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1.
  • Andersen, Soren Juhl, et al. (author)
  • Statistics of LES Simulations of Large Wind Farms
  • 2016
  • In: SCIENCE OF MAKING TORQUE FROM WIND (TORQUE 2016). - : IOP PUBLISHING LTD.
  • Conference paper (peer-reviewed)abstract
    • Numerous large eddy simulations are performed of large wind farms using the actuator line method, which has been fully coupled to the aero-elastic code, Flex5. The higher order moments of the flow field inside large wind farms is examined in order to determine a representative reference velocity. The statistical moments appear to collapse and hence the turbulence inside large wind farms can potentially be scaled accordingly. The thrust coefficient is estimated by two different reference velocities and the generic C-T expression by Frandsen. A reference velocity derived from the power production is shown to give very good agreement and furthermore enables the very good estimation of the thrust force using only the steady C-T-curve, even for very short time samples. Finally, the effective turbulence inside large wind farms and the equivalent loads are examined.
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2.
  • Arnqvist, Johan, 1985-, et al. (author)
  • Investigation of Turbulence Accuracy When Modeling Wind in Realistic Forests Using LES
  • 2019
  • In: Progress In Turbulence Viii. - Cham : SPRINGER INTERNATIONAL PUBLISHING AG. - 9783030221966 - 9783030221959 ; , s. 291-296
  • Conference paper (peer-reviewed)abstract
    • This study presents an evaluation of wind field simulations, in neutral atmospheric conditions, above a heterogeneous forest. The calculations were performed with Large-Eddy Simulation (LES) code OpenFOAM, with explicit modelling of the forest through drag coefficient and forest density. The findings indicate that a large modelling domain is needed in order to reproduce the measurements in different wind directions, since the effect of far upwind forest characteristics influence the wind and turbulence profiles. It is further shown that even though the low resolution of the LES simulations lead to slightly misrepresented single point turbulence characteristics, two point turbulence characteristics are well predicted due to spatial filtering of the small scales.
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3.
  • Asmuth, Henrik, et al. (author)
  • Actuator line simulations of wind turbine wakes using the lattice Boltzmann method
  • 2020
  • In: Wind Energy Science. - : Copernicus GmbH. - 2366-7443 .- 2366-7451. ; 5:2, s. 623-645
  • Journal article (peer-reviewed)abstract
    • The high computational demand of large-eddy simulations (LESs) remains the biggest obstacle for a wider applicability of the method in the field of wind energy. Recent progress of GPU-based (graphics processing unit) lattice Boltzmann frameworks provides significant performance gains alleviating such constraints. The presented work investigates the potential of LES of wind turbine wakes using the cumulant lattice Boltzmann method (CLBM). The wind turbine is represented by the actuator line model (ALM). The implementation is validated and discussed by means of a code-to-code comparison to an established finite-volume Navier–Stokes solver. To this end, the ALM is subjected to both laminar and turbulent inflow while a standard Smagorinsky sub-grid-scale model is employed in the two numerical approaches. The resulting wake characteristics are discussed in terms of the first- and second-order statistics as well the spectra of the turbulence kinetic energy. The near-wake characteristics in laminar inflow are shown to match closely with differences of less than 3 % in the wake deficit. Larger discrepancies are found in the far wake and relate to differences in the point of the laminar-turbulent transition of the wake. In line with other studies, these differences can be attributed to the different orders of accuracy of the two methods. Consistently better agreement is found in turbulent inflow due to the lower impact of the numerical scheme on the wake transition. In summary, the study outlines the feasibility of wind turbine simulations using the CLBM and further validates the presented set-up. Furthermore, it highlights the computational potential of GPU-based LBM implementations for wind energy applications. For the presented cases, near-real-time performance was achieved using a single, off-the-shelf GPU on a local workstation.
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4.
  • Asmuth, Henrik, et al. (author)
  • Assessment of Weak Compressibility in Actuator Line Simulations of Wind Turbine Wakes
  • 2020
  • In: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 1618
  • Journal article (peer-reviewed)abstract
    • The trend of increasing rotor diameters and tip-speeds has brought about concerns of non-negligible compressibility effects in wind turbine aerodynamics. The investigation of such effects on wakes is particularly difficult when using actuator line models (ALM). This is because crucial regions of the flow, i.e. the direct vicinity of the blade, are not simulated but represented by body forces. To separately assess the impact of compressibility on the wake and the ALM itself, we conduct large-eddy simulations (LES) where the forces of the ALM are prescribed and based on the local sampled velocity (standard procedure), respectively. The LES are based on the weakly-compressible Lattice Boltzmann Method (LBM). Further to the comparison of (near-)incompressible to compressible simulations we investigate cases with artificially increased compressibility. This is commonly done in weakly-compressible approaches to reduce the computational demand. The investigation with prescribed forces shows that compressibility effects in the wake flow are negligible. Small differences in the wake velocity (of max. 1%) are found to be related to local compressibility effects in the direct vicinity of the ALM. Most significantly, compressibility is found to affect the sampled velocity and thereby accuracy of the ALM.
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5.
  • Asmuth, Henrik, et al. (author)
  • How Fast is Fast Enough? : Industry Perspectives on the Use of Large-eddy Simulation in Wind Energy
  • 2023
  • In: WAKE CONFERENCE 2023. - : Institute of Physics Publishing (IOPP).
  • Conference paper (peer-reviewed)abstract
    • The use of graphics processing units (GPUs) has facilitated unprecedented performance gains for computational fluids dynamics in recent years. In many industries this has enabled the integration of large-eddy simulation (LES) in the engineering practice. Flow modelling in the wind industry though still primarily relies on models with significantly lower fidelity. This paper seeks to investigate the reasons why wind energy applications of LES are still an exception in the industrial practice. On that account, we present a survey among industry experts on the matter. The survey shows that the large runtimes and computational costs of LES are still seen as a main obstacle. However, other reasons such as a lack of expertise and user experience, the need for more validation, and lacking trust in the potential benefits of LES reveal that computational efficiency is not the only concern. Lastly, we present an exemplary simulation of a generic offshore wind farm using a GPU-resident Lattice Boltzmann LES framework. The example shows that the runtime requirements stated by a large part of the respondents can already now be fulfilled with reasonable hardware effort.
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6.
  • Asmuth, Henrik, et al. (author)
  • The Actuator Line Model in Lattice Boltzmann Frameworks : Numerical Sensitivity and Computational Performance
  • 2019
  • In: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 1256
  • Journal article (peer-reviewed)abstract
    • The growing use of large-eddy simulations for the modelling of wind farms makes the need for efficient numerical frameworks more essential than ever. GPU-accelerated implementations of the Lattice Boltzmann Method (LBM) have shown to provide significant performance gains over classical Navier-Stokes-based computational fluid dynamics. Yet, their use in the field of wind energy remains limited to date. In this fundamental study the cumulant LBM is scrutinised for actuator line simulations of wind turbines. The numerical sensitivity of the method in a simple uniform inflow is investigated with respect to spatial and temporal resolution as well as the width of the actuator line’s regularisation kernel. Comparable accuracy and slightly better stability properties are shown in relation to a standard Navier-Stokes implementation. The results indicate the overall suitability of the cumulant LBM for wind turbine wake simulations. The potential of the LBM for future wind energy applications is clarified by means of a brief comparison of computational performance.
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7.
