Computational fluid dynamics simulations and wind tunnel measurements of unsteady wind loads on a scaled model of a very large optical telescope: A comparative study

Thorough theoretical and experimental investigations were performed to analyze the main characteristics of unsteady flows past a 1:100-scale wind tunnel (WT) model of a very large optical telescope housed within a spherical enclosure. The investigations were focused on the prediction and measurements of unsteady pressures on the inner and outer surfaces of the enclosure and on the telescope primary mirror. The WT measurements were performed essentially to provide aerodynamic data on the telescope structure and also to build a database for correlation with numerical simulation of the flow using computational fluid dynamics (CFD). Unsteady viscous flow solutions were computed for different telescope orientations using the lattice Boltzmann method coupled with the RNG k-ε turbulence model. For WT testing, unsteady pressure measurements were performed in an open jet WT for different telescope orientations and wind speeds, using a number of pressure taps distributed around the inner and the outer surfaces of the enclosure and on the primary mirror surface. A smoke stream visualization technique was also used to study the flow behavior around and inside of the telescope enclosure. The flow solutions were computed using the WT flow conditions. Correlations were obtained between CFD and WT data in terms of the mean pressure coefficients on the enclosure and the primary mirror surfaces, and for their standard deviations. Power spectral density analyses were also carried out for a number of pressure signals collected on the primary mirror surface. Both CFD solutions and WT measurements demonstrated that the flow inside and outside the enclosure was unsteady and massively separated on the back of the enclosure. The mean values and standard deviations of the pressure coefficients on the enclosure and the primary mirror surfaces correlated well with the experimental data. Using the WT Mach number in the simulation, the shear layer over the enclosure opening and the resulting acoustic wave effects were well captured, and there was excellent agreement between the CFD results and the WT measurements.