This study investigates the aerodynamic interaction between two neighbouring offshore wind farms operating in a Conventionally Neutral Boundary Layer (CNBL), where the atmospheric boundary layer (ABL) is neutrally stable, capped by a stable inversion layer and a less stable free
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This study investigates the aerodynamic interaction between two neighbouring offshore wind farms operating in a Conventionally Neutral Boundary Layer (CNBL), where the atmospheric boundary layer (ABL) is neutrally stable, capped by a stable inversion layer and a less stable free atmosphere above. While previous studies have investigated: 1) wake effects between two neighbouring wind farms in a Truly Neutral Boundary Layer (TNBL) with neutral conditions throughout the ABL and above, and 2) the impact of a single wind farm on the flow field in a CNBL, there is a lack of research on the interaction between neighbouring wind farms in a CNBL. Improving our understanding of wind farm interactions in a realistic atmosphere is important for offshore wind farm planning and operation. In this study the performance of the two wind farms under two atmospheric conditions, a CNBL and a TNBL. While the TNBL assumes neutral conditions throughout the ABL and above, the CNBL represents a more complex and realistic modelling approach.
When the wind approaches a wind farm in a TNBL, the combined induction of the turbines creates a zone with increased pressure just before the start of the wind farm, redirecting the flow laterally and vertically. In a CNBL, the vertical component of the redirected wind interacts with a stable inversion layer aloft, which creates an enhanced high pressure area at the start of the wind farm, a phenomena commonly known as global blockage. At the end of the wind farm, the wind is directed down towards the low pressure region in the wake of the wind farm, lowering the inversion layer and creating a zone with even lower pressure. The combination of the two processes described, reduces the wind speed at the start of the wind farm and accelerates it towards the end.
In this study, we look at the interaction of two neighbouring wind farms at varying distances in both a TNBL and CNBL using the relatively fast Multi-Scale Coupled model (Stipa, Ajay, Allaerts, & Brinkerhoff, 2024) which uses a simplified mesoscale model to determine a background flow field which is then used to drive an engineering wake model. Each wind farm consists of a regularly aligned array of five turbines in the spanwise and ten turbines in the streamwise direction with a five rotor diameter (5D) spacing in both dimensions. The turbines are based on the 5 MW NREL reference (Jonkman et al., 2009) and simulations are run at a wind speed of 9 m/s at hub height, on the plateau of the thrust curve. For the CNBL simulations, we choose a Froude number of 1.0 for the inversion layer, resulting in maximum inversion layer displacement due to a phenomena called choking, and a Froude number of 0.10 for the free atmosphere.
Key findings:
1. For infinitely spaced wind farms in a CNBL, lower power output is seen compared to a TNBL due to stronger global blockage effects at the start of each wind farm despite a small speed-up towards the end of the farm.
2. For small wind farm separations (up to ±72D), the speed-up effect at the downstream end of the first wind farm enhances the power output of the second wind farm compared to a TNBL, whilst the blockage effect of the second wind farm negatively impacts the first wind farm.
3. The combined output of the two wind farms at small wind farm spacings is higher than when the farms are infinitely spaced, though lower than in a TNBL.