  • Asmuth, Henrik, et al. (author)
  • Wall-modeled lattice Boltzmann large-eddy simulation of neutral atmospheric boundary layers
  • 2021
  • In: Physics of fluids. - : American Institute of Physics (AIP). - 1070-6631 .- 1089-7666. ; 33:10, s. 105111-105111
  • Journal article (peer-reviewed)abstract
    • The lattice Boltzmann method (LBM) sees a growing popularity in the field of atmospheric sciences and wind energy, largely due to itsexcellent computational performance. Still, LBM large-eddy simulation (LES) studies of canonical atmospheric boundary layer flows remainlimited. One reason for this is the early stage of development of LBM-specific wall models. In this work, we discuss LBM–LES of isothermalpressure-driven rough-wall boundary layers using a cumulant collision model. To that end, we also present a novel wall modeling approach,referred to as inverse momentum exchange method (iMEM). The iMEM enforces a wall shear stress at the off-wall grid points by adjustingthe slip velocity in bounce-back boundary schemes. In contrast to other methods, the approach does not rely on the eddy viscosity, nor doesit require the reconstruction of distribution functions. Initially, we investigate different aspects of the modeling of the wall shear stress, i.e.,an averaging of the input velocity as well as the wall-normal distance of its sampling location. Particularly, sampling locations above the firstoff-wall node are found to be an effective measure to reduce the occurring log-layer mismatch. Furthermore, we analyze the turbulence statis-tics at different grid resolutions. The results are compared to phenomenological scaling laws, experimental, and numerical references. Theanalysis demonstrates a satisfactory performance of the numerical model, specifically when compared to a well-established mixed pseudo-spectral finite difference (PSFD) solver. Generally, the study underlines the suitability of the LBM and particularly the cumulant LBM forcomputationally efficient LES of wall-modeled boundary layer flows.
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8.
  • Asmuth, Henrik, et al. (author)
  • Wind Turbine Response in Waked Inflow: A Modelling Benchmark Against Full-Scale Measurements
  • 2022
  • In: Renewable energy. - : Elsevier. - 0960-1481 .- 1879-0682. ; 191, s. 868-887
  • Journal article (peer-reviewed)abstract
    • Predicting the power and loads of wind turbines in waked inflow conditions still presents a major modelling challenge. It requires the accurate modelling of the atmospheric flow conditions, wakes of upstream turbines and the response of the turbine of interest. Rigorous validations of model frameworks against measurements of utility-scale wind turbines in such scenarios remain limited to date. In this study, six models of different fidelity are compared against measurements from the DanAero experiment. The two benchmark cases feature a full-wake and partial-wake scenario, respectively. The simulations are compared against local pressure forces and inflow velocities measured on several blade sections of the downstream turbine, as well as met mast measurements and standard SCADA data. Regardless of the model fidelity, reasonable agreements are found in terms of the wake characteristics and turbine response. For instance, the azimuth variation of the mean aerodynamic forces acting on the blade was captured with a mean relative error of 15–20%. While various model-specific deficiencies could be identified, the study highlights the need for further full-scale measurement campaigns with even more extensive instrumentation. Furthermore, it is concluded that validations should not be limited to integrated and/or time-averaged quantities that conceal characteristic spatial or temporal variations.
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9.
  • Barney, Andrew (author)
  • Energy planning for islands : Guiding island energy transition and decision-making
  • 2024
  • Doctoral thesis (other academic/artistic)abstract
    • The importance and benefits of transitioning away from fossil fuel based energy systems is becoming ever clearer. The transition is especially crucial for geographic islands that typically have limited, or no, access to mainland energy networks. This limited access means that they must rely on costly, economically and environmentally, imported fossil fuels for energy production. This isolation and reliance on imported fuels makes islands particularly energy insecure, which is only expected to worsen because of climate change. At the same time, much of the current guidance and policy developed to help plan for islands’ energy transition is based on the conditions and circumstances present on the mainland or on energy planning that focuses primarily on technical or economic parameters. To plan for a smoother, more successful transition away from fossil fuel based energy production on islands, more data is needed about islanders themselves and their local circumstances as well as planning guidance flexible enough to fit the variable needs of island planners that goes beyond a techno-economic focus.In this PhD Thesis and in response to these transition issues, an energy planning platform, REACT-DECARB, has been developed to specifically address the needs of island energy planners. This platform seeks to be holistic in its approach to facilitate the island energy transition planning procedure by considering typical island characteristics and energy transition processes while also including key technical, social, environmental and economic dimensions and granting planners flexibility in how they incorporate them. Additionally, surveys and interviews with island residents are conducted to gain an understanding of islanders’ motivations, priorities and awareness in relation to energy and energy transition as well as to determine if these can be of guidance to island energy transition planners.The applications of the REACT-DECARB platform on geographic islands representing eight different countries from around the EU with varying populations, land areas and climates identified energy transition opportunities and obstacles specific to the islands where the platform was applied as well as to other islands seeking to transition their energy systems. Moreover, these applications of the planning platform demonstrated its ability to help island planners in the development of holistic energy transition paths. The platform takes island energy transition planning beyond a focus on techno-economics by including a wider range of planning dimensions allowing them to decide the methods best suited to their island’s needs. Further, the interactions with island residents demonstrated that their relationships to energy could be informative in the development of energy transition plans as well as likely being critical to their success. Islanders’ understandings, motivations, priorities and awareness were found to be not only directly useful in guiding planners during the designing of island energy transition plans but can also serve to inform planners in how best to educate local residents about and engage them in local energy transition projects. Ultimately, the work in this Thesis contributes to the efforts to make sure islands’, and islanders’, needs and perspectives are considered and included as a part of the wider energy transition.
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10.
  • Barney, Andrew (author)
  • Integrated regional energy planning for islands : Development of a decision-aiding methodological framework and application
  • 2022
  • Licentiate thesis (other academic/artistic)abstract
    • Clear and inclusive planning guidance for island planners seeking transitions away from traditional fossil fuel based energy systems is currently needed. At present, the guidance from the scientific community has largely been provided piecemeal and with significant focus on the specifics of single islands and, more often than not, on proposed energy systems’ techno-economics. To address these gaps in applicability and encourage a more balanced energy project analysis the REACT-DECARB integrated energy planning and decision-aiding platform has been developed. REACT-DECARB seeks to work as a comprehensive planning platform for island energy transition and this thesis investigates the ability of the platform to provide the support to planners needed to facilitate their transition planning. To do so the core concepts, descriptions and applications of decision making as well as a number of energy planning tools are presented. The platform is then described and its individual steps are applied to eight geographic islands within the EU. These applications of the platform identified key opportunities for the islands, such as significant RES potential, while also finding potential obstacles, such as land use conflicts. Further, it highlighted the high economic cost of seeking full electricity autonomy and of using substantial battery capacities. The platform additionally identified the importance local preferences can potentially have on the ranking of future energy scenarios as well as the impact lifetime project environmental costs can have on overall system environmental impacts. In summation, the REACT-DECARB platform is found to be an effective tool in directing island energy decarbonisation efforts by guiding local energy planners in what should be included in their planning and how these key elements can be incorporated in the decisions made. Further, the platform is found to be flexible enough to be applied to more than the specific circumstances of a single island and allows planners to adjust their approaches to its different steps as needed and to better match their capabilities.
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11.
  • Bastankhah, Majid, et al. (author)
  • A fast-running physics-based wake model for a semi-infinite wind farm
  • 2024
  • In: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 985
  • Journal article (peer-reviewed)abstract
    • This paper presents a new generation of fast-running physics-based models to predict the wake of a semi-infinite wind farm, extending infinitely in the lateral direction but with finite size in the streamwise direction. The assumption of a semi-infinite wind farm enables concurrent solving of the laterally averaged momentum equations in both streamwise and spanwise directions. The developed model captures important physical phenomena such as vertical top-down transport of energy into the farm, variable wake recovery rate due to the farm-generated turbulence and also wake deflection due to turbine yaw misalignment and Coriolis force. Of special note is the model's capability to predict and shed light on the counteracting effect of Coriolis force causing wake deflections in both positive and negative directions. Moreover, the impact of wind farm layout configuration on the flow distribution is modelled through a parameter called the local deficit coefficient. Model predictions were validated against large-eddy simulations extending up to 45 km downstream of wind farms. Detailed analyses were performed to study the impacts of various factors such as incoming turbulence, wind farm size, inter-turbine spacing and wind farm layout on the farm wake.
