Estimating the effect of wind loading on extremely large telescope performance using computational fluid dynamics

Abstract

Wind loading is one of the critical parameters influencing the performance of large telescopes, with potentially more dramatic consequences for proposed future giant telescopes. This study describes a strategy for modeling the effects of wind loading on extremely large telescopes such as the Thirty Meter Telescope (TMT). The optical performance of the telescope is estimated by an integrated model, which incorporates the telescope structure, optics, and control. To model the dynamic force variation on the telescope, a Finite Element Analysis (FEA) model of the telescope is created along with an unsteady Computational Fluid Dynamics (CFD) model of the airflow around the enclosure-telescope configuration, which should have a suitable level of geometric fidelity. Numerical simulations using the CFD model are performed for a chosen wind speed and telescope orientation (azimuth, zenith), through which the dynamic force pattern on the primary and secondary mirrors as well as on the secondary support structure can be determined. Finally the force pattern is applied to the FEA model. This can be achieved either by applying temporally and spatially filtered white noise forces with random distribution deducted from the CFD analysis, or by considering the dynamic force pattern itself from the unsteady CFD calculations. Since the FEA and CFD models usually have different resolution requirements and consequently different, non-uniform spatial sampling grids, a key part of the interface is the conversions necessary to transfer the forces from CFD surface cells to structural nodes.