Impact of free-stream turbulence and thrust coefficient on wind turbine-generated wakes

This study investigates the influence of free-stream turbulence (FST) and the thrust coefficient ( $C_T$ ) on wind turbine wakes. Wakes generated at $C_T \in \{0.5, 0.7,0.9\}$ are exposed to turbulent inflows with varying FST intensities ( $1\,\% \lesssim {\textit{TI}}_{\infty } \lesssim 11\,\%$ ) and integral length scales ( $0.1 \lesssim {\mathcal L}_x/\!D \lesssim 2$ , $D$ is the rotor diameter). For high- ${\textit{TI}}_{\infty }$ inflows, a flow region in the wake is observed where a mean momentum deficit persists despite the turbulence intensity having already homogenised with that of the free stream, challenging traditional wake definitions. A ‘turning point’ in the mean wake width evolution is identified, beyond which wakes spread at slower rates. Near-field ( $x\!/\!D \lesssim 7$ ) wake growth rate increases with higher ${\textit{TI}}_{\infty }$ and $C_T$ , while far-field ( $x\!/\!D \gtrsim 15$ ) wake growth rate decreases with higher ${\textit{TI}}_{\infty }$ – a finding with profound implications for wind turbine wake modelling that also aligns with the entrainment behaviours observed in bluff- and porous-body wakes exposed to FST. Increasing ${\mathcal L}_x$ delays wake recovery onset and reduces the mean wake width, with minimal effect on the spreading rate. Both $C_T$ and FST influence the high- and low-frequency wake dynamics, with varying contributions in the near and far fields. For low- ${\textit{TI}}_{\infty }$ and small- ${\mathcal L}_x$ inflows, wake meandering is minimal, sensitive to $C_T$ and appears to be triggered by a shear-layer instability. Wake meandering is enhanced for high- ${\textit{TI}}_{\infty }$ and large- ${\mathcal L}_x$ inflows, with the integral length scale playing a leading role. This emphasises the complex role of FST integral length scale: while increasing ${\mathcal L}_x$ amplifies meandering, it does not necessarily translate to larger mean wake width due to the concurrent suppression of entrainment rate.