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12.
  • Breton, Simon-Philippe, et al. (author)
  • Validation of the actuator disc and actuator line techniques for yawed rotor flows using the New MEXICO experimental data
  • 2017
  • Conference paper (peer-reviewed)abstract
    • Experimental data acquired in the New MEXICO experiment on a yawed 4.5m diameter rotor model turbine are used here to validate the actuator line (AL) and actuator disc (AD) models implemented in the Large Eddy Simulation code EllipSys3D in terms of loadingand velocity field. Even without modelling the geometry of the hub and nacelle, the AL and AD models produce similar results that are generally in good agreement with the experimental data under the various configurations considered. As expected, the AL model does better at capturing the induction effects from the individual blade tip vortices, while the AD model can reproduce the averaged features of the flow. The importance of using high quality airfoil data (including 3D corrections) as well as a fine grid resolution is highlighted by the results obtained. Overall, it is found that both models can satisfactorily predict the 3D velocity field and blade loading of the New MEXICO rotor under yawed inflow.
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13.
  • Diaz, Gonzalo Pablo Navarro, et al. (author)
  • Actuator line model using simplified force calculation methods
  • 2023
  • In: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 8:3, s. 363-382
  • Journal article (peer-reviewed)abstract
    • To simulate transient wind turbine wake interaction problems using limited wind turbine data, two new variants of the actuator line technique are proposed in which the rotor blade forces are computed locally using generic load data. The proposed models, which are extensions of the actuator disk force models proposed by Navarro Diaz et al. (2019a) and Sorensen et al. (2020), only demand thrust and power coefficients and the tip speed ratio as input parameters. In the paper the analogy between the actuator disk model (ADM) and the actuator line model (ALM) is shown, and from this a simple methodology to implement local forces in the ALM without the need for knowledge of blade geometry and local airfoil data is derived. Two simplified variants of ALMs are proposed, an analytical one based on Sorensen et al. (2020) and a numerical one based on Navarro Diaz et al. (2019a). The proposed models are compared to the ADM using analogous data, as well as to the classical ALM based on blade element theory, which provides more detailed force distributions by using airfoil data. To evaluate the local force calculation, the analysis of a partial-wake interaction case between two wind turbines is carried out for a uniform laminar inflow and for a turbulent neutral atmospheric boundary layer inflow. The computations are performed using the large eddy simulation facility in Open Source Field Operation and Manipulation (OpenFOAM), including Simulator for Wind Farm Applications (SOWFA) libraries and the reference National Renewable Energy Laboratory (NREL) 5 MW wind turbine as the test case. In the single-turbine case, computed normal and tangential force distributions along the blade showed a very good agreement between the employed models. The two new ALMs exhibited the same distribution as the ALM based on geometry and airfoil data, with minor differences due to the particular tip correction needed in the ALM. For the challenging partially impacted wake case, both the analytical and the numerical approaches manage to correctly capture the force distribution at the different regions of the rotor area, with, however, a consistent overestimation of the normal force outside the wake and an underestimation inside the wake. The analytical approach shows a slightly better performance in wake impact cases compared to the numerical one. As expected, the ALMs gave a much more detailed prediction of the higher-frequency power output fluctuations than the ADM. These promising findings open the possibility to simulate commercial wind farms in transient inflows using the ALM without having to get access to actual wind turbine and airfoil data, which in most cases are restricted due to confidentiality.
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14.
  • Eriksson, Ola, 1979-, et al. (author)
  • Impact of Wind Veer and the Coriolis Force for an Idealized Farm to Farm Interaction Case
  • 2019
  • In: Applied Sciences. - : MDPI AG. - 2076-3417. ; 9:5
  • Journal article (peer-reviewed)abstract
    • The impact of the Coriolis force on the long distance wake behind wind farms is investigated using Large Eddy Simulations (LES) combined with a Forced Boundary Layer (FBL) technique. When using the FBL technique any mean wind shear and turbulent fluctuations can be added with body forces. The wind shear can also include the mean wind veer due to the Coriolis force. The variation of the Coriolis force due to local deviations from the mean profile, e.g., from wakes, is not taken into account in the FBL. This can be corrected for with an extra source term in the equations, hereon defined as the Coriolis correction. For a row of 4 turbines it is shown that the inclusion of the wind veer turns the wake to the right, while including the Coriolis correction turns it to the left. When including both wind veer and Coriolis correction the impact of wind veer dominates. For an idealized farm to farm interaction case, two farms of 4 * 4 turbines with 6 km in between, it can be seen that when including wind veer and the Coriolis correction a approximately 3% increase in the relative production for a full wake direction can be seen and only a slightly smaller increase can be seen when including only wind veer. The results indicate that FBL can be used for studies of long distance wakes without including a Coriolis correction but efforts need to be taken to use a wind shear with a correct mean wind veer.
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15.
  • Eriksson, Ola, 1979-, et al. (author)
  • Large-eddy simulations of the evolution of imposed turbulence in forced boundary layers in a very long domain
  • 2020
  • In: Wind Energy. - : Wiley. - 1095-4244 .- 1099-1824. ; 23:6, s. 1482-1493
  • Journal article (peer-reviewed)abstract
    • The technique of using imposed turbulence in combination with a forced boundary layer in order to model the atmospheric boundary layer is analyzed for a very long domain using large-eddy simulations with different combinations of prescribed velocity profiles and pregenerated turbulence fields based on the Mann model. The ambient flow is first studied in the absence of wind turbines. The velocity profiles undergo a transition throughout the domain with a velocity increase of 10% to 15% close to the ground far downstream in the domain. The turbulence characteristics close to the turbulence plane are, as expected, similar to those of the added Mann turbulence. The turbulence will then undergo a transition throughout the domain to finally reach a balance with the shear profile at a certain downstream distance. This distance is found to depend on the turbulence level of the added Mann turbulence planes. A lower Mann turbulence level generally results in a shorter "balancing" distance. Secondly, a row of 10 turbines is imposed in the simulations at different distances from the plane of turbulence in order to determine how the distance affects wake conditions and power production levels. Our results show that a "balancing" distance is needed between the turbulence plane and the first turbine in the row in order to ensure nonchanging ambient conditions throughout the turbine row. This introduces an increase in the computational costs. The computational cost for the forced boundary technique is normally lower compared with using precursor simulations, for longer domains; however, this needs to be verified further.
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16.
  • Eriksson, Ola, 1979-, et al. (author)
  • The Long distance wake behind Horns Rev I studied using large eddy simulations and a wind turbine parameterization in WRF
  • 2017
  • In: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 854
  • Journal article (peer-reviewed)abstract
    • The aim of the present paper is to obtain a better understanding of long distance wakes generated by wind farms as a first step towards a better understanding of farm to farm interaction. The Horns Rev I (HR) wind farm is considered for this purpose, where comparisons are performed between microscale Large Eddy Simulations (LES) using an Actuator Disc model (ACD), mesoscale simulations in the Weather Research and Forecasting Model (WRF) using a wind turbine parameterization, production data as well as wind measurements in the wind farm wake. The LES is manually set up according to the wind conditions obtained from the mesoscale simulation as a first step towards a meso/microscale coupling.The LES using an ACD are performed in the EllipSys3D code. A forced boundary layer (FBL) approach is used to introduce the desired wind shear and the atmospheric turbulence field from the Mann model. The WRF uses a wind turbine parameterization based on momentum sink. To make comparisons with the LESs and the site data possible an idealized setup of WRF is used in this study.The case studied here considers a westerly wind direction sector (at hub height) of 270 ± 2.5 degrees and a wind speed of 8 ± 0.5 m/s. For both the simulations and the site data a neutral atmosphere is considered. The simulation results for the relative production as well as the wind speed 2 km and 6 km downstream from the wind farm are compared to site data. Further comparisons between LES and WRF are also performed regarding the wake recovery and expansion.The results are also compared to an earlier study of HR using LES as well as an earlier comparison of LES and WRF. Overall the results in this study show a better agreement between LES and WRF as well as better agreement between simulations and site data.The procedure of using the profile from WRF as inlet to LES can be seen as a simplified coupling of the models that could be developed further to combine the methods for cases of farm to farm interaction.
