The researchers found the best turbine diameter for downwind turbines by considering wind shear and tower shade effects. They created a math model to accurately calculate energy distribution on the turbine's swept area. The study showed that wind shear has little impact on turbine diameter, while tower shade effect and tower height play a significant role. There is a direct relationship between turbine diameter and tower diameter, with tower height having the most influence. The study concluded that there is an optimal match between turbine diameter and tower height to maximize energy output in downwind turbines.