A WRF-LES study of turbine inflow and wake characterization

Measurements of both the wind turbine inflow and its wake from a floating wind turbine are here analyzed to understand the role of the vertical wind shear and the veer on both inflow and wake characteristics. The inflow measurements correspond to those from a nacelle-mounted Doppler wind lidar that retrieved radial velocities up to three rotor diameters in front of the turbine, measuring thus within a large portion of the atmosphere. The wake measurements correspond to those from a scanning lidar that measured up to ten rotor diameters in both plan position indicator and range height indicator modes, thus covering spatially a good portion of the wake. For a significant number of periods, both inflow and wake measurements reveal the presence of a clear low-level jet (LLJ) with noses close to turbine hub height (about 100 m). The presence of LLJs complicates inflow modeling as, depending on the inflow model and measurement coverage, negative wind shears might appear.

We therefore study the ability of the Weather Research and Forecasting (WRF) model in simulating similar inflow conditions as those observed under the above LLJ episodes. Since turbulence and the presence of the turbine itself are highly important, we use the ability of the WRF model to perform large-eddy simulations (LES) for these conditions and we model the turbine by using a generalized actuator disk (GAD). The WRF-LES simulations reveal the turbulence and atmospheric stability conditions that are required to replicate the inflow conditions that lead to these shallow LLJs. Further, after similar LLJ conditions (as those measured) are achieved with the WRF-LES simulations, a similar turbine as the real floating turbine is implemented in the simulation via the GAD. Here, we then analyzed the differences and similarities between the wake measurements and the wake simulations  and their characteristics.