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17.
  • Forsting, Alexander R. Meyer, et al. (author)
  • On the accuracy of predicting wind-farm blockage
  • 2023
  • In: Renewable energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; 214, s. 114-129
  • Journal article (peer-reviewed)abstract
    • To assess the uncertainty in blockage quantification, this study proposes a comparison of farm blockage predictions from wind-tunnel experiments, Reynolds Averaged Navier-Stokes based simulations using multiple numerical setups, and analytical models. The influence of the numerical setup is demonstrated to be small if a consistent definition of blockage (able to sort out systematic errors) is used. The effect of domain confinement and turbulence intensity is investigated assessing their range of variability. Different analytical models performed similarly in comparison to the numerical data, demonstrating the best accuracy for realistic spacing between the turbines and supporting their use as reliable engineering tools.
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18.
  • Hallgren, Christoffer, et al. (author)
  • Brief communication : On the definition of the low-level jet
  • 2023
  • In: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 8:11, s. 1651-1658
  • Journal article (peer-reviewed)abstract
    • Low-level jets (LLJs) are examples of non-logarithmic wind speed profiles affecting wind turbine power production, wake recovery, and structural/aerodynamic loading. However, there is no consensus regarding which definition should be applied for jet identification. In this study we argue that a shear definition is more relevant to wind energy than a falloff definition. The shear definition is demonstrated and validated through the development of a European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation reanalysis (ERA5) LLJ climatology for six sites. Identification of LLJs and their morphology, frequency, and intensity is critically dependent on the (i) vertical window of data from which LLJs are extracted and (ii) the definition employed.
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19.
  • Hallgren, Christoffer, et al. (author)
  • Classification and properties of non-idealized coastal wind profiles - an observational study
  • 2022
  • In: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 7:3, s. 1183-1207
  • Journal article (peer-reviewed)abstract
    • Non-idealized wind profiles frequently occur over the Baltic Sea and are important to take into consideration for offshore wind power, as they affect not only the power production but also the loads on the structure and the behavior of the wake behind the turbine. In this observational study, we classified non-idealized profiles as the following wind profiles having negative shear in at least one part of the lidar wind profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum and negative profiles. Using observations spanning over 3 years, we show that these non-idealized profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28-300 m layer) mostly occurred during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the strong shear zone of low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favorable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase in the wind speed from below.
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20.
  • Hallgren, Christoffer, et al. (author)
  • Looking for an Offshore Low-Level Jet Champion among Recent Reanalyses : A Tight Race over the Baltic Sea
  • 2020
  • In: Energies. - : MDPI. - 1996-1073. ; 13:14
  • Journal article (peer-reviewed)abstract
    • With an increasing interest in offshore wind energy, focus has been directed towards large semi-enclosed basins such as the Baltic Sea as potential sites to set up wind turbines. The meteorology of this inland sea in particular is strongly affected by the surrounding land, creating mesoscale conditions that are important to take into consideration when planning for new wind farms. This paper presents a comparison between data from four state-of-the-art reanalyses (MERRA2, ERA5, UERRA, NEWA) and observations from LiDAR. The comparison is made for four sites in the Baltic Sea with wind profiles up to 300 m. The findings provide insight into the accuracy of reanalyses for wind resource assessment. In general, the reanalyses underestimate the average wind speed. The average shear is too low in NEWA, while ERA5 and UERRA predominantly overestimate the shear. MERRA2 suffers from insufficient vertical resolution, which limits its usefulness in evaluating the wind profile. It is also shown that low-level jets, a very frequent mesoscale phenomenon in the Baltic Sea during late spring, can appear in a wide range of wind speeds. The observed frequency of low-level jets is best captured by UERRA. In terms of general wind characteristics, ERA5, UERRA, and NEWA are similar, and the best choice depends on the application.
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21.
  • Hallgren, Christoffer, et al. (author)
  • Machine learning methods to improve spatial predictions of coastal wind speed profiles and low-level jets using single-level ERA5 data
  • 2024
  • In: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 9:4, s. 821-840
  • Journal article (peer-reviewed)abstract
    • Observations of the wind speed at heights relevant for wind power are sparse, especially offshore, but with emerging aid from advanced statistical methods, it may be possible to derive information regarding wind profiles using surface observations. In this study, two machine learning (ML) methods are developed for predictions of (1) coastal wind speed profiles and (2) low-level jets (LLJs) at three locations of high relevance to offshore wind energy deployment: the US Northeastern Atlantic Coastal Zone, the North Sea, and the Baltic Sea. The ML models are trained on multiple years of lidar profiles and utilize single-level ERA5 variables as input. The models output spatial predictions of coastal wind speed profiles and LLJ occurrence. A suite of nine ERA5 variables are considered for use in the study due to their physics-based relevance in coastal wind speed profile genesis and the possibility to observe these variables in real-time via measurements. The wind speed at 10  ma.s.l. and the surface sensible heat flux are shown to have the highest importance for both wind speed profile and LLJ predictions. Wind speed profile predictions output by the ML models exhibit similar root mean squared error (RMSE) with respect to observations as is found for ERA5 output. At typical hub heights, the ML models show lower RMSE than ERA5 indicating approximately 5 % RMSE reduction. LLJ identification scores are evaluated using the symmetric extremal dependence index (SEDI). LLJ predictions from the ML models outperform predictions from ERA5, demonstrating markedly higher SEDIs. However, optimization utilizing the SEDI results in a higher number of false alarms when compared to ERA5.
  •  
22.
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23.
  • Hallgren, Christoffer, et al. (author)
  • The smoother the better? : A comparison of six post-processing methods to improve short-term offshore wind power forecasts in the Baltic Sea
  • 2021
  • In: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 6:5, s. 1205-1226
  • Journal article (peer-reviewed)abstract
    • With a rapidly increasing capacity of electricity generation from wind power, the demand for accurate power production forecasts is growing. To date, most wind power installations have been onshore and thus most studies on production forecasts have focused on onshore conditions. However, as offshore wind power is becoming increasingly popular it is also important to assess forecast quality in offshore locations. In this study, forecasts from the high-resolution numerical weather prediction model AROME was used to analyze power production forecast performance for an offshore site in the Baltic Sea. To improve the AROME forecasts, six post-processing methods were investigated and their individual performance analyzed in general as well as for different wind speed ranges, boundary layer stratifications, synoptic situations and in low-level jet conditions. In general, AROME performed well in forecasting the power production, but applying smoothing or using a random forest algorithm increased forecast skill. Smoothing the forecast improved the performance at all wind speeds, all stratifications and for all synoptic weather classes, and the random forest method increased the forecast skill during low-level jets. To achieve the best performance, we recommend selecting which method to use based on the forecasted weather conditions. Combining forecasts from neighboring grid points, combining the recent forecast with the forecast from yesterday or applying linear regression to correct the forecast based on earlier performance were not fruitful methods to increase the overall forecast quality.
  •  
24.
  • Hallgren, Christoffer, et al. (author)
  • The winds are twisting: analysis of strong directional shear across the rotor plane using coastal lidar measurements and ERA5
  • Other publication (other academic/artistic)abstract
    • The change of wind direction with height (the directional shear) affects both the power production from a wind turbine, wake effects and aerodynamic loading. In this study, a climatology of the relative occurrence of strong directional shear over Scandinavia is created using 43 years of hourly ERA5 data covering the height range of a modern wind turbine and at wind speeds of operation. It is shown that strong directional shear (≥15° over the rotor) is occurring 20-30% of the time over land and 10-25% of the time over the extended Baltic Sea. The height of the atmospheric boundary-layer and the wind speed at hub height are identified as the most important predictors for strong directional shear, with low boundary-layer heights and weak winds being the main causes. Associated with this, a strong land-sea seasonality is observed. Further, ERA5 is validated against lidar soundings from two coastal sites, both indicating a major underestimation in the distribution of the directional shear in ERA5. Especially in strongly stratified boundary-layers ERA5 struggles, with 25% of the data having errors exceeding 24° and 28° for Östergarnsholm and Utö respectively.
