation telescopes are equipped with relatively small primary mirror areas that limit the light-gathering power and angular resolution. This obviously causes a detrimental effect and restrictions on the telescope performance in capturing details of distant celestial objects, which are key elements for understanding the origin of the universe. To overcome this limitation, astrophysicists are building much larger optical telescopes to allow large mirror reflecting areas. Consequently, massive and stiff supporting structures are required. As the telescopes grow in size, structural buffeting, unsteady wind loads, thermal mass, and other issues become crucial factors that must be considered in the early design phases. The enclosure requires an opening without shielding to avoid smearing of light and electromagnetic waves, and diffraction. The boundary layer flow that starts to build up from the stagnation point on the enclosure surface up to the upstream edge of the opening, must detach and by doing so forms a free strong shear layer across the opening. Under some flow conditions, the shear layer becomes unstable and rolls up into a series of strong and distinct vortices, which impinge against the aft edge of the opening. As a result, acoustic waves are