Influence of Upwind Length Scales on Wake Meandering*

Wake meandering, a large-scale periodic transverse motion of the far wake of a wind turbine affects the overall variability in power production. Two distinct mechanisms have separately hypothesized its formation: (1) Large (much greater than the size of the diameter) upwind structures in the atmospheric boundary layer and (2) turbine-scale bluff body effects related to the size of the rotor. This study focuses on what ranges of upwind length scales interact with the turbine to affect wake meandering. We carry out a series of large-eddy simulations under a range of inflow scenarios to investigate how the distribution of upwind length scales changes the downwind spectra and spanwise velocity statistics. Initially, a precursory simulation of an atmospheric boundary layer is performed. The upwind length scale distribution is manipulated by a high by-pass filtering approach to produce four different upwind length scale scenarios with the largest length scale cutoff ranging from three times the boundary layer height to the turbine diameter. Wind turbine simulations for each scenario employ the actuator surface with the nacelle model to produce similar near-wake characteristics. The far wake exhibit differences to the instantaneous and averaged features of wake meandering. The comparison of the upwind to far wake power spectral densities shows that energy transfers between select length scales.