  •  
25.
  • Hanssen-Bauer, O. W., et al. (author)
  • Comparison of three DWM-based wake models at above-rated wind speeds
  • 2023
  • In: WAKE CONFERENCE 2023. - : Institute of Physics Publishing (IOPP).
  • Conference paper (peer-reviewed)abstract
    • In this study we investigate three mid-fidelity wind turbine wake models based on the dynamic wake meandering (DWM) model principle, and compare their performance with a reference dataset, produced with large-eddy simulations using the actuator line model. The models are compared with respect to flow field, power, and loads on a row of four 5MW reference turbines experiencing above-rated wind conditions. In general, the DWM models show fairly good agreement with large-eddy simulation for the time-averaged flow fields, blade forces and power, with increasing differences along the turbine row. Also when comparing fatigue loads of blade root moments, the differences between the models increase further into the row, with deviations up to 25 % of the reference case. However, while the development in blade root moment fatigue along the turbine row is predominantly driven by the energy content at the frequency corresponding to the turbine's rotational period (1P) for the DWM models, the large-eddy simulation results suggest that the key drivers for the blade root and tower loads are the increase in meandering and energy at higher frequencies (> 1P) deeper into the turbine row. For the tower loads, the DWM models highly underestimate the fatigue for the waked turbines. From these results, we suggest priorities for future model developments so that robust model implementations can be used in wind farm design and operation.
  •  
26.
  • Hanssen-Bauer, O. W., et al. (author)
  • Dependence of wind turbine loads on inlet flow field
  • 2020
  • In: Science of making torque from wind (TORQUE 2020), pts 1-5. - : IOP Publishing.
  • Conference paper (peer-reviewed)abstract
    • In wind farm simulations, the inflow wind field plays a crucial role in the accuracy of both power production, structural load predictions and the turbulent wake development behind wind turbines. Three different inflow wind field generation techniques, namely the Mann model, a reduced order based model described herein and LES data, are used in this study to characterise the relation between the inflow and the structural response of the wind turbine. In addition, the wake development under different inflow conditions are studied. The turbulence statistics of the reduced-order model and the LES data are similar to each other while the Mann turbulence has different turbulence profiles and spectral characteristics. An in-house developed aeroelastic code, 3Dfloat, is used for structural response analysis. The differences between the inflow fields are mainly attributed to the turbulence intensity profiles, and differences in their spectral characteristics.
  •  
27.
  • Ivanell, Stefan, 1974-, et al. (author)
  • Micro-scale model comparison (benchmark) at the moderately complex forested site Ryningsnäs
  • 2018
  • In: Wind Energy Science. - : Copernicus GmbH. - 2366-7443 .- 2366-7451. ; 3:2, s. 929-946
  • Journal article (peer-reviewed)abstract
    • This article describes a study in which modellers were challenged to compute the wind field at a forested site with moderately complex topography. The task was to model the wind field in stationary conditions with neutral stratification by using the wind velocity measured at 100 m at a metmast as the only reference. Detailed maps of terrain elevation and forest densities were provided as the only inputs, derived from airborne laser scans (ALSs) with a resolution of 10 m x 10 m covering an area of 50 km x 50 km, that closely match the actual forest and elevation of the site. The participants were free to apply their best practices for the simulation to decide the size of the domain, the value of the geostrophic wind, and every other modelling parameter. The comparison of the results with the measurements is shown for the vertical profiles of wind speed, shear, wind direction, and turbulent kinetic energy. The ALS-based data resulted in reasonable agreement of the wind profile and turbulence magnitude. The best performance was found to be that of large-eddy simulations using a very large domain. For the Reynolds-averaged Navier-Stokes type of models, the constants in the turbulence closure were shown to have a great influence on the yielded turbulence level, but were of much less importance for the wind speed profile. Of the variety of closure constants used by the participating modellers, the closure constants from Sogachev and Panferov (2006) proved to agree best with the measurements. Particularly the use of C-mu approximate to 0.03 in the k-epsilon model obtained better agreement with turbulence level measurements. All except two participating models used the full detailed ground and forest information to model the forest, which is considered significant progress compared to previous conventional approaches. Overall, the article gives an overview of how well different types of models are able to capture the flow physics at a moderately complex forested site.
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28.
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29.
  • Ivanell, Stefan S. A., 1974- (author)
  • Numerical computations of wind turbine wakes
  • 2009
  • Doctoral thesis (other academic/artistic)abstract
    • Numerical simulations of the Navier-Stokes equations are performed to achieve a better understanding of the behaviour of wakes generated by wind turbines. The simulations are performed by combining the in-house developed computer code EllipSys3D with the actuator line and disc methodologies. In the actuator line and disc methods the blades are represented by a line or a disc on which body forces representing the loading are introduced. The body forces are determined by computing local angles of attack and using tabulated aerofoil coefficients. The advantage of using the actuator disc technique is that it is not necessary to resolve blade boundary layers. Instead the computational resources are devoted to simulating the dynamics of the flow structures. In the present study both the actuator line and disc methods are used. Between approximately six to fourteen million mesh points are used to resolve the wake structure in a range from a single turbine wake to wake interaction in a farm containing 80 turbines. These 80 turbines are however represented by 20 actuator discs due to periodicity because of numerical limitations. In step one of this project the objective was to find a numerical method suitable to study both the flow structures in the wake behind a single wind turbine and to simulate complicated interaction between a number of turbines. The study resulted in an increased comprehension of basic flow features in the wake, but more importantly in the use of a numerical method very suitable for the upcoming purpose. The second objective of the project was to study the basic mechanisms controlling the length of the wake to obtain better understanding of the stability properties of wakes generated by wind turbine rotors. The numerical model was based on large eddy simulations of the Navier-Stokes equations using the actuator line method to generate the wake and the tip vortices. To determine critical frequencies the flow is disturbed by inserting a harmonic perturbation. The results showed that instability is dispersive and that growth occurs only for specific frequencies and mode types. The study also provides evidence of a relationship between the turbulence intensity and the length of the wake. The relationship however needs to be calibrated with measurements. In the last project objective, full wake interaction in large wind turbine farms was studied and verified to measurements. Large eddy simulations of the Navier-Stokes equations are performed to simulate the Horns Rev off-shore wind farm 15 km outside the Danish west coast. The aim is to achieve a better understanding of the wake interaction inside the farm. The simulations are performed by using the actuator disc methodology. Approximately 13.6 million mesh points are used to resolve the wake structure in the park containing 80 turbines. Since it is not possible to simulate all turbines, the 2 central columns of turbines have been simulated with periodic boundary conditions. This corresponds to an infinitely wide farm with 10 turbines in downstream direction. Simulations were performed within plus/minus 15 degrees of the turbine alignment. The infinitely wide farm approximation is thus reasonable. The results from the CFD simulations are evaluated and the downstream evolution of the velocity field is depicted. Special interest is given to what extent production is dependent on the inflow angle and turbulence level. The study shows that the applied method captures the main production variation within the wind farm. The result further demonstrates that levels of production correlate well with measurements. However, in some cases the variation of the measurement data is caused by the different measurement conditions during different inflow angles.
  •  
30.
