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

Abstract

This study investigates how variations in freestream turbulence (FST) and the thrust coefficient ($C_T$) influence wind turbine wakes. Wakes generated at $C_T \in \{0.5, 0.7,0.9\}$ are exposed to turbulent inflows with varying FST intensity ($1\% \lesssim TI_{\infty} \lesssim 11\%$) and integral length scale ($0.1 \lesssim \mathcal{L}_x/D \lesssim 2$, $D$ is the rotor diameter). For high-$TI_{\infty}$ inflows, a flow region within the wake is observed several diameters downstream, where a mean momentum deficit persists despite the turbulence intensity having already homogenised with the freestream, 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 $TI_{\infty}$ and $C_T$, while far-field ($x/D \gtrsim 15$) wake growth rate decreases with higher $TI_{\infty}$ -- a finding with profound implications for wind turbine wake modelling that also bridges the gap with 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 high- and low-frequency wake dynamics, with varying contributions in the near and far fields. For low-$TI_{\infty}$ and small-$\mathcal{L}_x$ inflows, wake meandering is minimal, sensitive to $C_T$, and appears to be triggered by shear layer instabilities. Wake meandering is enhanced for high-$TI_{\infty}$ and large-$\mathcal{L}_x$ inflows and is dominated by background turbulence. 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.