Velocity measurements around low-profile, tilted, solar arrays mounted on large flat-roofs, for wall normal wind directions

An investigation of the wind field around roof-mounted solar arrays has been undertaken, utilizing synchronized time-resolved particle image velocimetry and pressure measurements, in order to better understand the flow structures and aerodynamic mechanisms which cause the peak wind loads. The study focused on wall normal wind directions, which result in critical loads on panels within the separation bubble. The mean flow is not significantly altered above the array as compared to that for a bare roof. However, the array has a significant effect on the turbulence above and around the panels in the separation bubble. When panels are installed on the roof, both Reynolds normal and Reynolds shear stresses are markedly reduced when compared to the flow over the bare roof in the region of the separated shear layer. Ensemble averaged flow fields, conditioned on peak panel uplift, were used to investigate the mechanisms associated with peak uplifts on the array. As the tilt angle of the solar array is increased, a progression from purely vortex driven suctions, which cause peak uplifts on a bare roof, towards local flow driven uplifts is observed. For small tilt angle (2°) arrays this local flow is established by large-scale building generated vortices, while for the larger tilt angle (20°) arrays the instantaneous reattachment of the (building generated) separated shear layer sets up the local flow. For the larger tilt angles, south wind peak uplifts are driven by large vertical gusts, while peaks for northern winds are the result of streamwise gusts. The interaction of these large-scale features with the panels, in the form of locally oriented drag, results in the peak uplifting loads.