Impact of Surface Topography and Atmospheric Stability on Interactions of Wind Turbine Wakes

As wind farms grow in size, their aerodynamic performance becomes more dependent on wind turbine wake interactions. While the power and loads of turbines within the farm are regulated by wake interactions, these interactions themselves depend on atmospheric stability, surface topography and other environmental and operational factors. Atmospheric stability can lead to conditions in which wind direction varies substantially with height across the rotor. Surface topography can further modify the local wind direction across a wind farm for a specific height above sea level. Thus, if the turbines are placed in complex terrain at different elevations, at any given time they can face wind coming from different directions. Accordingly, wakes under these conditions will not follow a straight path down the row of turbines, as has been the focus of wind turbine wake studies to date. Instead, wakes will be directed in various directions, leading to complex flow patterns and potentially costly consequences to the farm. To better understand the influence of atmospheric stability and complex terrain in wake interactions, a pulsed scanning Doppler LiDAR is placed at a wind farm in complex terrain. Line-of-sight (LOS) wind speed measurements collected from the LiDAR are post-processed to quantify the variation of wind direction horizontally and vertically over the wind farm and validated against meteorological tower measurements. Different atmospheric conditions like Ekman spiral, low-level jet and convective rolls are observed, which appear to have a substantial impact on wake interactions. At the end, impacts of upstream turbine wakes on downstream turbines are analyzed in terms of wake recovery as a function of atmospheric stability.