Near and far wake energy transfer mechanisms behind a wind turbine*

Wake meandering, a large diffuse far wake coherent structure, and near wake strong vortical coherent structures induced by blades and the center-body nacelle exhibit regular Strouhal number characteristics in the wake of a wind turbine. The coherent structures are subject to turbulence kinetic energy transfer mechanisms as they evolve, break down, and diffuse. The interactions between coherent scales manifest as triads under a sum-zero frequency condition. To elucidate the interactions, we employ large-eddy simulation of a utility-scale wind turbine with a resolution sufficient to capture spatio-temporal evolution of dominant vortical and diffuse coherent structures and scales in the wake. A precursory inflow is created with a broad range of upwind length-scales similar to an atmospheric boundary layer. The bispectrum, dominant triadic interactions, and scale-specific coherent kinetic energy budget terms are quantified with the scale-specific energy transfer method, based on triple decomposition and dynamic mode decomposition. The triadic interactions present a "web'' or network of connections that transfer kinetic energy among upwind, turbine, and the wake meandering scales. This is observed by the modulation of upwind scales by the wind turbine, the breakdown and diffusion of coherent vortical structures, and the genesis of wake meandering.