Investigating the physical mechanisms that modify wind plant blockage in stable boundary layers

Abstract

Abstract. Wind plants slow down the approaching wind, a phenomenon known as blockage. Wind plant blockage undermines turbine performance for front-row turbines and potentially for turbines deeper into the array. We use large-eddy simulations to characterize blockage upstream of a finite-size wind plant in flat terrain for different atmospheric stability conditions and investigate the physical mechanisms modifying the flow upstream of the turbines. To examine the influence of atmospheric stability, we compare simulations of two stably stratified boundary layers using the Weather Research and Forecasting model in large-eddy simulation mode, representing wind turbines using the generalized actuator disk approach. For a wind plant, a faster cooling rate at the surface, which produces stronger stably stratified flow in the boundary layer, amplifies blockage. As a novelty, we investigate the physical mechanisms amplifying blockage by evaluating the different terms in the momentum conservation equation within the turbine rotor layer. The velocity deceleration upstream of a wind plant is caused by an adverse pressure gradient and momentum advection out of the turbine rotor layer. The cumulative deceleration of the flow upstream of the front-row turbines instigates vertical motions. The horizontal flow is diverted vertically, reducing momentum availability in the turbine rotor layer. Although the adverse pressure gradient upstream of the wind plant remains unchanged with atmospheric stability, vertical advection of horizontal momentum is amplified in the more strongly stable boundary layer, mainly by larger shear of the horizontal velocity, thus increasing the blockage effect.

Numerical simulations for a parametric study of blockage effect on offshore wind farms

“…The reason for this is the placement of the additional turbines on the sides of the farm, which are at a far enough distance not to affect the turbines in the center. However, a slight decrease in the upstream velocity reduction with an increase in the number of turbines in a row also supports the findings of Sanchez Gomez 22 who points to a vertical advection of the momentum as a reason for the global blockage. The advection does not cause the loss of momentum with a wider wind farm due to bypassing the farm.…”

“…The evidence supporting the hypothesis that the global blockage effect was mostly caused by the superposition of individual turbine induction zones was also found for the investigated configurations. However, other authors 12,22,23 point out to other mechanisms impacting blockage, such as gravity waves or advection. This shows that blockage effect on a wind farm still needs further study.…”

“…The comparison of the LES with engineering models based on the assumption of a superposition shows that there are additional effects which have to be taken into account for very accurate modeling of blockage model 18 . Sanchez‐Gomez et al 22 based on the LES analysis point a vertical advection of the momentum as a mechanism driving the global blockage.…”

A parametric large-eddy simulation study of wind-farm blockage and gravity waves in conventionally neutral boundary layers

“…In SP-Wind the precursor domain width and height should match those of the main domain when they are run concurrently. Therefore, we adopt the tiling technique to extend the precursor flow fields in the y direction from 10 to 30 km (Sanchez Gomez et al 2023). In regard to the z direction, we extrapolate the flow fields from 3 to 25 km, using a constant geostrophic wind.…”

Implementation and Validation of a Generalized Actuator Disk Parameterization for Wind Turbine Simulations within the FastEddy ® Model

“…Turbulence-resolving simulations allow for accurate modeling of wind turbine wake dynamics (e.g., Y. T. Wu & Porté-Agel, 2012), predicting loads on their structural components (e.g., Chanprasert, Sharma, Cater, & Norris, 2022; M. J. Churchfield, Lee, Michalakes, & Moriarty, 2012), and understanding their interaction with the atmosphere (e.g., Maas, 2023;Sanchez Gomez, Lundquist, Mirocha, & Arthur, 2023;K. Wu & Porté-Agel, 2017).…”