Wind turbine wakes on escarpments: A wind-tunnel study

In this study, the wake behind a wind turbine located on an escarpment is investigated using particle-image velocimetry in a wind tunnel. Five different escarpment models are used, which vary in the windward side shape from forward facing steps (FFS) with different curvatures at the leading-edge to sinusoidal ramp shapes with varying slopes. The difference in the baseflow (flow without the turbine)resulting from the change in the geometry of the escarpment leads to significant differences in the average and dynamic characteristics of the turbine wake. The relatively high level of turbulence intensity in the baseflow induced by the FFS escarpments leads to a faster wake recovery accompanied by higher turbulence kinetic energy, compared with the ramp-shaped ones. The self-similar behavior of the velocity deficit profiles in the far wake is confirmed for all the cases; unlike turbine wakes over flat terrain, the wake growth rate is found to be larger in the vertical direction than in the lateral direction. Meandering of the wake is observed to be higher on the FFS escarpment with an upward wake trajectory, compared to the ramp-shaped one. Finally, an analytical model is assessed to predict the wake velocity deficit of the turbine.