  • Juhl Andersen, Søren, et al. (author)
  • Global trends in the performance of large wind farms based on high-fidelity simulations
  • 2020
  • In: Wind Energy Science. - : Copernicus GmbH. - 2366-7443 .- 2366-7451. ; 5:4, s. 1689-1703
  • Journal article (peer-reviewed)abstract
    • A total of 18 high-fidelity simulations of large wind farms have been performed by three different institutions using various inflow conditions and simulation setups. The setups differ in how the atmospheric turbulence, wind shear and wind turbine rotors are modeled, encompassing a wide range of commonly used modeling methods within the large eddy simulation (LES) framework. Various turbine spacings, atmospheric turbulence intensity levels and incoming wind velocities are considered. The work performed is part of the International Energy Agency (IEA) wind task Wakebench and is a continuation of previously published results on the subject. This work aims at providing a methodology for studying the general flow behavior in large wind farms in a systematic way. It seeks to investigate and further understand the global trends in wind farm performance, with a focus on variability.Parametric studies first map the effect of various parameters on large aligned wind farms, including wind turbine spacing, wind shear and atmospheric turbulence intensity. The results are then aggregated and compared to engineering models as well as LES results from other investigations to provide an overall picture of how much power can be extracted from large wind farms operating below the rated level. The simple engineering models, although they cannot capture the variability features, capture the general trends well. Response surfaces are constructed based on the large number of aggregated LES data corresponding to a wide range of large wind farm layouts. The response surfaces form a basis for mapping the inherently varying power characteristics inside very large wind farms, including how much the turbines are able to exploit the turbulent fluctuations within the wind farms and estimating the associated uncertainty, which is valuable information useful for risk mitigation.
  •  
31.
  • Korb, Henry, et al. (author)
  • Exploring the application of reinforcement learning to wind farm control
  • 2021
  • In: Journal of Physics, Conference Series. - : Institute of Physics Publishing (IOPP). - 1742-6588 .- 1742-6596. ; 1934
  • Journal article (peer-reviewed)abstract
    • Optimal control of wind farms to maximize power is a challenging task since the wake interaction between the turbines is a highly nonlinear phenomenon. In recent years the field of Reinforcement Learning has made great contributions to nonlinear control problems and has been successfully applied to control and optimization in 2D laminar flows. In this work, Reinforcement Learning is applied to wind farm control for the first time to the authors' best knowledge. To demonstrate the optimization abilities of the newly developed framework, parameters of an already existing control strategy, the helix approach, are tuned to optimize the total power production of a small wind farm. This also includes an extension of the helix approach to multiple turbines. Furthermore, it is attempted to develop novel control strategies based on the control of the generator torque. The results are analysed and difficulties in the setup in regards to Reinforcement Learning are discussed. The tuned helix approach yields a total power increase of 6.8 % on average for the investigated case, while the generator torque controller does not yield an increase in total power. Finally, an alternative setup is proposed to improve the design of the problem.
  •  
32.
  • Korb, Henry, et al. (author)
  • The characteristics of helically deflected wind turbine wakes
  • 2023
  • In: Journal of Fluid Mechanics. - : Cambridge University Press. - 0022-1120 .- 1469-7645. ; 965
  • Journal article (peer-reviewed)abstract
    • The helix approach is a new individual pitch control method to mitigate wake effects of wind turbines. Its name is derived from the helical shape of the wake caused by a rotating radial force exerted by the turbine. While its potential to increase power production has been shown in previous studies, the physics of the helical wake are not well understood to date. Open questions include whether the increased momentum in the wake stems from an enhanced wake mixing or from the wake deflection. Furthermore, its application to a row of more than two turbines has not been examined before. We study this approach in depth from both an analytical and numerical perspective. We examine large-eddy simulations (LES) of the wake of a single turbine and find that the helix approach exhibits both higher entrainment and notable deflection. As for the application to a row of turbines, we show that the phase difference between two helical wakes is independent of ambient turbulence. Examination of LES of a row of three turbines shows that power gains greatly depend on the phase difference between the helices. We find a maximum increase in the total power of approximately 10 % at a phase difference of 270°. However, we do not optimise the phase difference any further. In summary, we provide a set of analytical tools for the examination of helical wakes, show why the helix approach is able to increase power production, and provide a method to extend it to a wind farm.
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33.
  • Korb, Henry, 1996- (author)
  • The Lattice Boltzmann Method for Wind Farm Simulations: Validation and Application
  • 2023
  • Licentiate thesis (other academic/artistic)abstract
    • Many new challenges in wind energy require the use of large eddy simulation for accurate modeling of wind farm flows. However, the immense computational cost hinders its use in research and industry. The lattice Boltzmann method is the most promising candidate to date to achieve the highest level of accuracy while decreasing computational cost by orders of magnitude compared to traditional methods. In this thesis, I present further development of the lattice Boltzmann method for wind energy and compile various applications, such as industrial use, generation of training data for machine learning, and analysis of wind farm control paradigms. In order to evaluate the requirements of different industrial stakeholders, we conduct a survey among industry experts on the use of large eddy simulation and show that the run time requirements indicated by many respondents can be met with the current state of the lattice Boltzmann method. In a validation study, the lattice Boltzmann method is as accurate as traditional Navier-Stokes solvers, while reducing computational cost by one to more orders of magnitude. A convolutional neural network is trained to predict average flow velocities in the wake of a single turbine. The predictions exhibit very high accuracy at execution times similar to engineering models. The lattice Boltzmann method enables the generation of larger training sets at a feasible computational cost. A proof of concept is provided for the use of reinforcement learning to discover new, cooperative wind farm control mechanisms. In an extensive analysis of the helix approach, its physical mechanisms are elucidated and a thorough parameter study of the wake of a single turbine is provided. We also study the interaction of multiple helical wakes, providing a way to extend the approach from a pair of turbines to wind farms.
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34.
  • Korb, Henry, et al. (author)
  • Validation of a Lattice Boltzmann Solver Against Wind Turbine Response and Wake Measurements
  • 2023
  • In: WAKE CONFERENCE 2023. - : Institute of Physics Publishing (IOPP).
  • Conference paper (peer-reviewed)abstract
    • Two of the major limitations facing the adoption of large-eddy simulation (LES) to the industry today are a lack of validation against full-scale measurements and the high computational cost. The lattice Boltzmann method is an approach to conduct LES that is suitable for parallelization on graphics processing units, leading to reduction in energy-tosolution by multiple orders of magnitude compared to Navier-Stokes solvers. We validate the lattice Boltzmann solver VirtualFluids against the measurements published in the SWiFT benchmark and the results obtained with LES by the participants in the benchmark. We compare inflow, turbine response and wake quantities and show that our method yields similar results. While the other LES methods vary in the required energy by one order of magnitude, our methodology is always about one to two orders of magnitude more efficient. The benchmark allows for a comparison to a large number of models, however, the scale of the turbine is not representative of modern turbines and therefore important challenges of modern turbines, such as blade deflection, could not be validated.
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35.
  • Li, Huidong, et al. (author)
  • A sensitivity study of the WRF model in offshore wind modeling over the Baltic Sea
  • 2021
  • In: Geoscience Frontiers. - : Elsevier. - 1674-9871. ; 12:6
  • Journal article (peer-reviewed)abstract
    • Accurate wind modeling is important for wind resources assessment and wind power forecasting. To improve the WRF model configuration for the offshore wind modeling over the Baltic Sea, this study performed a sensitivity study of the WRF model to multiple model configurations, including domain setup, grid resolution, sea surface temperature, land surface data, and atmosphere-wave coupling. The simulated offshore wind was evaluated against LiDAR observations under different wind directions, atmospheric stabilities, and sea status. Generally, the simulated wind profiles matched observations, despite systematic underestimations. Strengthening the forcing from the reanalysis data through reducing the number of nested domains played the largest role in improving wind modeling. Atmosphere-wave coupling further improved the simulated wind, especially under the growing and mature sea conditions. Increasing the vertical resolution, and updating the sea surface temperature and the land surface information only had a slight impact, mainly visible during very stable conditions. Increasing the horizontal resolution also only had a slight impact, most visible during unstable conditions. Our study can help to improve the wind resources assessment and wind power forecasting over the Baltic Sea.
