Effects of turbulent inflow time scales on wind turbine wake behavior and recovery

Abstract

Wind turbines experience a range of turbulent time and length scales related to the atmospheric boundary layer, wakes of upstream turbines, and wind farm effects. This work aims to investigate the impact of turbulent scales on wake behavior and recovery, in isolation from overall turbulence intensity, shear, or buoyancy. Large eddy simulations of a single turbine are conducted using idealized single time scale inflows and full spectra turbulent inflows, varying the predominant time scale in the equivalent Strouhal number range of S t = 0.04 - 0.5 , while maintaining the same turbulence intensity and flow structures. Under idealized inflows, shorter inflow time scales result in a faster breakdown of tip vortices, while longer scales induce greater wake meandering. For full spectra turbulent inflows, shorter integral time scales result in a shorter near-wake region and enhanced wake recovery, while wake meandering occurs to a similar extent when adjusted for the near-wake breakdown location. A wake-generated turbulence region in the range of S t = 0.3 - 0.7 is identified in the streamwise velocity spectra, and inflows that contain more turbulent kinetic energy in this range show a faster redistribution from long inflow scales to smaller wake-generated turbulence and enhanced wake recovery. The improved wake recovery for the shortest integral time scale results in a 9% increase in mean rotor-averaged velocity and 35% increase in power at 12R downstream, compared to the longest integral time scale. Overall, it is shown that inflow turbulent scales have a significant impact on wake recovery, particularly through their impact on the near-wake breakdown.