Three-dimensional wind-turbine wake characterization via tomographic particle-image velocimetry

Wind turbines are often sited on different topographical features. In the current work, we performed wind-tunnel measurements of the wake behind a single wind turbine sited on two escarpments differing in the windward side shape using tomographic particle-image velocimetry. The escarpments are classified as forward facing step (FFS) and ramp-shape. The turbine sited on the FFS experiences an inflow with high flow shear and turbulence due to flow separation from the escarpment leading edge compared to the one on the ramp-shaped escarpment. As a consequence, the wake characteristics behind the turbine are strongly affected by the shape of the topography. The velocity deficit in the wake of the turbine is relatively higher in the forward facing step shape, but it also shows a faster recovery. The rotation of the wake is stronger for the turbine on the ramp-shaped escarpment, whereas the meandering of the wake is higher for the FFS case. The spatial coherence is observed to be higher in the near wake of the turbine sited on the FFS escarpment, while it is very similar in the far wake for both cases. Instantaneous vortices identified by the Q-criterion show that the development of tip and hub vortices is affected by the topography as well.