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

The design of future large optical telescopes must take into account the wind-induced vibration of the telescope structure that is caused by large-scale flow structures and turbulence inside the telescope enclosure when the aperture in the enclosure dome is open. However, estimating the resulting degradation in image quality is difficult due to our relatively poor understanding of the flow inside the enclosure. Data has been collected in a wind-tunnel test of an empty telescope enclosure to understand the flow-field around the region near the dome opening where the secondary mirror and supporting structure would be subjected to wind loads. Digital particle image velocimetry (DPIV) data was collected in a vertical plane near the dome opening to obtain mean velocity and fluctuation kinetic energy, and hot-wire data was collected along the telescope axis to obtain temporal spectra of the velocity. Data was collected both with and without additional venting, in order to understand its influence on the flow. The temporal spectrum of the broadband turbulence is of von Karman type. The flow field also includes tonal shear layer modes, with the mode selection influenced by the enclosure Helmholtz mode, and the overall amplitude strongly influenced by the vented area. In addition to its direct use in telescope modelling and design, the data presented herein is of particular value in validation of computational fluid dynamic (CFD) analyses, so that CFD can be used with confidence in future design work.