Effect of freestream turbulence on the coherent dynamics of a wind turbine wake

The wake of a model wind turbine exposed to incoming freestream turbulence (FST) with a variety of turbulent characteristics is studied through particle image velocimetry experiments. The FST cases were produced using different passive turbulence generating grids. The cases spanned turbulent intensities ( $T_i$ ) in the range $1.3\,\% \lesssim T_i\lesssim 14\,\%$ and only considered short integral length scales $L_v \lesssim 0.2D$ (where $D$ is the turbine diameter). Increasing $T_i$ and $L_v$ in this range resulted in an earlier breakdown of the tip vortices which is shown to be associated with increased wake meandering amplitudes. For all the FST cases considered, the initiation of wake meandering was found to be related to a turbine instability that is excited by the nacelle’s shedding, even for the highest FST levels. The amplitudes of wake meandering were similar for all the cases in the near wake ( $x\lt 2D$ ), but the amplitudes in the far wake ( $x\gt 4D$ ) were discernibly higher for all the FST cases compared with the no grid case (lowest $T_i$ ), primarily due to the early break down of the tip vortices. Deeper insights into the origins, and subsequent evolution, of the various coherent motions (characterised by particular frequencies) in the presence of FST are obtained through analysis of the multiscale triple-decomposed coherent kinetic energy budgets. The wake meandering modes in the presence of FST are shown to better utilise the mean velocity shear, extracting more energy from the mean flow while other sources such as nonlinear triadic interactions and diffusion also become important.