  •  
36.
  • Li, Zhongtian, et al. (author)
  • Impact of turbine availability and wake effect on the application of dynamic thermal rating of wind farm export transformers
  • 2024
  • In: Energy Reports. - : Elsevier BV. - 2352-4847. ; 11, s. 1399-1411
  • Journal article (peer-reviewed)abstract
    • Dynamic thermal rating allows transformers to operate beyond the nameplate rating according to the actual weather and loading conditions. This paper proposes a methodology to improve the application of this technology in the design of new transformers or in the operation of existing transformers connected to wind farms by accurately predicting their load profiles, accounting for the influence of wake effect and turbine availability. Specifically, the variation of turbine availability due to the intermittent wind is considered in the load profile estimation. Additionally, a correction method, which can be incorporated into any wake model, is proposed to improve the accuracy of wake loss computation. A case study shows that the wake effect and the changing turbine availability shorten the time that the transformers maintain at full load, thereby reducing the aging rate of the wind farm export transformers. The findings suggest that considering these two factors in the DTR application can benefit the longevity and efficiency of wind farm exported transformers.
  •  
37.
  • Li, Zhongtian, et al. (author)
  • Sizing Transformer Considering Transformer Thermal Limits and Wind Farm Wake Effect
  • 2021
  • In: 2021 IEEE PES Innovative Smart Grid Technologies - Asia, ISGT Asia 2021. - : Institute of Electrical and Electronics Engineers (IEEE). - 9781665433396
  • Conference paper (peer-reviewed)abstract
    • Compared to the wind farm itself, wind farm transformers are often oversized with regard to their capacity and lifetime. One of the reasons is that power transformers are normally sized at planning stage according to their rated power limits instead of their thermal limits. The thermal limits are usually considered only in operation. In this paper, a new method is proposed to take thermal limits into account and size the wind farm transformer at planning stage based on the expected life of the transformer insulation. An analytical model of wind turbine wakes loss is combined with the transformer thermal model to calculate the expected lifetime of the transformer insulation more accurately. Different from the previous approaches, the proposed method considers both the wake effect and the time-varying ambient temperature. Results show that compared to using the constant temperature and predicted power output without wake loss consideration, the expected lifetime of transformer insulation evaluated after involving these two factors is closer to the result evaluated based on the measured wind power.
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38.
  • Mohammadi, Mohammad Mehdi, et al. (author)
  • An actuator sector model for wind power applications : a parametric study
  • 2024
  • In: Wind Energy Science. - : Copernicus Publications. - 2366-7443 .- 2366-7451. ; 9:6, s. 1305-1321
  • Journal article (peer-reviewed)abstract
    • This paper investigates different actuator sector model implementation alternatives and how they compare to actuator line results. The velocity sampling method, tip/smearing correction, and time step are considered. A good agreement is seen between the line and sector model in the rotor plane and the wake flow. Using the sector model, it was possible to reduce the computational time by 75 % compared to the actuator line model as it is possible to run the simulations with a larger time step without compromising the accuracy considerably. The results suggest that the proposed velocity sampling method produces the closest results to the line model with different tip speed ratios. Moreover, the vortex-based smearing correction applied to the sector model results in the lowest error values, among the considered methods, to correct the radial load distributions. Also, it is seen that reducing the time step compared to the one used for the actuator disc/sector does not provide an advantage considering the increased computational time.
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39.
  • Navarro Diaz, Gonzalo, et al. (author)
  • Wind turbine wake inflow over a heterogeneous forest : comparison between measurement and LES simulation
  • 2021
  • In: Journal of Physics, Conference Series. - : Institute of Physics Publishing (IOPP). - 1742-6588 .- 1742-6596. ; 1934
  • Journal article (peer-reviewed)abstract
    • In this work a new step in understanding the wind turbine (WT) wake behavior on forested areas is made. For this analysis, a pair of real scale WTs located on a site with moderately complex terrain and heterogeneous forest is simulated using Large Eddy Simulation (LES). This simulation is compared with met mast and power output measurements of two WTs in Ryningsnas, Sweden, considering near neutral stratification in the atmospheric boundary layer (ABL). Three validation steps are followed; first, the undisturbed wind profile is compared with met mast data and another similar LES code. Then, the wake for each WT wake impacting on the met mast at different directions is addressed. A feature of this pair of WTs is that these have different hub heights, but the same rated power and rotor diameter, which helps provide insight into how the tip clearance over the forest affects the operation and wake characteristics. Finally, power output deficits when the WTs are operating in each others wakes are compared to observed power deficits. For these simulations SOWFA, the OpenFOAM project for wind farms simulation in ABL, is used. In this code, three new additions are made; the forest model, the mesh modification for complex terrain and the representation of the WT using an actuator disc model with local force adaptation for wind farm flows. The simulation results show a good performance on quantitatively and qualitatively capturing the velocity in the wake, but for TKE the simulation underestimates the magnitude, and fails to match the measured structure of the wake for one of two WTs. The power deficit on the impacted WTs is well captured, despite the complexity related to turbines with different hub heights. This study makes one of the first steps on validating LES simulations for wind farms in forest.
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40.
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41.
  • Olivares-Espinosa, Hugo, et al. (author)
  • Assessment of Turbulence Modelling in the Wake of an Actuator Disk with a Decaying Turbulence Inflow
  • 2018
  • In: Applied Sciences. - : MDPI. - 2076-3417. ; 8:9
  • Journal article (peer-reviewed)abstract
    • The characteristics of the turbulence field in the wake produced by a wind turbine model are studied. To this aim, a methodology is developed and applied to replicate wake measurements obtained in a decaying homogeneous turbulence inflow produced by a wind tunnel. In this method, a synthetic turbulence field is generated to be employed as an inflow of Large-Eddy Simulations performed to model the flow development of the decaying turbulence as well as the wake flow behind an actuator disk. The implementation is carried out on the OpenFOAM platform, resembling a well-documented procedure used for wake flow simulations. The proposed methodology is validated by comparing with experimental results, for two levels of turbulence at inflow and disks with two different porosities. It is found that mean velocities and turbulent kinetic energy behind the disk are well estimated. The development of turbulence lengthscales behind the disk resembles what is observed in the free flow, predicting the ambient turbulence lengthscales to dominate across the wake, with little effect of shear from the wake envelope. However, observations of the power spectra confirm that shear yields a boost to the turbulence energy within the wake noticeable only in the low turbulence case. The results obtained show that the present implementation can successfully be used in the modelling and analysis of turbulence in wake flows.
  •  
42.
  • Olivares-Espinosa, Hugo, et al. (author)
  • Assessment of wind fields over forested sites with LES and a nacelle lidar
  • 2019
  • In: Wake conference. - : IOP Publishing.
  • Conference paper (peer-reviewed)abstract
    • The capability of a model based on Large-Eddy Simulations (LES) to reproduce turbulence measurements over a forest of heterogeneous density is assessed. Measurements are obtained from an experimental campaign with a continuous wave lidar mounted on the nacelle of a wind turbine (not considered in this work) that scans over a cone in the upstream direction. The measurements are then compared with the results of the LES of the atmospheric boundary layer in neutral stability conditions. The model comprises a full description of the forest over a large area upstream of the lidar by using plant area density data obtained with airborne laser scans, which also provides the terrain elevation. Although the relatively restricted mesh refinement of the LES leads to a limited representation of turbulence towards higher frequencies, comparisons with the measurements show that the model is capable of reproducing the turbulence levels and spatial coherence in the hypothetical rotor plane. Results permit to conclude that the LES-based model is a suitable tool to identify and predict the microscale effects that terrain features have in the wind resource for sites of high complexity. This work exemplifies the challenges associated to the process and interpretation of data from the employed lidar and its setup, for which a filtering technique potentially useful in future studies is presented.
