Large-eddy simulation of a 15 GW wind farm: Flow effects, energy budgets and comparison with wake models

Maas, Oliver

doi:10.3389/fmech.2023.1108180

Cited by 4 publications

(1 citation statement)

References 38 publications

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“…The studies mentioned above showed that the presence of AGWs have two main implications, namely an adverse pressure gradient upwind the wind farm, which is responsible for global blockage, and a favorable pressure gradient inside the wind farm that is beneficial for wake recovery. Moreover, Centurelli et al (2021) and Maas (2023) showed that LES results strongly differ from reduced-order wake models when thermal stratification is considered. To assess the impact of inversion height, strength and lapse rate on wind farm blockage, Lanzilao and Meyers (2023) conducted an LES parametric study, concluding that the overall effect of AGWs on wind farm operation is either beneficial or detrimental depending on the specific structure of the potential temperature profile.…”

Section: Introductionmentioning

confidence: 99%

An LES Model for Wind Farm-Induced Atmospheric Gravity Wave Effects Inside Conventionally Neutral Boundary Layers

Stipa,

Ahmed Khan,

Allaerts

et al. 2024

Preprint

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Abstract. The interaction of large wind farm clusters with the thermally-stratified atmosphere has emerged as an important physical process that impacts the productivity of wind farms. Under stable conditions, this interaction triggers the creation of atmospheric gravity waves (AGWs) due to the vertical displacement of the boundary layer by the wind farm. AGWs induce horizontal pressure gradients within the boundary layer that alter the wind speed distribution within the farm, influencing both wind farm power generation and wake development. Additional factors, such as the growth of an internal boundary layer originating from the wind farm entrance and increased turbulence due to the wind turbines, further contribute to wake evolution. Recent studies have highlighted the considerable computational cost associated with simulating gravity wave effects within large eddy simulations (LES), as a significant portion of the free atmosphere must be resolved due to the large vertical spatial scales involved. Additionally, specialized boundary conditions are required to prevent wave reflections from contaminating the solution. In this study, we introduce a novel methodology to model the effects of AGWs without extending the LES computational domain into the free atmosphere. The proposed approach addresses the wave reflection problem inherently, eliminating the need for these specialized boundary conditions. We utilize the recently developed multi-scale coupled (MSC) model of Stipa et al. (2023b) to estimate the vertical boundary layer displacement triggered by the wind farm, and apply the deformation to the domain of an LES that extends only to the inversion layer. We validate our AGW modeling technique for two distinct free atmosphere stability conditions, comparing it to the traditional approach in which AGWs are fully resolved using a domain extending several kilometers into the free atmosphere. The proposed approach accurately captures AGW effects on the row-averaged thrust and power distribution of wind farms while demanding less than 15 % of the computational resources compared to traditional methods. This can be further reduced in cases of conventionally neutral boundary layers, since there is no longer a need for solving the potential temperature equation. The developed approach is used to compare global blockage and pressure disturbances obtained from the simulated cases against a solution characterized by zero boundary layer displacement, which represents a limiting case of very strong free atmosphere stratification. Finally, we discuss the implications of making such "rigid-lid" approximation, instead of considering the full gravity wave solution, when predicting wind farm power.

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Section: Introductionmentioning

confidence: 99%

An LES Model for Wind Farm-Induced Atmospheric Gravity Wave Effects Inside Conventionally Neutral Boundary Layers

Stipa,

Ahmed Khan,

Allaerts

et al. 2024

Preprint

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A meso–micro atmospheric perturbation model for wind farm blockage

