Numerical Tower Shadow Modeling for a Downwind Wind Turbine Truss Tower

The numerical simulation of offshore wind turbines needs a realistic tower shadow model, especially when studying downwind wind turbine designs. Using computational fluid dynamic simulations of the wind flow field around a two-dimensional tubular tower and corresponding truss tower cross-sections we show that current tower shadow models do not correctly represent the mean wind speeds close to such complex geometries. Interestingly, the multi-member extension of Powles’ tower shadow model with the square-root law is found to be adequate for the truss tower at moderate distances, and we suggest parameter values that better reproduce the velocities encountered by the blades in such circumstances. Finally, velocity spectra show that the fluctuations behind the two towers are almost an order of magnitude smaller for the truss tower. This suggests that downwind wind turbines on truss towers might suffer less from fatigue damage than downwind machines on tubular towers.