  •  
43.
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44.
  • Simisiroglou, Nikolaos, et al. (author)
  • Validation of the actuator disc approach in PHOENICS using small scale model wind turbines
  • 2016
  • In: Science Of Making Torque From Wind (Torque 2016). - : IOP Publishing.
  • Conference paper (peer-reviewed)abstract
    • In this study two wind turbine setups are investigated numerically: (a) the flow around a single model wind turbine and (b) the wake interaction between two in-line model wind turbines. This is done by using Reynolds averaged Navier-Stokes (RANS) and an actuator disc (ACD) technique in the computational fluid dynamics code PHOENICS. The computations are conducted for the design condition of the rotors using four different turbulence closure models. The computed axial velocity field as well as the turbulent kinetic energy are compared with PIV measurements. For the two model wind turbine setup, the thrust and power coefficient are also computed and compared with measurements. The results show that this RANS ACD method is able to predict the overall behaviour of the flow with low computational effort and that the turbulence closure model has a direct effect on the predicted wake development.
  •  
45.
  • Simisiroglou, Nikolaos, et al. (author)
  • Validation of the actuator disc approach using small-scale model wind turbines
  • 2017
  • In: Wind Energy Science. - : Copernicus GmbH. - 2366-7443 .- 2366-7451. ; 2, s. 587-601
  • Journal article (peer-reviewed)abstract
    • The aim of the present study is the validation of the implementation of an actuator disc (ACD) model in the computational fluid dynamics (CFD) code PHOENICS. The flow behaviour for three wind turbine cases is investigated numerically and compared to wind tunnel measurements: (A) the flow around a single model wind turbine, (B) the wake interaction between two in-line model wind turbines for a uniform inflow of low turbulence intensity and (C) the wake interaction between two in-line model wind turbines at different separation distances in a uniform or sheared inflow of high turbulence intensity. This is carried out using Reynolds-averaged Navier–Stokes (RANS) simulations and an ACD technique in the CFD code PHOENICS. The computations are conducted for the design condition of the rotors using four different turbulence closure models and five different thrust distributions. The computed axial velocity field as well as the turbulence kinetic energy are compared with hot-wire anemometry (HWA) measurements. For the cases with two in-line wind turbines, the thrust coefficient is also computed and compared with measurements. The results show that for different inflow conditions and wind turbine spacings the proposed method is able to predict the overall behaviour of the flow with low computational effort. When using the k-ε and Kato–Launder k-ε turbulence models the results are generally in closer agreement with the measurements.
  •  
46.
  • Simisiroglou, Nikolaos, et al. (author)
  • Wind farm power production assessment : a comparative analysis of two actuator disc methods and two analytical wake models
  • 2018
  • In: Wind Energy Science. - : Copernicus GmbH. - 2366-7443 .- 2366-7451. ; 2018, s. 1-13
  • Journal article (other academic/artistic)abstract
    • The aim of the present study is to perform a comparative analysis of two actuator disc methods (ACD) and two analytical wake models for wind farm power production assessment. To do so wind turbine power production data from the Lillgrund offshore wind farm in Sweden is used. The measured power production for individual wind turbines is compared with results from simulations, done in the WindSim software, using two ACD methods (old and new) and two analytical wake models widely used within the wind industry (Jensen and Larsen wake models). It was found that the new ACD method and the Larsen model outperform the other method and model in most cases. Furthermore, results from the new ACD method show a clear improvement in the estimated power production in comparison to the old ACD method. The Jensen method seems to overestimate the power deficit for all cases. The new ACD method, despite it's simplicity, is capable of capturing the power production within the given error margin although it tends to underestimate the power deficit.
  •  
47.
  • Simisiroglou, Nikolaos, et al. (author)
  • Wind Farm Power Production Assessment : Introduction of a New Actuator Disc Method and Comparison with Existing Models in the Context of a Case Study
  • 2019
  • In: Applied Sciences. - : MDPI AG. - 2076-3417. ; 9:3
  • Journal article (peer-reviewed)abstract
    • The aim of the present study is to perform a comparative analysis of two actuator disc methods (ACD) and two analytical wake models for wind farm power production assessment. To do so, wind turbine power production data from the Lillgrund offshore wind farm in Sweden is used. The measured power production for individual wind turbines is compared with results from simulations, done in the WindSim software, using two ACD methods (ACD (2008) and ACD (2016)) and two analytical wake models widely used within the wind industry (Jensen and Larsen wake models). It was found that the ACD (2016) method and the Larsen model outperform the other method and model in most cases. Furthermore, results from the ACD (2016) method show a clear improvement in the estimated power production in comparison to the ACD (2008) method. The Jensen method seems to overestimate the power deficit for all cases. The ACD (2016) method, despite its simplicity, can capture the power production within the given error margin although it tends to underestimate the power deficit.
  •  
48.
  • Simisiroglou, Nikolaos (author)
  • Wind power wake modelling : Development and application of an actuator disc method for industrial utilization.
  • 2018
  • Doctoral thesis (other academic/artistic)abstract
    • As a wind turbine extracts energy from the wind it creates a region downstream where the wind velocity is decreased and the urbulence intensity is increased, this region is commonly called the wake region. Today’s wind farms include a large number of wind turbines position in tight layouts. These tight layouts result in increased power losses due to wakes, rendering accurate wind turbine wake modelling crucial in developing cost effective projects.The primary aim of this study is to create a method capable of conducting full-scale wind farm wake computations accurately in a time efficient manner by taking into account the computational resources and data availability of a typical industrial user. As a first step of this study, an actuator disc (ACD) method (old ACD) used within WindSim, is evaluated against power production data from the Lillgrund offshore wind. This study is followed by the development of a new ACD method. The new ACD method differs from the previous ACD method in terms of how the thrust distribution and the power production is calculated. A series of validation studies are performed on this newly introduced ACD method. These consist of validating the method against two cases with known analytical solutions, the research code EllipSys3D which uses Large Eddy Simulation (LES) based computations with an ACD approach and three differentwind tunnel set–ups. Lastly, a comparative analysis of the two ACD methods (old and new) and two analytical wake models is done using wind turbine power production data from Lillgrund.Results from the validation studies show that this new ACD method is able to predict the overall behaviour of the flow with low computational effort while also taking into account the availability of data for a typical industrial user. One may say that the new ACD method in RANS, which has much lower computational requirements than the ACD method in LES at the cost of lower accuracy, represents a good compromise. Lastly, the results from the new ACD method show a clear improvement in the estimated power production for the Lillgrund wind farm in comparison to the old ACD method.
  •  
49.
  • Sörensen, Jens Nörkaer, et al. (author)
  • Analytical body forces in numerical actuator disc model of wind turbines
  • 2020
  • In: Renewable energy. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0960-1481 .- 1879-0682. ; 147:Part 1, s. 2259-2271
  • Journal article (peer-reviewed)abstract
    • An analytical model for representing body forces in numerical actuator disc models of wind turbines is developed and validated. The model is based on the assumption that the rotor disc is subject to a constant circulation modified for tip and root effects. The model comprises expressions for both the axial and the azimuthal force distributions, and is generalized to be utilized for all kinds of inflow, including wind shear, turbulence, and shadow effects in wind farms. The advantage of the model is that it does not depend on any detailed knowledge concerning the wind turbine being analysed, but only requires knowledge regarding the rated wind speed and nameplate capacity. To validate the analytical model, results are compared to numerically generated results using detailed information regarding geometry and airfoil data for the 2 MW Tjaereborg wind turbine and the 10 MW DTU reference turbine. The comparisons show very good agreement between the loadings using the new analytical model and the airfoil data based method for the two tested wind turbines, demonstrating that the analytical model is a simple and reliable way of introducing body forces in actuator disc simulations without any prior knowledge of the wind turbine being analysed.
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