Devesse,

Lanzilao,

Meyers

2024

J. Fluid Mech.

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As wind farms continue to grow in size, mesoscale effects such as blockage and gravity waves become increasingly important. Allaerts & Meyers (J. Fluid Mech., vol. 862, 2019, pp. 990–1028) proposed an atmospheric perturbation model (APM) that can simulate the interaction of wind farms and the atmospheric boundary layer while keeping computational costs low. The model resolves the mesoscale flow, and couples to a wake model to estimate the turbine inflow velocities at the microscale. This study presents a new way of coupling the mesoscale APM to a wake model, based on matching the velocity between the models throughout the farm. This method performs well, but requires good estimates of the turbine-level velocity fields by the wake model. Additionally, we investigate the mesoscale effects of a large wind farm, and find that aside from the turbine forces and increased turbulence levels, the dispersive stresses due to subgrid flow heterogeneity also play an important role at the entrance of the farm, and contribute to the global blockage effect. By using the wake model coupling, we can explicitly incorporate these stresses in the model. The resulting APM is validated using 27 prior large-eddy simulations of a large wind farm under different atmospheric conditions. The APM and large-eddy simulation results are compared on both mesoscale and turbine scale, and on turbine power output. The APM captures the overall effects that gravity waves have on wind farm power production, and significantly outperforms standard wake models.

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A large-eddy simulation (LES) model for wind-farm-induced atmospheric gravity wave effects inside conventionally neutral boundary layers

Stipa,

Ahmed Khan,

Allaerts

et al. 2024

Wind Energ. Sci.

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Abstract. The interaction of large wind farm clusters with the thermally stratified atmosphere has emerged as an important physical process that impacts the productivity of wind farms. Under stable conditions, this interaction triggers atmospheric gravity waves (AGWs) in the free atmosphere due to the vertical displacement of the atmospheric boundary layer (ABL) by the wind farm. AGWs induce horizontal pressure gradients within the ABL that alter the wind speed distribution within the farm, influencing both wind farm power generation and wake development. Additional factors, such as the growth of an internal boundary layer originating from the wind farm entrance and increased turbulence due to the wind turbines, further contribute to wake evolution. Recent studies have highlighted the considerable computational cost associated with simulating gravity wave effects within large-eddy simulations (LESs), as a significant portion of the free atmosphere must be resolved due to the large vertical spatial scales involved. Additionally, specialized boundary conditions are required to prevent wave reflections from contaminating the solution. In this study, we introduce a novel methodology to model the effects of AGWs without extending the LES computational domain into the free atmosphere. The proposed approach addresses the wave reflection problem inherently, eliminating the need for these specialized boundary conditions. We utilize the recently developed multi-scale coupled (MSC) model of Stipa et al. (2024b) to estimate the vertical ABL displacement triggered by the wind farm, and we apply the deformation to the domain of an LES that extends only to the inversion layer. The accuracy in predicting the AGW-induced pressure gradients is equivalent to the MSC model. The AGW modeling technique is verified for two distinct free-atmosphere stability conditions by comparing it to the traditional approach in which AGWs are fully resolved using a domain that extends several kilometers into the free atmosphere. The proposed approach accurately captures AGW effects on the row-averaged thrust and power distribution of wind farms while demanding 12.7 % of the computational resources needed for traditional methods. Moreover, when conventionally neutral boundary layers are studied, there is no longer a need to solve the potential temperature equation, as stability is neutral within the boundary layer. The developed approach is subsequently used to compare the global blockage and pressure disturbances obtained from the simulated cases against a solution characterized by zero boundary layer displacement, which represents the limiting case of very strong stratification above the boundary layer. This approximation, sometimes referred to as the “rigid lid”, is compared against the full AGW solution using the MSC model. This is done for different values of inversion strength and free atmosphere lapse rate, evaluating the ability of the “rigid lid” to predict blockage, wake effects, and overall wind farm performance.

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Large-eddy simulation of a 15 GW wind farm: Flow effects, energy budgets and comparison with wake models

Cited by 4 publications

References 38 publications

An LES Model for Wind Farm-Induced Atmospheric Gravity Wave Effects Inside Conventionally Neutral Boundary Layers

An LES Model for Wind Farm-Induced Atmospheric Gravity Wave Effects Inside Conventionally Neutral Boundary Layers

A meso–micro atmospheric perturbation model for wind farm blockage

A large-eddy simulation (LES) model for wind-farm-induced atmospheric gravity wave effects inside conventionally neutral boundary layers

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