The spectral signature of wind turbine wake meandering: A wind tunnel and field‐scale study

Abstract

Field‐scale and wind tunnel experiments were conducted in the 2D to 6D turbine wake region to investigate the effect of geometric and Reynolds number scaling on wake meandering. Five field deployments took place: 4 in the wake of a single 2.5‐MW wind turbine and 1 at a wind farm with numerous 2‐MW turbines. The experiments occurred under near‐neutral thermal conditions. Ground‐based lidar was used to measure wake velocities, and a vertical array of met‐mounted sonic anemometers were used to characterize inflow conditions. Laboratory tests were conducted in an atmospheric boundary layer wind tunnel for comparison with the field results. Treatment of the low‐resolution lidar measurements is discussed, including an empirical correction to velocity spectra using colocated lidar and sonic anemometer. Spectral analysis on the laboratory‐ and utility‐scale measurements confirms a meandering frequency that scales with the Strouhal number St = fD/U based on the turbine rotor diameter D. The scaling indicates the importance of the rotor‐scaled annular shear layer to the dynamics of meandering at the field scale, which is consistent with findings of previous wind tunnel and computational studies. The field and tunnel spectra also reveal a deficit in large‐scale turbulent energy, signaling a sheltering effect of the turbine, which blocks or deflects the largest flow scales of the incoming flow. Two different mechanisms for wake meandering—large scales of the incoming flow and shear instabilities at relatively smaller scales—are discussed and inferred to be related to the turbulent kinetic energy excess and deficit observed in the wake velocity spectra.

A Review on the Meandering of Wind Turbine Wakes

“…However, the magnitude of the PSD at the peak frequency of the inflow is significantly reduced by the turbine until about 3D downwind of the turbine, where the peak PSD of the inflow large eddies recover to values slightly lower than that upwind of the turbine. This observation is consistent with the field measurement of full-scale wind turbines and was termed as a sheltering effect of the turbine in [71], as shown in Figure 7. In [71], Heisel et al studied the spectral characteristics of wake meandering using both wind tunnel and field-scale measurements.…”

“…This observation is consistent with the field measurement of full-scale wind turbines and was termed as a sheltering effect of the turbine in [71], as shown in Figure 7. In [71], Heisel et al studied the spectral characteristics of wake meandering using both wind tunnel and field-scale measurements. It is noted that, in this review, we refer to turbines such as the EOLOS turbine of 96 m rotor diameter as utility scale, the SWiFT turbine of 27 m rotor diameter and above as field scale, and turbines employed in wind tunnels (e.g., the G-1 wind turbine model of rotor diameter 1.1 m [66] and the miniature model turbine of rotor diameter 0.13 m [64]) as laboratory scale.…”

“…A deficit of the energy of the large-scale eddies (as shown in Figure 7a,b for the field-scale measurements) is observed in both measurements, which is caused by the transfer of energy from the large-scale eddies to the small scales in the turbine wake, which was also observed in numerical simulations, as shown in Figure 6. Heisel et al [71] postulated that the wake meandering is driven by two different mechanisms, i.e., one by the large-scale eddies and the other by the shear layer instability as bluff bodies, at two separate scales, which is consistent with and demonstrated by the LES results of Yang and Sotiropoulos [70], as shown in Figure 6. In the study of wake meandering of an infinitely long row of wind turbines, Andersen et al [72] observed three distinct peak frequencies and claimed that the two lower frequencies are related to turbine spacing, and the third one is related to the vortex shedding of the wind turbine as a bluff body with a Strouhal number of 0.19.…”

“…Howard et al [64] systematically investigated the characteristics of the wake meandering of a model turbine (D = 0.13 m) and found that the Strouhal number of the hub vortex is about 0.7 and the Strouhal number of the large-scale oscillations is about 0.3. With the data of their own measurements and those in the literature, Heisel et al [71] found that the values of the Strouhal number from laboratory-scale and field-scale experiments are in the same range independent of Reynolds number. Foti et al [67] investigated the characteristics of wake meandering in the Horns Rev wind farm using LES with actuator surface models for the turbine blades and nacelle.…”

“…show the wake energy excess or wake energy deficit compared with that of inflow. Reproduced from [71]. Reprinted with permission.…”