Impact of freestream turbulence integral length scale on wind farm flows and power generation

Abstract

The impact of freestream turbulence integral length scale on wind farm flow and power production is investigated by conducting Large Eddy Simulations on wind farms with two spacings, $S_x=8R$ and $S_x=12R$ (turbine radius $R$). The integral length scale of inflow turbulence $L_u$ is varied, $L_u\in [3.2R,12.0R]$, while maintaining identical turbulence intensity and velocity. Shorter integral length scales lead to a faster near wake breakdown and improved wake recovery in the wake of the first turbine, causing substantial increases in the second turbine power output; 42% and 18% for the two spacings. Over the first four turbines, total power output increases by 8.6% and 6.0% respectively. Spectra, cross-correlations and entrainment scales are also examined and show that the first turbine breaks down inflow scales and wake-generated turbulence dominates the inflow to the second turbine. Further into the turbine row, dominant flow structures and entrainment scales are associated with both wake turbulence and larger wind farm-generated structures matching the turbine spacing. These results show that the freestream turbulence integral length scale has a significant impact on wind farm flows and power generation, mainly by impacting the development of wakes in the farm entrance.