21 Feb 2023
 | 21 Feb 2023
Status: this preprint is currently under review for the journal WES.

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

Miguel Sanchez Gomez, Julie K. Lundquist, Jeffrey D. Mirocha, and Robert S. Arthur

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 sets in motion a secondary circulation. 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 momentum advection is amplified in the more strongly stable boundary layer, mainly by larger shear of the horizontal velocity, thus increasing the blockage effect.

Miguel Sanchez Gomez et al.

Status: open (until 28 Mar 2023)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on wes-2023-20', Karim Ali, 22 Feb 2023 reply
  • RC1: 'Comment on wes-2023-20', Dries Allaerts, 07 Mar 2023 reply
  • RC2: 'Comment on wes-2023-20', Anonymous Referee #2, 14 Mar 2023 reply
  • RC3: 'Comment on wes-2023-20', Bleeg James, 24 Mar 2023 reply

Miguel Sanchez Gomez et al.

Miguel Sanchez Gomez et al.


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Short summary
The wind slows down as it approaches a wind plant, this phenomenon is called blockage. As a result, the turbines in the wind plant produce less power than initially anticipated. We investigate wind plant blockage for two atmospheric conditions. Blockage is larger for a wind plant compared to a stand-alone turbine. Also, blockage increases with atmospheric stability. Wind farm blockage is amplified by the vertical transport of momentum as the wind approaches the front-row turbines in the array.