Data assimilation of generic boundary layer flows for wind turbine applications – an LES study

Providing date- and site-specific turbulent inflow fields for large-eddy simulations (LESs) of the flow through wind turbines becomes increasingly important for reliable estimates of power production. In this study, data assimilation techniques are applied to adapt the atmospheric inflow field towards previously defined wind profiles. A standard and a modified version of the Newtonian relaxation technique and an assimilation method based on the vibration equation are implemented in the geophysical flow solver EULAG. The extent to which they are able to adapt mean horizontal wind velocities towards target profiles and the impact on atmospheric turbulence of an idealized LES are investigated. The sensitivity of the methods to grid refinement is analyzed. The method based on the vibration equation is suited for fine grids (dx = dy = dz = 5 m), which is a common grid resolution for wind energy studies. Furthermore, the vibration method is used to nudge the zonal and meridional inflow velocities of an idealized atmospheric simulation towards velocity profiles representing a weakly stably stratified atmospheric boundary layer (ABL) at the wind farm site WiValdi at Krummendeich, Germany. On-site wind measurements and the output of mesoscale simulations are evaluated to define the target velocity profile. The assimilation method based on the vibration equation is able to adapt the zonal and meridional velocity components of an atmospheric flow, while negative effects on the atmospheric turbulence could be reduced. In a final step, the assimilated flow field is taken as inflow for a wind turbine simulation, which then shows the characteristic structures of a wake in the ABL. This study shows the suitability of the vibration method for adapting inflow fields for wind energy purposes and presents the advantages and disadvantages of the method.