Role of coherent structures in mean kinetic energy entrainment by finite-sized wind farms and lessons from canopy flows

It is now well-recognized that in the presence of an asymptotically large wind farm, the ABL adjusts to this ‘enhanced' roughness to form a fully developed ‘wind turbine array boundary layer' (WTABL), where the driving force of the ABL is in balance with the drag induced by the presence of the wind turbines. It has been shown that mean kinetic energy (MKE) from air aloft is transported to the wind farm wake region through turbulent motions and serves as the dominant mechanism for the recovery of the wakes (Calaf et al. 2010). A better understanding of the underlying mechanisms governing the kinetic energy transport is therefore of much interest to the wind-energy community.

Applying Proper Orthogonal Decomposition (POD) on data obtained from Large Eddy Simulations (LES) of infinitely large wind farms, stream-wise counter-rotating vortices were found to be the responsible mechanism of MKE entrainment (Verhulst and Meneveau 2014).

In the current study, the POD based analysis is extended to data from LES of finite-sized wind farms in a neutrally stratified ABL. The simulations are performed for an extremely long pressure-driven channel flow with an embedded wind farm occupying a maximum of 10% of the streamwise length. Different wind farm configurations (aligned/staggered, streamwise and spanwise spacing) are simulated to draw correlations with spatial distribution of the coherent structures.

The presence of streamwise counter-rotating rollers above wind farms hints towards a flow similarity with canopy flows where similar coherent structures are found. The analogy between wind farms and canopy flows will be explored additionally using the quadrant-hole analysis.