Abstract:
Vortex shedding and the resultant transient loadings on a medium-sized heliostat are investigated in this paper. Reynolds-Averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) of a simplified version of an operational heliostat, the LH-2, is used as a validation case for mean loads and to define the orientations used for fluid-structure interaction (FSI): maximum drag in the upright position, and maximum torque-tube moment at an elevation of 60° relative to the vertical. Optimized Atmospheric Boundary Layer profiles for mean flow and turbulence intensity were implemented as the inlet flow boundary condition for both the RANS and Stress-Blended Eddy Simulation (SBES) simulations, with the latter using the synthetic turbulence technique, the Vortex Method, at the inlet. The SBES results show a strong likeness to the experimental results of Peterka et al. (1986) with a comparable mean and peak loading distribution. The transient SBES CFD pressure was implemented in a one-way FSI simulation to obtain the structural response of the heliostat to the transient wind loading. The results show that the response of the heliostat conformed to and depended on the mode shapes and frequencies of the heliostat structure more so than the vortex shedding frequencies. For the upright case, the Strouhal number obtained was within 8% of that obtained from an experimental study in literature, with the main vortex-shedding frequency shown to be 0.5 Hz. When excited with this wind, the structure responded with the third modal frequency of 2.7 Hz. The 60° elevation case also responded mainly with the third mode of 2.6 Hz being excited, but the response was significantly influenced by a combination of higher modes located around 6 Hz. The results from the transient structural analysis using the temporal SBES heliostat surface pressure fields as input indicate that the method holds promise in predicting the transient response of heliostats. Importantly it can be concluded that due to the difference in frequencies between the vortex shedding and modal frequencies, the structure is safe from self-excitation.
