Effects of thermal stability and incoming boundary-layer flow in the structure of wind turbine wake(s): a wind tunnel study

Wind turbine wakes have been the subject of intensive research, both experimentally and numerically, during the last few decades. Spatial characteristics of wind-turbine wakes are expected to depend on the incoming atmospheric boundary layer flow statistics (e.g., mean velocity distribution, turbulent stresses and turbulent fluxes) and atmospheric stability. On the other hand, large wind farms are supposed to affect the turbulence properties of the boundary layer around of them.

In this study, a model wind turbine was placed in the boundary layer wind tunnel of the Saint Anthony Falls Laboratory at the University of Minnesota. The structure and behavior of the wind turbine wake were studied under different thermal stratifications (neutral and stable). Thermal stratification levels in the boundary layer were achieved by controlling the temperature of both the tunnel floor and the air flow. A triple-wire (x-wire and cold wire) anemometer was used to characterize the turbulent wake downwind of the turbine at different locations. In addition, the flow structure around a wind farm was studied using an array of 10 by 3 aligned model wind turbines. Here, the flow was characterized at different locations inside the wind farm as well as above the turbines. Main results suggest that the turbulent flow around the wind farm can be characterized in two broad regions. The first, located below the turbines top tip height, has a direct effect on the performance of the turbine. Here the mean flow statistics appears to reach equilibrium as close as 3-4 turbines downwind the first (in the aligned case). In the second region, which is located above the first, flow adjusts slowly. Here two layers were found: an internal boundary layer where flow starts to adjust to the new farm conditions and an equilibrium layer, where flow statistics are adjusted.